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Two types of oil modified tips as force sensors to detect adhesion forces between oil and membrane surfaces in fluid
Sensors and Actuators A 267 (2017) 127–134
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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Two types of oil modified tips as force sensors to detect adhesion forces between oil and membrane surfaces in fluid Jing Han a , Linyan Xu a,∗ , Shuangbei Qian a , Xiaodong Hu a , Tong Guo a , Sen Wu a , Yanan Liu b , Zhongyi Jiang b a b
State Key Laboratory of Precision Measuring Technology and Instruments, Tianjin University, Tianjin 300072, China Key Laboratory for Green Chemical Technology of Ministry of Education, Tianjin University, Tianjin 300072, China
a r t i c l e
i n f o
Article history: Received 10 November 2016 Received in revised form 5 September 2017 Accepted 1 October 2017 Available online 7 October 2017 Keywords: Antifouling property Probe modification Force sensor Adhesion force
a b s t r a c t The surface resistance to oil is very important for filtration efficiency, film cleaning and life span of ultrafiltration membranes. The atomic force microscope (AFM) has provided unprecedented opportunities to study interface interactions. In this paper, the probe with an oil droplet modified was used as a force sensor for evaluating the oil resistance of membrane surfaces. The adhesion forces between two types of modified tips and a serial of membrane surfaces in fluid was detected via force curves. On the one hand, the AFM probe with an oil droplet immobilized directly was used to study the adhesion forces between a series of PVF/F127 membranes and a hexadecane droplet. On the other hand, a tip attached polystyrene (PS) microsphere covered with a layer of oil was employed. It was found that the oil antifouling performance of membranes was improved effectively through blending F127 with PVF. As the F127 additive varied from 0% to 60%, adhesion force detected by the AFM probe with an hexadecane droplet immobilized showed a decreasing tendency from 2.84 nN to 0.73 nN. Meanwhile, the average interaction force detected by the second modified tip decreased from 3.39 nN to 0.54 nN. The measured force behavior was in agreement with experimental observations of contact angle measurement, which indicated that the blend membranes had better antifouling. © 2017 Published by Elsevier B.V.
1. Introduction With the development of the petrochemical industry, emissions of oily wastewater are increasing and environmental problems have become a great challenge. Membrane technology is regarded as the most efficient approach for advanced wastewater treatment and reuse, owing to its advantages of low energy consumption, high separation efficiency, low environmental pollution, flexible and simple process and strong versatility. However, oil droplets may directly come into contact with membrane surfaces in reality. The inherent oil fouling often leads to a cascade of events, such as adhesion, accumulation, spreading, coalescence and migration of suspended oil foulants, which is much more complicated than the fouling caused by soluble organic macromolecules and insoluble inorganic matter [1]. As a result, the gradual coalescence and migration of oil droplets form a continuous oil film and cause a sharp
∗ Corresponding author. E-mail address: [email protected] (L. Xu). https://doi.org/10.1016/j.sna.2017.10.003 0924-4247/© 2017 Published by Elsevier B.V.
decline of permeation flux temporarily or permanently, leading to a reduced life span of the membranes. Superhydrophilic-superoleophobic films utilize “waterremoving” method [2] that effectively prevents the adhesion of oil droplets and could be applied to the mixture with small percentages of oil, and it is usually selected because of its low energy consumption. Researches show that the hydrophilicchemical materials and micro/nano composite structure are the key factors to design antifouling membrane surfaces [3,4]. Surface modification, physical blending and chemical grafting are mostly adapted to improve the membrane hydrophilicity and fouling resistance. Among these methods, blending has been extensively applied due to its versatile controlling conditions. Filtration test [5,6] and adsorption test [7] evaluate the antifouling performance of membranes on the macroscopic level. Filtration test usually takes ∼3 h for one membrane, which is timeconsuming. Adsorption test has the same problem. Contact angle measurement [8] evaluates the antifouling property of films by measuring the geometrical shape of oil droplets on films, which can only evaluate the hydrophilic property of materials. It belongs to point measurement method. And the result has certain contin-
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gency and randomness. Pore analysis [9,10] and surface roughness evaluation [11] are methods to explore the relationship between structural parameters and antifouling property of membrane surfaces. They can only be used as a reference and comparison of other methods. The methods discussed above are all on the macroscopic level. However, a quantitative and accurate measurement of the force between the oil and the sample membranes in microscopic aspect can be made. The atomic force microscope (AFM) can be used to estimate the interaction between membrane surfaces and pollutants modified on the tip of the special AFM probes, which is one of the most direct methods to assess the antifouling property of membranes. Hence, AFM probes are sensitive force sensors to convert interaction force between them to the deflection of the cantilevers. Finally, the interaction force is calculated by several parameters. It is well-known that a bovine serum albumin (BSA) protein immobilized tip must be obtained by a series of complicated steps, such as vacuum oxygen plasma method, chemically modification and so on. It is used to evaluate the bio-antifouling properties of the membrane. The new methods discussed in this paper aim at evaluate the oil-antifouling performance of the membrane and they are more simplified and efficient. However, the approach to modify an oil droplet on the tip of the cantilever is different from the traditional method of protein modification. The protein cluster can be fixed on the tip by the strong interaction of chemical bond [11,12], which is generally considered to be a rigid connection with the same structure. The oil droplets would not be stable in air, so the oil droplet modification must be carried out in the liquid phase. After the modification, it must ensure that the fixed position of the oil drop is always the same during the whole experiment. In addition, oil droplets are flexible structures. It is worth discussing whether the loss of the oil droplet during the testing process will have an influence on the results. The size of AFM cantilevers and the micron dimension of oil droplets are well matched. The cantilever with a micron oil droplet modified can be used as a high sensitive sensor for the measurement of adhesion between oil pollutants and films, which can be expected to be an effective method of membrane antifouling evaluation in microscopic aspect. In this paper, two methods are adopted to transfer a target oil droplet on the tip of a bare cantilever. A series of polyvinyl formal (PVF) membranes with different percent of additive F127 were prepared via non-solvent induction phase separation process. F127 was usually used as a surface segregation additive to construct a hydrophilic brush layer on the membrane surface and to further improve the antifouling performance of PVF membranes. The adhesion forces between the blend membranes and the oil droplets (hexadecane) on the AFM tips were measured in order to compare the antifouling property of the membranes. But the shortcoming of these methods lies in the wide range of the spring constant of the commercial AFM probes. So it is necessary to calibrate the AFM cantilevers and only the data from just the
same cantilever is comparable. Moreover, comparison experiment of contact angle measurement has also been done for analysis and comparison. 2. Material and methods 2.1. Materials Polyvinyl formal (PVF, MW = 35 kDa, acetalization degree = 80%) purchased from Tokyo Chemical Industry Corp. (Japan) was dried at 60 ◦ C for 12 h before use. Triblock copolymer Pluronic F127 (EO100 PO65 -EO100 ) with Polyethylene oxide (PEO) content of 70 wt% was purchased from Sigma Chemical Company (USA). Preparation of PVF/Pluronic F127 membranes was described in the previous study[13]. NP-O10 probes made of silicon nitride were purchased from Bruker Corp. (Germany). HA-C/tipless probes made of polysilicon were purchased from NT-MDT Corp. (Russia). Hexadecane were obtained at >99% (Sigma Aldrich). Polystyrene (PS) microspheres and special glue were provided by School of Material, Tianjin University. All silicon wafers were carefully cleaned with ethanol through ultrasonic cleaning machine (KQ3200E, China). Other chemicals were purchased from Kewei Chemicals Corp. (China). 2.2. Interaction forces between an oil droplet-immobilized tip and membrane surfaces 2.2.1. Preparation of oil droplets Two micropipettors (0.5–10 L and 20–200 L) were used to prepare hexadecane droplets in the following manner. Firstly, a silicon wafer was cleaned by ultrasonic cleaning machine. Then the hexadecane was sucked into a 0.5–10 L micropipettor and pure water was sucked into a 20–200 L micropipettor. Finally, it is better to direct micropipettor containing hexadecane approximately 4 cm over the top of the slide and to add pure water onto the silicon wafer quickly after squeezing it, in order to create a fine spray of hexadecane droplets which was dispersed over a clean silicon wafer. It was found that this method resulted in a fairly uniform coverage of the slide with discrete 10–40 m diameter oil droplets. 2.2.2. Attachment of oil droplets onto the AFM cantilever The AFM head holding NP-O10 tipless cantilever (Si3 N4 ) was positioned directly above a droplet on the surface of the silicon wafer. Then it was lowered down in order to sandwich the water between the cantilever holder for fluid operation and the silicon wafer. And it was advised to adjust the value of setpoint during this period. Once engulfment of the hexadecane droplet had been achieved, the AFM head and cantilever were retracted away from the surface of the slide, where upon the oil droplet was found to detach itself from the slide surface. This method allowed the
Fig. 1. Images of water droplets on the probe surface in air.
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Fig. 2. Schematics illustrating the conceptual designs. (a) Precise location of bare cantilever onto a hexadecane droplet. (b) The cantilever with an attached oil droplet pulled away from the silicon surface. (c) The direct modified cantilever approaches/retracts away the sample membrane to detect adhesion force between the oil and the membrane surface. (d) The indirect modified cantilever approaches/retracts away the sample membrane.
droplet to be placed with high precision at the end of the cantilever. Repeated experiments showed that silicon wafers were more convenient for oil droplets to transfer to cantilevers than glass slides. Besides, two kinds of probes (HA-C/tipless and NP-O10) were used. It was found that NP-O10 tipless cantilever was easier to achieve attachment of oil droplets onto the AFM cantilever than HA-C/tipless because NP-O10 was more oleophilic, in accordance with the larger water contact angle of it (Fig. 1). 2.2.3. AFM force curve measurement of membranes Each membrane was immersed in water for 24 h in advance. Force measurement was made between the modified tip and the membrane surfaces by an atomic force microscope (Dimension Icon, Bruker Corp., Germany) in fluid. Firstly, the sensitivity S of the cantilever should be calibrated before AFM force curve measurement, which can be illustrated in Eq. (1). S=
z U
(1)
Where Z is the cantilever deflection signal; U is voltage applied to the piezo scanner in the vertical direction. The detector sensitivity was determined by pressing the bare cantilever against the silicon surface 10 times in water environment, and the average value of the experimental results was calculated afterwards. The details of AFM probe calibration are shown in Section 3.2. The nominal value of the spring constant of the cantilevers provided by the supplier is not accurate for every specific probe, which is calculated based on the geometry of the cantilever designed and the approximate material parameters estimated. At present, many calibration methods have been proposed, such as the Sader method [14,15], the thermal noise method [16] and the balance method [17]. In this study, the nominal spring constant of the NP-O10 tipless can-
tilever used was 0.12 N/m. However, the exact values of 10 probes in the same box were found to be in the range of 0.11-0.18 N/m using a thermal frequency method carried out in fluid. The deviation between maximum and minimum was 81.8%. Secondly, the silicon wafer with the mixture of water and drops of oil (hexadecane) was placed in a sink with the sample membranes stuck in. Then the tip was modified with oil droplet according to the method mentioned in Sections 2.2.1 and 2.2.2. It can be seen from Fig. 2a–b. Thirdly, water was added into the sink about 0.5 mm high and then the modified tip was positioned directly above the surface of the PVF membrane. Fourthly, the tip approached the membrane surface. An interaction force was occurred between the tip and the membrane surface, as shown in Fig. 2c. A speed of 0.2 m/s was applied to obtain all the force curves. They were collected from at least six positions on the same sample. Approximately 60 extending/retracting cycles were performed for each membrane surface versus the same tip. Raw data were collected as a detector voltage output, which reflects the cantilever deflection versus relevant piezo motion. The other four PVF/F127 membranes with different ratios (10%, 20%, 40%, 60%) were examined in turns and the experiments were repeated. Conversion of the cantilever deflection voltage data (V) to force (nN) was performed using the standard procedure of multiplication by the AFM photodiode detector sensitivity (nm/V) and the measured cantilever spring constant (N/m) [18]. Finally, the adhesion force F between tip and membrane surface could be calculated by Eq. (2). F = k · Z = k · S · U
(2)
Here, Z is the deflection of the cantilever; U is the PSD output voltage of the cantilever, which could be derived from the data in the vertical axis of the AFM force curve.
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used to AFM force curve measurement, as shown in Fig. 2d. Similar method was described in Section 2.2.3. 100 force curves were done in this part.
2.4. Contact angle measurement on PVF/F127membranes
Fig. 3. Force-curve records monitoring cantilever deflection versus distance for a NP-O10 tipless cantilever.
2.3. Interaction forces between membrane surfaces and a PS microsphere covered with a layer of oil
The contact angle is an important parameter in surface science. It is a regular measurement of the surface hydrophobicity. Contact angle between water and the membrane surface was measured in a contact angle meter (Erma Contact Angle Meter, Japan). Each value was obtained immediately after dropping water on the membrane surface. Furthermore, underwater wetting properties of oil droplets (hexadecane) on membrane surfaces were examined. It must be noticed that each membrane should be immersed in water for 24 h in advance. The measurements were taken at five different locations on the same sample, and the average value of the experimental results was calculated afterwards.
3. Results and discussion 3.1. Spring constant calibration of cantilevers
As mentioned in Section 2.2.1, preparation of oil droplets with size more than tens of micrometer has strong randomicity. Due to the deformation of oil droplets, the calibration of elasticity constant of the probe will be affected. However, the method discussed here can ensure the better uniformity of the oil droplet size and the accuracy of the elasticity constant of the probe because of the rigid microsphere. Apart from an oil droplet-immobilized tip, another modified tip described in the following manner can also be used for AFM force curve measurement. Firstly, a small amount of glue was placed on a glass slide. Then the AFM head holding NP-O10 tipless cantilever (Si3 N4 ) was positioned directly on the edge of glue, so that only the tip of the probe adhered to the glue when the AFM head was engaged. Secondly, a PS microsphere with a known diameter (less than 10 m) was attached to the front end of a bare cantilever. Thirdly, the tip should be immersed into oil environment in order to achieve adsorption of oil. Finally, the modified tip was
The sensitivity changes with different cantilever geometric dimensions, as well as the position and quality of the laser spot on the cantilever. Therefore, the installation position of a tip and the laser spot should not be moved during the whole calibration process in order to avoid the changes of the sensitivity. As mentioned in Section 2.2.3, sensitivity was determined by pressing the bare cantilever against the silicon surface at least 10 times in fluid. One pair of the force curves were shown in Fig. 3, and the average value of the sensitivity for 10 repeated measurements was 96.821 nm/V. It can be seen from Fig. 3 that there is no obvious adhesion force between the tip and the silicon wafer. Presently, the thermal noise method has become the standard method of AFM experiments. Thermal frequency range was set to be 1–100 kHz covering the nominal frequency of the cantilever. After adjusting the parameters, the experimental result of spring constant was 0.155 N/m.
Fig. 4. Optical micrographs illustrating the attachment of oil droplets to a tipless AFM cantilever under water. (a) Precise location of the cantilever onto a hexadecane droplet. (The focus was on the oil droplets.) (b) The cantilever pulled away from the silicon surface with an attached oil droplet. (c) Relocation of the cantilever with an attached droplet. (The focus was on the probe.) (d)-(f) transference of a dichloroethane droplet onto the end of a tipless cantilever.
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Fig. 5. Force-curve records of an oil droplet immobilized tip against (a) PVF membrane, (b) 10% additive F127, (c) 20% additive F127, (d) 40% additive F127, and (e) 60% additive F127. (f) Sum up of the adhesion forces with different ratios (0%, 10%, 20%, 40%, 60%).
3.2. Oil droplet-immobilized AFM tips Fig. 4a–c showed a series of images captured through AFM, illustrating the transfer of a hexadecane droplet onto the end of the cantilever. In the first stage of the process, the cantilever was precisely located on the selected oil droplet in order to obtain proper position to achieve the attachment of oil droplets (Fig. 4a). The size of oil droplets should be moderate. Note that several other droplets attached to the silicon wafer were also in focus. Once the engulfment of the cantilever into an oil droplet was achieved, the cantilever pulled away from the surface with an attached oil droplet. A little residual oil remained on the surface (Fig. 4b). Fig. 4c was focused on the cantilever, and hence those droplets that were still attached to the surface were now out of focus. But it could not be seen from Fig. 4c that the droplet was clearly attached to the cantilever, as a result of its small size. Fig. 4d–f demonstrated the attachment of a dichloroethane droplet. Fig. 4d showed that the diameter of the dichloroethane droplet was larger (∼75 m). There was a little oil trace on the silicon surface after the attachment, shown in Fig. 4e. It can be clearly seen from Fig. 4f that the droplet was attached to the cantilever. After all the force curve experiments, the modified tip was the same. Therefore, it is obvious that the attached droplet is stable and that the cantilever can be repositioned and used to adhesion force measurement. However, hexadecane was used to prepare droplets in this study, owing to its convenience of the method described in Section 2.2.1, while it was more difficult for volatile dichloroethane to create a fine spray of droplets. 3.3. AFM force curve measurement of an oil droplet-immobilized tip The antifouling performance of membranes is dominated by the interaction between the foulants and the membrane surfaces. Meanwhile, there is considerable amount of literature showing that the magnitude of the adhesion force correlates closely with the fouling propensity of the membranes [19–21]. However, different
from the previous researches [22,23], the adhesion force measurement by AFM was carried out in fluid. As the probe approaches or retracts towards the membrane surface, the resulting interaction caused by the hydrodynamic and adhesion force generates a total interaction force. A theoretical prediction of experimental forces is provided by the hydrodynamic theory of Chan et al. [24,25]. Therefore, it should be noted that the approaching/retracting speed of the probe on AFM force curve measurement has a great influence on the results of the experiment in fluid. When the velocity is under 1 m/s, the resulting interaction can be regarded as quasi-static interaction [26]. There is a negligible hydrodynamic contribution to the adhesion force measurement. To ensure the accuracy of the data, slower nominal velocities (0.5 m/s, 0.2 m/s and 0.1 m/s) were investigated. When it was 0.5 m/s, the adhesion force was about 2.28 nN. And it was around 2.84 nN at a slower velocity of 0.2 m/s. There was no big difference between 0.2 m/s and 0.1 m/s for detected adhesion forces. So it was reasonable to choose 0.2 m/s due to its obvious experimental results and moderate time consumption. Cautions must therefore be taken in direct comparison with adhesion forces of different membranes. Only the data from just the same cantilever and the same speed of extending/retracting is comparable. Otherwise the results will be inaccurate. Conversion of the AFM output to a force was calculated by Eq. (2). Fig. 4a–e show the interaction forces between a hexadecane droplet immobilized tip and the PVF membranes with 0–60 percentage of F127 additive. X-axis represents the displacement of the AFM piezo scanner in vertical direction, and Y-axis represents the interaction forces between the tip and the membranes. Obviously, the overlap of the approach portions of the force curves indicates a negligible hydrodynamic contribution to the force. There is no obvious attractive force in the extending curve. While in the retracting curve, the interactions show a deep “attractive minimum” followed by a sharp pull-off force. It is used to evaluate the antifouling performance of the membranes. Fig. 4f shows the sum up of the adhesion forces with different ratios (0%, 10%, 20%, 40%, 60%). It can be seen from Fig. 4f that with the increasing addition of F127, the average interaction force decreases
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Fig. 6. A PS microsphere with a known diameter (less than 10 m) was used to modify a tipless cantilever. (a) Optical micrograph of two discrete PS microspheres. (b) Optical micrograph of a modified tip attached a PS microsphere.
obviously from 2.84 nN to 0.73 nN. The surface resistance to oil is characterized by the maximum debonding force during the unloading process. The loading process is not employed to detect the force. It just ensures that oil droplets can contact with the membrane surfaces. Therefore, the maximum loading force is controlled in 6 nN (Fig. 4a–e), which aims at avoiding mass loss and movement of the fixed oil droplet during the repeated loading/unloading process. In the experiment, the same oil modified tip was always used on a set of films during 300 loading/unloading process. Through good control, the movement of oil droplet have not yet been discovered. At the same time, it also shows that the affinity between the oil droplet and the cantilever is strong. The sample membranes have good oil antifouling, because the residual oil were not observed like in Fig. 4b during the test. But the process is possible to leave oil remnant which is not easy to be discovered on sample surfaces. So each testing point is repeatedly measured continuously. If the phenomenon of oil remaining on films occur, the adhesion will exhibit a monotonically increasing trend. In fact, the adhesion force detected in single point repeated measurement shows a random distribution. Hence, there is no need to consider the influence of residual oil in the test. In Fig. 5f, experimental results suggest that the PVF/F127 membranes show better antifouling properties than pristine PVF membrane. With the increasing of F127 additive percentage, the antifouling performance becomes more effective. The adhesion forces between membrane surfaces and foulants have a substantial impact on the membrane fouling. Pluronic F127 bearing hydrophilic PEO segments and hydrophobic PPO segments was considered as both excellent surface antifouling modifier [27]. Here, higher F127 contents in membranes indicated high surface coverage of hydrophilic material (PEO segments) on membranes, which resulted in the minimum adhesion force and an improved antifouling property. And the significant decrease in the adhesion force may originate from the energetic adhesion barrier created by the extended antifouling brushes on the membrane surfaces. The measured force behavior was in agreement with experimental observations of antifouling performance of the membranes against different proteins [13].
a PS microsphere coated a layer of oil and PVF membranes with different ratios (0%, 10%, 20%, 40%, 60%) of F127. Obviously, the average interaction force decreases from 3.39 nN to 0.54 nN with the increasing addition of F127. Therefore, the dependence of the pull-off forces on different ratios of F127 contents in membranes is still valid. In spite of the reducing diameter of oil droplet, the adhesion forces are quiet close comparing with Fig. 5f. It is supposed that when the AFM cantilever is retracted away from the surface of the membrane, oil may deform and stretch resulting in equivalent contact area with the membrane. Hence, the pull-off forces are not quite different at the moment of the oil departing from membrane surfaces. The two methods are considered to be equivalent under the condition that the size of oil droplet has the same magnitude. The difference between the two methods is that the diameter of oil droplets is from 10 microns to tens of microns, so one of them can be selected. The first method is to directly modify an oil droplet on the cantilever. Its difficulty lies in the control of oil droplet which should not move during the test. The second showing better oil droplet fixing effect is an indirect method with the help of the PS microsphere as the intermediate medium. The researchers made an oil droplet fixed in a large metal ring structure under the optical microscope. They combined the ring structure and force sensors to detect the adhesion force between oil droplets and films. Their experiments showed an adhesion force reached around 1 N in water. The method is feasible for the lateral comparison of antifouling performance of membranes.
3.4. AFM force curve measurement of a tip attached PS microsphere covered with a layer of oil Fig. 6a shows an image captured through AFM, illustrating two discrete PS microspheres with the size of less than 10 m. Fig. 6b obtained by an upright metallurgical microscope (Shanghai, China) shows a PS microsphere is accurately attached to the tip of the NP-O10 tipless cantilever. The sensitivity of the modified tip was 60.49 nm/V via the method mentioned in Section 2.2.3. And its spring constant was found to be 0.108 N/m through thermal noise method. Fig. 7 shows the sum up of the adhesion forces between
Fig. 7. Sum up of the adhesion forces between a PS microsphere coated a layer of oil and PVF membranes with different ratios (0%, 10%, 20%, 40%, 60%) of F127.
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Fig. 8. Results of contact angle measurement. (a) Image of a water droplet on a membrane surface in air. (b) Image of a hexadecane droplet on a membrane surface under water. (c) A summary of contact angles of PVF/F127 membranes in air and under water with different ratios.
3.5. Contact angle measurement on PVF/F127membrane The water contact angles in air and underwater wettabilities of oil (hexadecane) on the blending membrane surfaces were comprehensively characterized (Fig. 8). The PVF membrane possessed the highest water contact angle of 61.34◦ , while the PVF/F127 membranes had lower water contact angles. With the increasing of the F127 additive percentage, the water contact angles were remarkably decreased to 49.65◦ . However, the gradual increase in F127 content promoted an increase in underwater hexadecane contact angles from 116.13◦ to 124.67◦ . That’s to say, the contact angles of water in air decreased with increasing hydrophilicity of the membranes, and the results are contrasted with the contact angles of oil on these surfaces in water. It was known that membranes with higher hydrophilicity exhibited better antifouling property [4]. Comparing with Fig. 5f, the results also showed the equivalent tendency of the antifouling performance. The data of contact angle measurement agreed with AFM force curves very well, which could be an evidence of correctness and effectiveness of AFM force curve method. 3.6. Comparison of two modification methods The advantage of the direct method is that it is quick and convenient. But the size of oil droplets modified on the bare cantilever has strong randomicity. Besides, only some specific oil droplets can be transferred efficiently. Other types of oil droplets could be attached to the tip of a bare cantilever after the chemical modification of the cantilever. The indirect method can ensure the better uniformity of the oil droplet size and the accuracy of the elasticity constant of
the probe because of the rigid microsphere. Meanwhile, the different type of oils has no significant influence on this method, which is more feasible. However, a PS microsphere should be attached to the tip of the NP-O10 tipless cantilever in advance. And this obtained probe should be followed by an ultraviolet lamp for one night exposure, so that the microsphere can be fixed to ensure the follow-up test. In brief, the two methods based on AFM in this paper are more suitable for the accurate evaluation of the oil antifouling property of membranes, because they are more close to the actual process of the separation of water and oil.
4. Conclusions In summary, two novel and generic approaches to evaluate the oil antifouling property of membranes have been proposed. The adhesion force between the oil droplet and the prepared membrane surface measured by two types of modified tips was a significant parameter that allowed direct assessment of the oil adhesion behavior at the interface. As a result of the gradual increase in F127 content, adhesion force showed a decreasing tendency, which was an evidence for less direct contact of oil droplets. Besides, it was indicated that the membranes with increasing additive of F127 had a favourable antifouling performance, along with more underwater hexadecane contact angles. In the future, researches that explore the adhesion force of a variety of oils on the surface can be investigated. The practicality of such interaction forces measurement between oil and membranes will give rise to attractive prospects for oily wastewater treatment evaluation. And this study may set a simple and universal example to evaluate the surface antifouling
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performance and research interface behaviours for a wider range of relevant applications. Acknowledgments This project was supported by National Natural Science Foundation of China (Grant No. 51305298, No. 51675379) and Tianjin Research Program of Application Foundation and Advanced Technology (Grant No. 13JCQNJC04700). References [1] W. Chen, Y. Su, J. Peng, Y. Dong, X. Zhao, Z. Jiang, Adv. Funct. Mater. 21 (2011) 191–198. [2] A.K. Kota, Y. Li, J.M. Mabry, A. Tuteja, Adv. Mater. 24 (2012) 5838–5843. [3] Y. Huang, H. Li, L. Wang, Y. Qiao, C. Tang, C. Jung, Y. Yoon, S. Li, M. Yu, Adv. Mater. Interfaces 2 (2015) 1400433. [4] M. Liu, S. Wang, Z. Wei, Y. Song, L. Jiang, Adv. Mater. 21 (2009) 665–669. [5] X. Zhao, Y. Su, J. Cao, Y. Li, R. Zhang, Y. Liu, Z. Jiang, J. Mater. Chem. A 3 (2015) 7287–7295. [6] C. Zhao, X. Xu, J. Chen, F. Yang, J. Environ. Chem. Eng. 1 (2013) 349–354. [7] Y. -q. Wang, T. Wang, Y. -l. Su, F. -b. Peng, H. Wu, Z. -y. Jiang, J. Membr. Sci. 270 (2006) 108–114. [8] Y. Sui, Z. Wang, X. Gao, C. Gao, J. Membr. Sci. 413–414 (2012) 38–47. [9] C. Liao, J. Zhao, P. Yu, H. Tong, Y. Luo, Desalination 260 (2010) 147–152. [10] C. Liao, J. Zhao, P. Yu, H. Tong, Y. Luo, Desalination 285 (2012) 117–122. [11] Y. Liu, Y. Su, X. Zhao, R. Zhang, T. Ma, M. He, Z. Jiang, J. Membr. Sci. 499 (2016) 406–417. [12] E.C. Cho, D.H. Kim, K. Cho, Langmuir 24 (2008) 9974–9978. [13] Y. Liu, L. Xu, Y. Song, X. Fu, J. Zou, X. Hu, Z. Jiang, X. Zhao, RSC Adv. 5 (2015) 36894–36901. [14] J.E. Sader, J.W. Chon, P. Mulvaney, Rev. Sci. Instrum. 70 (1999) 3967–3969. [15] J. Cleveland, S. Manne, D. Bocek, P. Hansma, Rev. Sci. Instrum. 64 (1993) 403–405. [16] J.L. Hutter, J. Bechhoefer, Rev. Sci. Instrum. 64 (1993) 1868–1873. [17] M.-S. Kim, J.-H. Choi, Y.-K. Park, J.-H. Kim, Metrologia 43 (2006) 389. [18] S. Sundararajan, B. Bhushan, Sens. Actuators A. Phys. 101 (2002) 338–351. [19] Y. Mo, A. Tiraferri, N.Y. Yip, A. Adout, X. Huang, M. Elimelech, Environ. Sci. Technol. 46 (2012) 13253–13261. [20] D.-G. Kim, H. Kang, S. Han, J.-C. Lee, J. Mater. Chem. 22 (2012) 8654–8661. [21] C.J. Weinman, N. Gunari, S. Krishnan, R. Dong, M.Y. Paik, K.E. Sohn, G.C. Walker, E.J. Kramer, D.A. Fischer, C.K. Ober, Soft Matter 6 (2010) 3237–3243. [22] Y. Liu, Y. Su, X. Zhao, Y. Li, R. Zhang, Z. Jiang, J. Membr. Sci. 486 (2015) 195–206. [23] J. Zhang, Z. Xu, M. Shan, B. Zhou, Y. Li, B. Li, J. Niu, X. Qian, J. Membr. Sci. 448 (2013) 81–92. [24] R. r. Manica, J.N. Connor, R.R. Dagastine, S.L. Carnie, R.G. Horn, D.Y.C. Chan, Phys. Fluids 20 (2008) 032101.
[25] S.L. Carnie, D.Y. Chan, C. Lewis, R. Manica, R.R. Dagastine, Langmuir 21 (2005) 2912–2922. [26] H. Lockie, R. Manica, R.F. Tabor, G.W. Stevens, F. Grieser, D.Y. Chan, R.R. Dagastine, Langmuir 28 (2012) 4259–4266. [27] Y. -q. Wang, T. Wang, Y. -l. Su, F. -b. Peng, H. Wu, Z. -y. Jiang, Langmuir 21 (2005) 11856–11862.
Biographies Jing Han received her B.S. degree in measurement, control technology and instrumentation from Wuhan University of Technology, China, in 2013. And she received a master’s degree in instrument science and technology from Tianjin University, China. Her research interests include interface nanoforce measurement, and mechanical characterization of graphene. Linyan Xu received her Ph.D. degree in instrument science and technology from Tianjin University, China, in 2009. She was a visiting scholar at the PhysikalischTechnische Bundesanstalt (PTB) in 2010. She is currently an assistant professor with Tianjin University. Her research interests include probe modification, nanoforce measurement, AFM tip reconstruction, and RF-MEMS vibration measurement. Shuangbei Qian received his B.S. degree from the College of Precision Instrument and Opto-electronics Engineering, Tianjin University, Tianjin, China, in 2016. He is currently a master in Tianjin University. His current research interests include FET, molecule interaction and AFM. Xiaodong Hu received his Ph.D. degree in instrument science and technology from Tianjin University, China, in 2000. He is currently a professor with Tianjin University. His research interests are AFM, optical precision measuring technology, and X-ray 3D microscopic imaging. Tong Guo received his Ph.D. Degree in measuring technology and instruments of Tianjin University, China, in 2005. He is currently an associate professor with Tianjin University. His research interests are AFM, optical precision measuring technology, and nano fabrication. Sen Wu received his Ph.D. Degree in instrument science and technology at Tianjin University in 2012. He is currently an assistant professor with Tianjin University. His research interests include the AFM-based manipulation, nanoforce measurement, and the development of new types of AFM instrument. Yanan Liu received her B.S. degree in chemical engineering and technology from Dalian University of Technology, China, in 2013. She is now pursuing Ph.D. degree in Tianjin University, China. Her research interests include nanomaterials and membrane technology for water treatment. Zhongyi Jiang received his PhD degree from Tianjin University, China, in 1994. He was a visiting scholar at the University of Minnesota in 1997 and the California Institute of Technology in 2009. His research interests include biominetic and bioinspired membranes and membrane processes, biocatalysis, and photocatalysis.
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Two types of oil modified tips as force sensors to detect adhesion forces between oil and membrane surfaces in fluid Jing Han a , Linyan Xu a,∗ , Shuangbei Qian a , Xiaodong Hu a , Tong Guo a , Sen Wu a , Yanan Liu b , Zhongyi Jiang b a b
State Key Laboratory of Precision Measuring Technology and Instruments, Tianjin University, Tianjin 300072, China Key Laboratory for Green Chemical Technology of Ministry of Education, Tianjin University, Tianjin 300072, China
a r t i c l e
i n f o
Article history: Received 10 November 2016 Received in revised form 5 September 2017 Accepted 1 October 2017 Available online 7 October 2017 Keywords: Antifouling property Probe modification Force sensor Adhesion force
a b s t r a c t The surface resistance to oil is very important for filtration efficiency, film cleaning and life span of ultrafiltration membranes. The atomic force microscope (AFM) has provided unprecedented opportunities to study interface interactions. In this paper, the probe with an oil droplet modified was used as a force sensor for evaluating the oil resistance of membrane surfaces. The adhesion forces between two types of modified tips and a serial of membrane surfaces in fluid was detected via force curves. On the one hand, the AFM probe with an oil droplet immobilized directly was used to study the adhesion forces between a series of PVF/F127 membranes and a hexadecane droplet. On the other hand, a tip attached polystyrene (PS) microsphere covered with a layer of oil was employed. It was found that the oil antifouling performance of membranes was improved effectively through blending F127 with PVF. As the F127 additive varied from 0% to 60%, adhesion force detected by the AFM probe with an hexadecane droplet immobilized showed a decreasing tendency from 2.84 nN to 0.73 nN. Meanwhile, the average interaction force detected by the second modified tip decreased from 3.39 nN to 0.54 nN. The measured force behavior was in agreement with experimental observations of contact angle measurement, which indicated that the blend membranes had better antifouling. © 2017 Published by Elsevier B.V.
1. Introduction With the development of the petrochemical industry, emissions of oily wastewater are increasing and environmental problems have become a great challenge. Membrane technology is regarded as the most efficient approach for advanced wastewater treatment and reuse, owing to its advantages of low energy consumption, high separation efficiency, low environmental pollution, flexible and simple process and strong versatility. However, oil droplets may directly come into contact with membrane surfaces in reality. The inherent oil fouling often leads to a cascade of events, such as adhesion, accumulation, spreading, coalescence and migration of suspended oil foulants, which is much more complicated than the fouling caused by soluble organic macromolecules and insoluble inorganic matter [1]. As a result, the gradual coalescence and migration of oil droplets form a continuous oil film and cause a sharp
∗ Corresponding author. E-mail address: [email protected] (L. Xu). https://doi.org/10.1016/j.sna.2017.10.003 0924-4247/© 2017 Published by Elsevier B.V.
decline of permeation flux temporarily or permanently, leading to a reduced life span of the membranes. Superhydrophilic-superoleophobic films utilize “waterremoving” method [2] that effectively prevents the adhesion of oil droplets and could be applied to the mixture with small percentages of oil, and it is usually selected because of its low energy consumption. Researches show that the hydrophilicchemical materials and micro/nano composite structure are the key factors to design antifouling membrane surfaces [3,4]. Surface modification, physical blending and chemical grafting are mostly adapted to improve the membrane hydrophilicity and fouling resistance. Among these methods, blending has been extensively applied due to its versatile controlling conditions. Filtration test [5,6] and adsorption test [7] evaluate the antifouling performance of membranes on the macroscopic level. Filtration test usually takes ∼3 h for one membrane, which is timeconsuming. Adsorption test has the same problem. Contact angle measurement [8] evaluates the antifouling property of films by measuring the geometrical shape of oil droplets on films, which can only evaluate the hydrophilic property of materials. It belongs to point measurement method. And the result has certain contin-
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gency and randomness. Pore analysis [9,10] and surface roughness evaluation [11] are methods to explore the relationship between structural parameters and antifouling property of membrane surfaces. They can only be used as a reference and comparison of other methods. The methods discussed above are all on the macroscopic level. However, a quantitative and accurate measurement of the force between the oil and the sample membranes in microscopic aspect can be made. The atomic force microscope (AFM) can be used to estimate the interaction between membrane surfaces and pollutants modified on the tip of the special AFM probes, which is one of the most direct methods to assess the antifouling property of membranes. Hence, AFM probes are sensitive force sensors to convert interaction force between them to the deflection of the cantilevers. Finally, the interaction force is calculated by several parameters. It is well-known that a bovine serum albumin (BSA) protein immobilized tip must be obtained by a series of complicated steps, such as vacuum oxygen plasma method, chemically modification and so on. It is used to evaluate the bio-antifouling properties of the membrane. The new methods discussed in this paper aim at evaluate the oil-antifouling performance of the membrane and they are more simplified and efficient. However, the approach to modify an oil droplet on the tip of the cantilever is different from the traditional method of protein modification. The protein cluster can be fixed on the tip by the strong interaction of chemical bond [11,12], which is generally considered to be a rigid connection with the same structure. The oil droplets would not be stable in air, so the oil droplet modification must be carried out in the liquid phase. After the modification, it must ensure that the fixed position of the oil drop is always the same during the whole experiment. In addition, oil droplets are flexible structures. It is worth discussing whether the loss of the oil droplet during the testing process will have an influence on the results. The size of AFM cantilevers and the micron dimension of oil droplets are well matched. The cantilever with a micron oil droplet modified can be used as a high sensitive sensor for the measurement of adhesion between oil pollutants and films, which can be expected to be an effective method of membrane antifouling evaluation in microscopic aspect. In this paper, two methods are adopted to transfer a target oil droplet on the tip of a bare cantilever. A series of polyvinyl formal (PVF) membranes with different percent of additive F127 were prepared via non-solvent induction phase separation process. F127 was usually used as a surface segregation additive to construct a hydrophilic brush layer on the membrane surface and to further improve the antifouling performance of PVF membranes. The adhesion forces between the blend membranes and the oil droplets (hexadecane) on the AFM tips were measured in order to compare the antifouling property of the membranes. But the shortcoming of these methods lies in the wide range of the spring constant of the commercial AFM probes. So it is necessary to calibrate the AFM cantilevers and only the data from just the
same cantilever is comparable. Moreover, comparison experiment of contact angle measurement has also been done for analysis and comparison. 2. Material and methods 2.1. Materials Polyvinyl formal (PVF, MW = 35 kDa, acetalization degree = 80%) purchased from Tokyo Chemical Industry Corp. (Japan) was dried at 60 ◦ C for 12 h before use. Triblock copolymer Pluronic F127 (EO100 PO65 -EO100 ) with Polyethylene oxide (PEO) content of 70 wt% was purchased from Sigma Chemical Company (USA). Preparation of PVF/Pluronic F127 membranes was described in the previous study[13]. NP-O10 probes made of silicon nitride were purchased from Bruker Corp. (Germany). HA-C/tipless probes made of polysilicon were purchased from NT-MDT Corp. (Russia). Hexadecane were obtained at >99% (Sigma Aldrich). Polystyrene (PS) microspheres and special glue were provided by School of Material, Tianjin University. All silicon wafers were carefully cleaned with ethanol through ultrasonic cleaning machine (KQ3200E, China). Other chemicals were purchased from Kewei Chemicals Corp. (China). 2.2. Interaction forces between an oil droplet-immobilized tip and membrane surfaces 2.2.1. Preparation of oil droplets Two micropipettors (0.5–10 L and 20–200 L) were used to prepare hexadecane droplets in the following manner. Firstly, a silicon wafer was cleaned by ultrasonic cleaning machine. Then the hexadecane was sucked into a 0.5–10 L micropipettor and pure water was sucked into a 20–200 L micropipettor. Finally, it is better to direct micropipettor containing hexadecane approximately 4 cm over the top of the slide and to add pure water onto the silicon wafer quickly after squeezing it, in order to create a fine spray of hexadecane droplets which was dispersed over a clean silicon wafer. It was found that this method resulted in a fairly uniform coverage of the slide with discrete 10–40 m diameter oil droplets. 2.2.2. Attachment of oil droplets onto the AFM cantilever The AFM head holding NP-O10 tipless cantilever (Si3 N4 ) was positioned directly above a droplet on the surface of the silicon wafer. Then it was lowered down in order to sandwich the water between the cantilever holder for fluid operation and the silicon wafer. And it was advised to adjust the value of setpoint during this period. Once engulfment of the hexadecane droplet had been achieved, the AFM head and cantilever were retracted away from the surface of the slide, where upon the oil droplet was found to detach itself from the slide surface. This method allowed the
Fig. 1. Images of water droplets on the probe surface in air.
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Fig. 2. Schematics illustrating the conceptual designs. (a) Precise location of bare cantilever onto a hexadecane droplet. (b) The cantilever with an attached oil droplet pulled away from the silicon surface. (c) The direct modified cantilever approaches/retracts away the sample membrane to detect adhesion force between the oil and the membrane surface. (d) The indirect modified cantilever approaches/retracts away the sample membrane.
droplet to be placed with high precision at the end of the cantilever. Repeated experiments showed that silicon wafers were more convenient for oil droplets to transfer to cantilevers than glass slides. Besides, two kinds of probes (HA-C/tipless and NP-O10) were used. It was found that NP-O10 tipless cantilever was easier to achieve attachment of oil droplets onto the AFM cantilever than HA-C/tipless because NP-O10 was more oleophilic, in accordance with the larger water contact angle of it (Fig. 1). 2.2.3. AFM force curve measurement of membranes Each membrane was immersed in water for 24 h in advance. Force measurement was made between the modified tip and the membrane surfaces by an atomic force microscope (Dimension Icon, Bruker Corp., Germany) in fluid. Firstly, the sensitivity S of the cantilever should be calibrated before AFM force curve measurement, which can be illustrated in Eq. (1). S=
z U
(1)
Where Z is the cantilever deflection signal; U is voltage applied to the piezo scanner in the vertical direction. The detector sensitivity was determined by pressing the bare cantilever against the silicon surface 10 times in water environment, and the average value of the experimental results was calculated afterwards. The details of AFM probe calibration are shown in Section 3.2. The nominal value of the spring constant of the cantilevers provided by the supplier is not accurate for every specific probe, which is calculated based on the geometry of the cantilever designed and the approximate material parameters estimated. At present, many calibration methods have been proposed, such as the Sader method [14,15], the thermal noise method [16] and the balance method [17]. In this study, the nominal spring constant of the NP-O10 tipless can-
tilever used was 0.12 N/m. However, the exact values of 10 probes in the same box were found to be in the range of 0.11-0.18 N/m using a thermal frequency method carried out in fluid. The deviation between maximum and minimum was 81.8%. Secondly, the silicon wafer with the mixture of water and drops of oil (hexadecane) was placed in a sink with the sample membranes stuck in. Then the tip was modified with oil droplet according to the method mentioned in Sections 2.2.1 and 2.2.2. It can be seen from Fig. 2a–b. Thirdly, water was added into the sink about 0.5 mm high and then the modified tip was positioned directly above the surface of the PVF membrane. Fourthly, the tip approached the membrane surface. An interaction force was occurred between the tip and the membrane surface, as shown in Fig. 2c. A speed of 0.2 m/s was applied to obtain all the force curves. They were collected from at least six positions on the same sample. Approximately 60 extending/retracting cycles were performed for each membrane surface versus the same tip. Raw data were collected as a detector voltage output, which reflects the cantilever deflection versus relevant piezo motion. The other four PVF/F127 membranes with different ratios (10%, 20%, 40%, 60%) were examined in turns and the experiments were repeated. Conversion of the cantilever deflection voltage data (V) to force (nN) was performed using the standard procedure of multiplication by the AFM photodiode detector sensitivity (nm/V) and the measured cantilever spring constant (N/m) [18]. Finally, the adhesion force F between tip and membrane surface could be calculated by Eq. (2). F = k · Z = k · S · U
(2)
Here, Z is the deflection of the cantilever; U is the PSD output voltage of the cantilever, which could be derived from the data in the vertical axis of the AFM force curve.
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used to AFM force curve measurement, as shown in Fig. 2d. Similar method was described in Section 2.2.3. 100 force curves were done in this part.
2.4. Contact angle measurement on PVF/F127membranes
Fig. 3. Force-curve records monitoring cantilever deflection versus distance for a NP-O10 tipless cantilever.
2.3. Interaction forces between membrane surfaces and a PS microsphere covered with a layer of oil
The contact angle is an important parameter in surface science. It is a regular measurement of the surface hydrophobicity. Contact angle between water and the membrane surface was measured in a contact angle meter (Erma Contact Angle Meter, Japan). Each value was obtained immediately after dropping water on the membrane surface. Furthermore, underwater wetting properties of oil droplets (hexadecane) on membrane surfaces were examined. It must be noticed that each membrane should be immersed in water for 24 h in advance. The measurements were taken at five different locations on the same sample, and the average value of the experimental results was calculated afterwards.
3. Results and discussion 3.1. Spring constant calibration of cantilevers
As mentioned in Section 2.2.1, preparation of oil droplets with size more than tens of micrometer has strong randomicity. Due to the deformation of oil droplets, the calibration of elasticity constant of the probe will be affected. However, the method discussed here can ensure the better uniformity of the oil droplet size and the accuracy of the elasticity constant of the probe because of the rigid microsphere. Apart from an oil droplet-immobilized tip, another modified tip described in the following manner can also be used for AFM force curve measurement. Firstly, a small amount of glue was placed on a glass slide. Then the AFM head holding NP-O10 tipless cantilever (Si3 N4 ) was positioned directly on the edge of glue, so that only the tip of the probe adhered to the glue when the AFM head was engaged. Secondly, a PS microsphere with a known diameter (less than 10 m) was attached to the front end of a bare cantilever. Thirdly, the tip should be immersed into oil environment in order to achieve adsorption of oil. Finally, the modified tip was
The sensitivity changes with different cantilever geometric dimensions, as well as the position and quality of the laser spot on the cantilever. Therefore, the installation position of a tip and the laser spot should not be moved during the whole calibration process in order to avoid the changes of the sensitivity. As mentioned in Section 2.2.3, sensitivity was determined by pressing the bare cantilever against the silicon surface at least 10 times in fluid. One pair of the force curves were shown in Fig. 3, and the average value of the sensitivity for 10 repeated measurements was 96.821 nm/V. It can be seen from Fig. 3 that there is no obvious adhesion force between the tip and the silicon wafer. Presently, the thermal noise method has become the standard method of AFM experiments. Thermal frequency range was set to be 1–100 kHz covering the nominal frequency of the cantilever. After adjusting the parameters, the experimental result of spring constant was 0.155 N/m.
Fig. 4. Optical micrographs illustrating the attachment of oil droplets to a tipless AFM cantilever under water. (a) Precise location of the cantilever onto a hexadecane droplet. (The focus was on the oil droplets.) (b) The cantilever pulled away from the silicon surface with an attached oil droplet. (c) Relocation of the cantilever with an attached droplet. (The focus was on the probe.) (d)-(f) transference of a dichloroethane droplet onto the end of a tipless cantilever.
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Fig. 5. Force-curve records of an oil droplet immobilized tip against (a) PVF membrane, (b) 10% additive F127, (c) 20% additive F127, (d) 40% additive F127, and (e) 60% additive F127. (f) Sum up of the adhesion forces with different ratios (0%, 10%, 20%, 40%, 60%).
3.2. Oil droplet-immobilized AFM tips Fig. 4a–c showed a series of images captured through AFM, illustrating the transfer of a hexadecane droplet onto the end of the cantilever. In the first stage of the process, the cantilever was precisely located on the selected oil droplet in order to obtain proper position to achieve the attachment of oil droplets (Fig. 4a). The size of oil droplets should be moderate. Note that several other droplets attached to the silicon wafer were also in focus. Once the engulfment of the cantilever into an oil droplet was achieved, the cantilever pulled away from the surface with an attached oil droplet. A little residual oil remained on the surface (Fig. 4b). Fig. 4c was focused on the cantilever, and hence those droplets that were still attached to the surface were now out of focus. But it could not be seen from Fig. 4c that the droplet was clearly attached to the cantilever, as a result of its small size. Fig. 4d–f demonstrated the attachment of a dichloroethane droplet. Fig. 4d showed that the diameter of the dichloroethane droplet was larger (∼75 m). There was a little oil trace on the silicon surface after the attachment, shown in Fig. 4e. It can be clearly seen from Fig. 4f that the droplet was attached to the cantilever. After all the force curve experiments, the modified tip was the same. Therefore, it is obvious that the attached droplet is stable and that the cantilever can be repositioned and used to adhesion force measurement. However, hexadecane was used to prepare droplets in this study, owing to its convenience of the method described in Section 2.2.1, while it was more difficult for volatile dichloroethane to create a fine spray of droplets. 3.3. AFM force curve measurement of an oil droplet-immobilized tip The antifouling performance of membranes is dominated by the interaction between the foulants and the membrane surfaces. Meanwhile, there is considerable amount of literature showing that the magnitude of the adhesion force correlates closely with the fouling propensity of the membranes [19–21]. However, different
from the previous researches [22,23], the adhesion force measurement by AFM was carried out in fluid. As the probe approaches or retracts towards the membrane surface, the resulting interaction caused by the hydrodynamic and adhesion force generates a total interaction force. A theoretical prediction of experimental forces is provided by the hydrodynamic theory of Chan et al. [24,25]. Therefore, it should be noted that the approaching/retracting speed of the probe on AFM force curve measurement has a great influence on the results of the experiment in fluid. When the velocity is under 1 m/s, the resulting interaction can be regarded as quasi-static interaction [26]. There is a negligible hydrodynamic contribution to the adhesion force measurement. To ensure the accuracy of the data, slower nominal velocities (0.5 m/s, 0.2 m/s and 0.1 m/s) were investigated. When it was 0.5 m/s, the adhesion force was about 2.28 nN. And it was around 2.84 nN at a slower velocity of 0.2 m/s. There was no big difference between 0.2 m/s and 0.1 m/s for detected adhesion forces. So it was reasonable to choose 0.2 m/s due to its obvious experimental results and moderate time consumption. Cautions must therefore be taken in direct comparison with adhesion forces of different membranes. Only the data from just the same cantilever and the same speed of extending/retracting is comparable. Otherwise the results will be inaccurate. Conversion of the AFM output to a force was calculated by Eq. (2). Fig. 4a–e show the interaction forces between a hexadecane droplet immobilized tip and the PVF membranes with 0–60 percentage of F127 additive. X-axis represents the displacement of the AFM piezo scanner in vertical direction, and Y-axis represents the interaction forces between the tip and the membranes. Obviously, the overlap of the approach portions of the force curves indicates a negligible hydrodynamic contribution to the force. There is no obvious attractive force in the extending curve. While in the retracting curve, the interactions show a deep “attractive minimum” followed by a sharp pull-off force. It is used to evaluate the antifouling performance of the membranes. Fig. 4f shows the sum up of the adhesion forces with different ratios (0%, 10%, 20%, 40%, 60%). It can be seen from Fig. 4f that with the increasing addition of F127, the average interaction force decreases
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Fig. 6. A PS microsphere with a known diameter (less than 10 m) was used to modify a tipless cantilever. (a) Optical micrograph of two discrete PS microspheres. (b) Optical micrograph of a modified tip attached a PS microsphere.
obviously from 2.84 nN to 0.73 nN. The surface resistance to oil is characterized by the maximum debonding force during the unloading process. The loading process is not employed to detect the force. It just ensures that oil droplets can contact with the membrane surfaces. Therefore, the maximum loading force is controlled in 6 nN (Fig. 4a–e), which aims at avoiding mass loss and movement of the fixed oil droplet during the repeated loading/unloading process. In the experiment, the same oil modified tip was always used on a set of films during 300 loading/unloading process. Through good control, the movement of oil droplet have not yet been discovered. At the same time, it also shows that the affinity between the oil droplet and the cantilever is strong. The sample membranes have good oil antifouling, because the residual oil were not observed like in Fig. 4b during the test. But the process is possible to leave oil remnant which is not easy to be discovered on sample surfaces. So each testing point is repeatedly measured continuously. If the phenomenon of oil remaining on films occur, the adhesion will exhibit a monotonically increasing trend. In fact, the adhesion force detected in single point repeated measurement shows a random distribution. Hence, there is no need to consider the influence of residual oil in the test. In Fig. 5f, experimental results suggest that the PVF/F127 membranes show better antifouling properties than pristine PVF membrane. With the increasing of F127 additive percentage, the antifouling performance becomes more effective. The adhesion forces between membrane surfaces and foulants have a substantial impact on the membrane fouling. Pluronic F127 bearing hydrophilic PEO segments and hydrophobic PPO segments was considered as both excellent surface antifouling modifier [27]. Here, higher F127 contents in membranes indicated high surface coverage of hydrophilic material (PEO segments) on membranes, which resulted in the minimum adhesion force and an improved antifouling property. And the significant decrease in the adhesion force may originate from the energetic adhesion barrier created by the extended antifouling brushes on the membrane surfaces. The measured force behavior was in agreement with experimental observations of antifouling performance of the membranes against different proteins [13].
a PS microsphere coated a layer of oil and PVF membranes with different ratios (0%, 10%, 20%, 40%, 60%) of F127. Obviously, the average interaction force decreases from 3.39 nN to 0.54 nN with the increasing addition of F127. Therefore, the dependence of the pull-off forces on different ratios of F127 contents in membranes is still valid. In spite of the reducing diameter of oil droplet, the adhesion forces are quiet close comparing with Fig. 5f. It is supposed that when the AFM cantilever is retracted away from the surface of the membrane, oil may deform and stretch resulting in equivalent contact area with the membrane. Hence, the pull-off forces are not quite different at the moment of the oil departing from membrane surfaces. The two methods are considered to be equivalent under the condition that the size of oil droplet has the same magnitude. The difference between the two methods is that the diameter of oil droplets is from 10 microns to tens of microns, so one of them can be selected. The first method is to directly modify an oil droplet on the cantilever. Its difficulty lies in the control of oil droplet which should not move during the test. The second showing better oil droplet fixing effect is an indirect method with the help of the PS microsphere as the intermediate medium. The researchers made an oil droplet fixed in a large metal ring structure under the optical microscope. They combined the ring structure and force sensors to detect the adhesion force between oil droplets and films. Their experiments showed an adhesion force reached around 1 N in water. The method is feasible for the lateral comparison of antifouling performance of membranes.
3.4. AFM force curve measurement of a tip attached PS microsphere covered with a layer of oil Fig. 6a shows an image captured through AFM, illustrating two discrete PS microspheres with the size of less than 10 m. Fig. 6b obtained by an upright metallurgical microscope (Shanghai, China) shows a PS microsphere is accurately attached to the tip of the NP-O10 tipless cantilever. The sensitivity of the modified tip was 60.49 nm/V via the method mentioned in Section 2.2.3. And its spring constant was found to be 0.108 N/m through thermal noise method. Fig. 7 shows the sum up of the adhesion forces between
Fig. 7. Sum up of the adhesion forces between a PS microsphere coated a layer of oil and PVF membranes with different ratios (0%, 10%, 20%, 40%, 60%) of F127.
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Fig. 8. Results of contact angle measurement. (a) Image of a water droplet on a membrane surface in air. (b) Image of a hexadecane droplet on a membrane surface under water. (c) A summary of contact angles of PVF/F127 membranes in air and under water with different ratios.
3.5. Contact angle measurement on PVF/F127membrane The water contact angles in air and underwater wettabilities of oil (hexadecane) on the blending membrane surfaces were comprehensively characterized (Fig. 8). The PVF membrane possessed the highest water contact angle of 61.34◦ , while the PVF/F127 membranes had lower water contact angles. With the increasing of the F127 additive percentage, the water contact angles were remarkably decreased to 49.65◦ . However, the gradual increase in F127 content promoted an increase in underwater hexadecane contact angles from 116.13◦ to 124.67◦ . That’s to say, the contact angles of water in air decreased with increasing hydrophilicity of the membranes, and the results are contrasted with the contact angles of oil on these surfaces in water. It was known that membranes with higher hydrophilicity exhibited better antifouling property [4]. Comparing with Fig. 5f, the results also showed the equivalent tendency of the antifouling performance. The data of contact angle measurement agreed with AFM force curves very well, which could be an evidence of correctness and effectiveness of AFM force curve method. 3.6. Comparison of two modification methods The advantage of the direct method is that it is quick and convenient. But the size of oil droplets modified on the bare cantilever has strong randomicity. Besides, only some specific oil droplets can be transferred efficiently. Other types of oil droplets could be attached to the tip of a bare cantilever after the chemical modification of the cantilever. The indirect method can ensure the better uniformity of the oil droplet size and the accuracy of the elasticity constant of
the probe because of the rigid microsphere. Meanwhile, the different type of oils has no significant influence on this method, which is more feasible. However, a PS microsphere should be attached to the tip of the NP-O10 tipless cantilever in advance. And this obtained probe should be followed by an ultraviolet lamp for one night exposure, so that the microsphere can be fixed to ensure the follow-up test. In brief, the two methods based on AFM in this paper are more suitable for the accurate evaluation of the oil antifouling property of membranes, because they are more close to the actual process of the separation of water and oil.
4. Conclusions In summary, two novel and generic approaches to evaluate the oil antifouling property of membranes have been proposed. The adhesion force between the oil droplet and the prepared membrane surface measured by two types of modified tips was a significant parameter that allowed direct assessment of the oil adhesion behavior at the interface. As a result of the gradual increase in F127 content, adhesion force showed a decreasing tendency, which was an evidence for less direct contact of oil droplets. Besides, it was indicated that the membranes with increasing additive of F127 had a favourable antifouling performance, along with more underwater hexadecane contact angles. In the future, researches that explore the adhesion force of a variety of oils on the surface can be investigated. The practicality of such interaction forces measurement between oil and membranes will give rise to attractive prospects for oily wastewater treatment evaluation. And this study may set a simple and universal example to evaluate the surface antifouling
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performance and research interface behaviours for a wider range of relevant applications. Acknowledgments This project was supported by National Natural Science Foundation of China (Grant No. 51305298, No. 51675379) and Tianjin Research Program of Application Foundation and Advanced Technology (Grant No. 13JCQNJC04700). References [1] W. Chen, Y. Su, J. Peng, Y. Dong, X. Zhao, Z. Jiang, Adv. Funct. Mater. 21 (2011) 191–198. [2] A.K. Kota, Y. Li, J.M. Mabry, A. Tuteja, Adv. Mater. 24 (2012) 5838–5843. [3] Y. Huang, H. Li, L. Wang, Y. Qiao, C. Tang, C. Jung, Y. Yoon, S. Li, M. Yu, Adv. Mater. Interfaces 2 (2015) 1400433. [4] M. Liu, S. Wang, Z. Wei, Y. Song, L. Jiang, Adv. Mater. 21 (2009) 665–669. [5] X. Zhao, Y. Su, J. Cao, Y. Li, R. Zhang, Y. Liu, Z. Jiang, J. Mater. Chem. A 3 (2015) 7287–7295. [6] C. Zhao, X. Xu, J. Chen, F. Yang, J. Environ. Chem. Eng. 1 (2013) 349–354. [7] Y. -q. Wang, T. Wang, Y. -l. Su, F. -b. Peng, H. Wu, Z. -y. Jiang, J. Membr. Sci. 270 (2006) 108–114. [8] Y. Sui, Z. Wang, X. Gao, C. Gao, J. Membr. Sci. 413–414 (2012) 38–47. [9] C. Liao, J. Zhao, P. Yu, H. Tong, Y. Luo, Desalination 260 (2010) 147–152. [10] C. Liao, J. Zhao, P. Yu, H. Tong, Y. Luo, Desalination 285 (2012) 117–122. [11] Y. Liu, Y. Su, X. Zhao, R. Zhang, T. Ma, M. He, Z. Jiang, J. Membr. Sci. 499 (2016) 406–417. [12] E.C. Cho, D.H. Kim, K. Cho, Langmuir 24 (2008) 9974–9978. [13] Y. Liu, L. Xu, Y. Song, X. Fu, J. Zou, X. Hu, Z. Jiang, X. Zhao, RSC Adv. 5 (2015) 36894–36901. [14] J.E. Sader, J.W. Chon, P. Mulvaney, Rev. Sci. Instrum. 70 (1999) 3967–3969. [15] J. Cleveland, S. Manne, D. Bocek, P. Hansma, Rev. Sci. Instrum. 64 (1993) 403–405. [16] J.L. Hutter, J. Bechhoefer, Rev. Sci. Instrum. 64 (1993) 1868–1873. [17] M.-S. Kim, J.-H. Choi, Y.-K. Park, J.-H. Kim, Metrologia 43 (2006) 389. [18] S. Sundararajan, B. Bhushan, Sens. Actuators A. Phys. 101 (2002) 338–351. [19] Y. Mo, A. Tiraferri, N.Y. Yip, A. Adout, X. Huang, M. Elimelech, Environ. Sci. Technol. 46 (2012) 13253–13261. [20] D.-G. Kim, H. Kang, S. Han, J.-C. Lee, J. Mater. Chem. 22 (2012) 8654–8661. [21] C.J. Weinman, N. Gunari, S. Krishnan, R. Dong, M.Y. Paik, K.E. Sohn, G.C. Walker, E.J. Kramer, D.A. Fischer, C.K. Ober, Soft Matter 6 (2010) 3237–3243. [22] Y. Liu, Y. Su, X. Zhao, Y. Li, R. Zhang, Z. Jiang, J. Membr. Sci. 486 (2015) 195–206. [23] J. Zhang, Z. Xu, M. Shan, B. Zhou, Y. Li, B. Li, J. Niu, X. Qian, J. Membr. Sci. 448 (2013) 81–92. [24] R. r. Manica, J.N. Connor, R.R. Dagastine, S.L. Carnie, R.G. Horn, D.Y.C. Chan, Phys. Fluids 20 (2008) 032101.
[25] S.L. Carnie, D.Y. Chan, C. Lewis, R. Manica, R.R. Dagastine, Langmuir 21 (2005) 2912–2922. [26] H. Lockie, R. Manica, R.F. Tabor, G.W. Stevens, F. Grieser, D.Y. Chan, R.R. Dagastine, Langmuir 28 (2012) 4259–4266. [27] Y. -q. Wang, T. Wang, Y. -l. Su, F. -b. Peng, H. Wu, Z. -y. Jiang, Langmuir 21 (2005) 11856–11862.
Biographies Jing Han received her B.S. degree in measurement, control technology and instrumentation from Wuhan University of Technology, China, in 2013. And she received a master’s degree in instrument science and technology from Tianjin University, China. Her research interests include interface nanoforce measurement, and mechanical characterization of graphene. Linyan Xu received her Ph.D. degree in instrument science and technology from Tianjin University, China, in 2009. She was a visiting scholar at the PhysikalischTechnische Bundesanstalt (PTB) in 2010. She is currently an assistant professor with Tianjin University. Her research interests include probe modification, nanoforce measurement, AFM tip reconstruction, and RF-MEMS vibration measurement. Shuangbei Qian received his B.S. degree from the College of Precision Instrument and Opto-electronics Engineering, Tianjin University, Tianjin, China, in 2016. He is currently a master in Tianjin University. His current research interests include FET, molecule interaction and AFM. Xiaodong Hu received his Ph.D. degree in instrument science and technology from Tianjin University, China, in 2000. He is currently a professor with Tianjin University. His research interests are AFM, optical precision measuring technology, and X-ray 3D microscopic imaging. Tong Guo received his Ph.D. Degree in measuring technology and instruments of Tianjin University, China, in 2005. He is currently an associate professor with Tianjin University. His research interests are AFM, optical precision measuring technology, and nano fabrication. Sen Wu received his Ph.D. Degree in instrument science and technology at Tianjin University in 2012. He is currently an assistant professor with Tianjin University. His research interests include the AFM-based manipulation, nanoforce measurement, and the development of new types of AFM instrument. Yanan Liu received her B.S. degree in chemical engineering and technology from Dalian University of Technology, China, in 2013. She is now pursuing Ph.D. degree in Tianjin University, China. Her research interests include nanomaterials and membrane technology for water treatment. Zhongyi Jiang received his PhD degree from Tianjin University, China, in 1994. He was a visiting scholar at the University of Minnesota in 1997 and the California Institute of Technology in 2009. His research interests include biominetic and bioinspired membranes and membrane processes, biocatalysis, and photocatalysis.