- Email: [email protected]
Liquid marble particle wetting separation
Colloid and Interface Science Communications 35 (2020) 100237
Contents lists available at ScienceDirect
Colloid and Interface Science Communications journal homepage: www.elsevier.com/locate/colcom
Rapid Communication
Liquid marble particle wetting separation a
a
a
a
b
Eric Shen Lin , Dwayne Chung Kim Chung , Jian Wern Ong , Hassan Ali Abid , Linghui Peng , ⁎ Xuchuan Jiangb, Oi Wah Liewc, Tuck Wah Nga,
T
a
Laboratory for Optics & Applied Mechanics, Department of Mechanical & Aerospace Engineering, Monash University, Clayton VIC3800, Australia Department of Chemical Engineering, Monash University, Clayton VIC3800, Australia Cardiovascular Research Institute, Yong Loo Lin School of Medicine, National University of Singapore, National University Health System, Centre for Translational Medicine, 14 Medical Drive, 117599, Singapore
b c
ABSTRACT
Liquid marbles to separate wetting (silver nanowires) from non-wetting (graphite and talcum) particles is described here. Two types of liquid marbles are formed by coating the dry mixture test material on pendant or sessile drops. The silver nanowire particles are separated from the graphite or talcum shell by entering into the liquid core of the marbles. The liquid core is extracted into capillary tubes where the presence of silver nanoparticles was confirmed by a blue to colourless change arising from silver-mediated catalytic reduction of methylene blue and sodium borohydride solution. The use of an inclined rotating surface to coat the pendant drop was found to create liquid marbles in a more controllable manner. A platform tilt angle of 15o followed by rotation was found to provide more uniform albeit only partially coating the bottom of the pendant drop.
1. Introduction Particle separation is important in a myriad of industrial and scientific endeavors. There is impetus for such processes to be conducted efficiently with minimal solid and liquid usage to limit contamination to the environment [1]. Membranes are widely used for particle separation but can be limited by fouling [2]. Coagulation-flocculation is an alternative method that circumvents fouling [3]. Coagulation destabilizes charged particles via chemical reactions while flocculation promotes agglomeration and settling. Both the membrane and coagulation-flocculation separation methods operate with the particles already present in solution. In cases where separation of particles from their anhydrous states are required, this may be achieved by exploiting their differences in wetting characteristics. Hence, a separation scheme that allows this to be achieved will be very useful. The ability of micro or nano scale particles to encapsulate a liquid drop to form a non-wetting soft solid called a liquid marble was first reported close to 20 years ago [4]. These coated droplets have been shown to be operable as miniature reactors [5,6] and micro sensors [7,8]. Liquid marble fabrication is simple, typically accomplished by first dispensing a liquid drop and moving it gently over a bed of particles. Removing the coating particles from the liquid marble to retrieve its interior contents has proven to be a challenge. Methods reported to achieve this include the use of optical radiation [9], pH-mediated disruption [10], electric fields [11], ultrasonic fields [12], and rolling the liquid marble over a film of oil [13]. The efficacies of these approaches
⁎
are dependent on some or all of a number of chemical-physical factors, such as the characteristics of the liquid (ionic, organic, etc.), the wetting properties of the liquids and particles, the delivery of motive forces through actuators, and the design of equipment necessary to provide the non-contacting fields. From a practical perspective, a method that obviates such considerations would be advantageous. This work investigates the use of liquid marbles created in the form of pendant and sessile drops to achieve particle separation on the basis of their differential wettability. The issues of usage ease and separation effectiveness are also assessed. 2. Materials and methods The schemes for particle separation with pendant and sessile drop liquid marbles are depicted in Figs. 1(A) and (B) respectively. With the former, liquid is dispensed from the proximal end of a capillary tube to form a pendant drop and lowered onto a powdered mixture of nanoparticles of different wettability. Separation is achieved with non-wetting particles coating the drop while the wetting particles are drawn into the liquid core. The liquid can then be drawn out from the liquid marble using the same capillary tube to recover the wetting particles. With the latter, a sessile drop is dispensed and maintained stably on a superhydrophobic (SH) substrate with a hole [14,15]. If the substrate is sufficiently thin, the liquid drop exhibits a sessile and an overhanging component above and below the hole respectively, and is rendered immovable by the pinning action of the solid phase at the edge of the
Corresponding author. E-mail address: [email protected] (T.W. Ng).
https://doi.org/10.1016/j.colcom.2020.100237 Received 10 December 2019; Received in revised form 14 January 2020; Accepted 21 January 2020 2215-0382/ © 2020 Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Colloid and Interface Science Communications 35 (2020) 100237
E.S. Lin, et al.
Fig. 1. The schemes of separating a dry mixture of hydrophobic and hydrophilic particles via liquid marbles formed from (A) pendant and (B) sessile drops. In both cases, the liquid core containing the hydrophilic particles are drawn out using a capillary tube. In the sessile drop approach, the dispensation of liquid onto a superhydrophobic substrate with a hole creates a liquid body with an overhanging component that is not coated by hydrophobic particles. This allows access of a capillary tube to effect drainage from the liquid core.
hole [15]. The sessile component of the drop is coated with the powder mixture to elicit the same particle separation behavior as described. The liquid core is then removed through the overhanging component using a capillary tube (wherein the thin SH substrate facilitates this). In the experiments, borosilicate capillary tubes of 0.68 mm and 1 mm inner and outer diameters were used. To create the SH substrate, holes of 2 mm diameter were drilled through a 0.1 mm thick copper plate that form the side walls of the wells. They were then ultra-sonicated in ethanol (70% v/v) and acetone for 3 min, and subsequently in deionized (DI) water for another 15 min. The substrates were oxidized in a solution containing 2 M NaOH and 1.5 M (NH4)2S2O8 for 5 min. The samples were then allowed to synthesize in an oven at 180 °C for 120 min, completing the phase transfer from hydroxides to oxides. At the end of the reaction, the substrates were removed from the solution, rinsed several times with DI water and ethanol and then dried with compressed air. In the final step, the substrates were silanized using FAS (1H,1H,2H,2H-perfluorodecyl-triethoxysilane) to obtain a lowsurface energy layer with good corrosion resistance and thermal stability. The substrates were immersed in FAS-ethanol solution for 30 min and finally dried in the oven at 150 °C for 10 min. From characterization studies done with a 20 μL drop on the surface, it was found that the equilibrium contact angle was 150.8o and the contact angle hysteresis was 20.7o. The silver nanowires were created by dissolving 1 g of Polyvinylpyrrolidone (PVP, Mw ~ 55,000 Sigma-Aldrich) and 0.014 g NaCl in 20 mL ethylene glycol (EG). 0.204 g of AgNO3 was dissolved in 10 mL EG. The PVP-NaCl-EG mixture was heated to 170 °C and then the AgNO3-EG cocktail was added in to the system at a rate of 5 mL/h with constant stirring. After that, the reaction was kept at 170 °C for 1 h. When the product was cooled down to room temperature, 70 mL acetone was added into the system. The mixture was put in the ultrasonic bath for 30 min. The product was centrifuged at a speed of 7000 rpm. Then acetone was added and the washing procedure was repeated again. The nanowires were finally dried in air under 80 °C. The non-wetting particles used were hydrous magnesium silicate (Talc) (Sigma Aldrich, 243,604, average size ≤10 μm) and graphite powder (Sigma Aldrich, 282,863, average size ≤20 μm). Previous research have indicated that the surface energies of these two types of particles were 54.8 mJ/m2 [16] and 31.5 mJ/m2 [17] respectively. The fabricated silver nanowires would act as the wetting material, since its surface energy of around 7200 mJ/m2 [18] is O(102) higher than that of Talc and graphite. To form the external shell of the liquid marble, talc or graphite powder spiked with 5% (by weight) of silver nanowires were used. Drops ranging from 10 to 30 μL by volume from a solution comprising 2 mM methylene blue (MB) and 0.1 M sodium borohydride (NaBH4) was used as the internal liquid phase of the marble. Particle separation is demonstrated when the silver nanoparticles separate from
the non-wetting shell and enters the liquid core where its presence catalyzes an NaBH4-MB redox reaction that results in a solution colour change from blue to colourless [19]. 3. Results and discussion Two methods for producing liquid marbles by sessile and pendant drop formation were investigated. With the sessile drop, the most effective way to coat the sessile drop was by depositing the particles over the surface of the drop using a plastic funnel (see Fig. 2A). Compressed air was then delivered at controlled pressures to disperse excess talc particles away (Fig. 2B). Figs. 2C to E shows the sequence with which the sessile drop was successfully encapsulated although achieving an external shell of uniform thickness was difficult. Interestingly, the powder does not move beyond the hole of the superhydrophobic powder. Hence, the “overhanging” component of the drop [15] remains uncoated (Fig. 2F) to allow access for a capillary tube to draw out the liquid core. The most efficient way to coat a pendant drop was to position it over a tilt/rotary stage covered with a thin layer of particles (Fig. 3A). The stage is tilted to a fixed angle θ, and lifted towards the drop by means of a lifting stage assembly. When the particles come into contact with the drop, the stage was rotated around to coat the drop. It can be seen that the height h1 to which the particles are able to coat the drop relative to its height h0 is dependent on θ up to 15o (see Figs. 3B – C). At higher θ angles (e.g. 30o), the hydrophobic particles tended to coat unevenly (see Fig. 3D). The ability of silver nanowires to hasten the degradation of 1 mL of the solution of methylene blue and sodium borohydride (diluted 10 times) placed in a cuvette is illustrated by colour change (see Supplementary Video). Nevertheless, in a liquid marble, the colour change of the liquid core arising from separation of the silver nanowires from the hydrophobic shell into it may not be as visually discernible especially with the particle coating is opaque. To demonstrate separation of silver nanoparticles from the shell into the liquid core, the liquid was drawn out from the pendant drop using the capillary tube and diluted 100 times with distilled water for analysis. As test controls, liquid extracted from the pendant drops without particle coverage (Fig. 4A) or coated with graphite powder alone (Fig. 4B) displayed clear blue tints indicating presence of MB in its oxidized state. On the other hand, liquid drawn out from the pendant drops coated with graphite and silver nanowire powder is colourless, showing catalytic reduction of MB by silver nanowires separated into the liquid core. Since the liquid core is withdrawn as soon as the coated pendant drop was formed, this indicates that the mechanism of wettability particle separation occurs very quickly as soon as the liquid droplet contacts the powder mixture. It is noteworthy that withdrawing the liquid core into the capillary tube at flowrates < 0.05 mL/s minimizes carryover of any 2
Colloid and Interface Science Communications 35 (2020) 100237
E.S. Lin, et al.
Fig. 2. Generation of a liquid marble from a sessile drop created on a superhydrophobic plate with a hole. The surface of the drop is coated with the hydrophobic particles by deposition through a funnel (A) positioned over the drop. Excess particles are removed via compressed air (B) with pressure control using a valve and manometer. Side view (C) of a drop pinned over a hole on the SH substrate showing the top sessile and bottom overhanging component. The sessile component is covered with excess particles to form a liquid marble (D). Top view of the liquid marble formed from the sessile drop after excess particles are removed (E). Bottom view of the overhanging component (F) shows the absence of hydrophobic shell formation over the droplet surface.
position over the hole is determined primarily by three phase contact line pinning on the superhydrophobic surfaces [15,21]. For these reasons, the pendant drop approach for liquid marble formation is more robust even though full encapsulation of the droplet surface was not achieved to afford maximal liquid-air interface for particle separation. It should also be noted that the inability to fully encapsulate the pendant drop with the non-wetting particles does not detract from the utility of this approach. Clearly, when the stage platform was in a horizontal position, the area of contact between the powder mixture and pendant drop is minimal. At a tilt angle of up to 15o, the coverage of particles over the bottom surface of the droplet was uniform, thus allowing a quantifiable coverage metric of h1/h0 to be used (see Fig. 3C). Fig. 5 presents box plots of measurements made at 5o and 15o using graphite and Talcum powder to form the shell. The data spread is high for both types of powder. At platform tilt of 15o, an average coverage metric of 0.25 was attained. The advantage of such a low coverage metric is that propensity for carryover of talcum or graphite powder into the capillary tube during the liquid evacuation process is greatly reduced. The non-wetting particles accumulated at the distal ends of capillary tubes with the pendant drop approach after liquid evacuation (see Fig. 6A), additionally makes it amenable for their accumulation by merging (see Fig. 6B). With the onset of drying, the non-wetting particles can be collected in the anhydrous form (see Fig. 6C). This is in contrast to the case where particles would typically attach to wall surfaces following liquid evacuation from receptacles like vials (see Fig. 6D). With the proper automation incorporated, the entire separation process can take under 10 s to complete. While the volumes of each drop are limited to the 10–30 μL range, it is possible to scale up the volume (and thus particle separation) processing without any sacrifice in time by using a spatial array of drops operating simultaneously. It is pertinent to mention that powdered mixtures used in pharmaceutical and cosmetic formulations comprising dried particles of different wettability can be inadvertently contaminated with unwanted hydrophilic material. The manufacture of talcum powder, for example, has been reported to contain heavy metal particle contaminants [22]. The high surface to volume ratios of the talc powder along with the large number of MgeOe and OH– bonds present produces conditions conducive for metallic nanoparticles to latch on. The effects of prolonged exposure of these heavy metal tainted powders on human health is still being investigated. However, there is a strong likelihood that findings similar to that of talcum powder contaminated by asbestos will be uncovered [23].
Fig. 3. Generation of a liquid marble from a pendant drop held in place by a capillary tube. A lifting stage is assembled below a tilt/rotary stage lined with hydrophobic particles over its platform surface (A). The surface is tilted to an angle of θ, raised to contact the drop, and then rotated to coat the particles on the drop. Varied coating heights h1 in relation to the height of the drop h0 were achieved with platform tilt angles of (B) 5o and (C) 15o. At a platform tilt angle of (D) 30o the coverage tended to be skewed to one side.
talcum or graphite particles into the capillary tube most of the time. Greater caution should be taken especially towards the end of liquid drawing process, where the propensity of talcum or graphite entering the tube is higher. The use of an absolute syringe displacement device is advantageous in this context [20]. Although both the sessile and pendant drop approaches could be used for creating liquid marbles, achieving uniform coating with the former is more challenging. This requires optimal airflow pressure (see Fig. 2B) sufficient for distributing the particles over the liquid surface but not so high as to displace the drop. In some instances, the strong airflow resulted in distortion of the drop to one side of the SH hole, allowing the non-wetting particles to breach the neck of the hole and extend over the overhanging component of the sessile drop. When this happens, complete encapsulation of both the sessile and overhanging component of the drop made extraction of the liquid core difficult. A further challenge is that the ability of the sessile drop to be held in 3
Colloid and Interface Science Communications 35 (2020) 100237
E.S. Lin, et al.
Fig. 4. Images of liquid extracted from pendant drops of MBNaBH4 solution using capillary tubes followed by 100 times dilution with distilled water. In the absence of any liquid marble created, the extracted solution (A) retains the distinctive blue colour of oxidized MB. The blue colour of oxidized MB is also observed from liquid extracted (B) from liquid marbles coated only with graphite powder. Liquid extracted from liquid marbles (C) coated with graphite and silver nanowire powder mixture is colourless due to reduction of MB by silver nanowire particles separating into the liquid core from the hydrophobic shell. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. When liquid is drawn out of the pendant liquid marble, the volume of the liquid core is reduced to create a smaller drop (A) at the distal end of the capillary tube covered by a thicker external shell of graphite. Two small pendant marbles can be brought into contact and merged (B) to facilitate further harvesting of the shell particles in the anhydrous state from a point source (C). When liquid is drained out of a typical receptacle (D), there is inevitable particle attachment to the wall that hampers harvesting.
Fig. 5. Box plots of h1/h0 (a metric for liquid marble coverage using the pendant drop approach) determined at platform tilt angles of 5o and 15o for liquid marbles formed from (A) graphite and (B) Talcum powder as the hydrophobic shell component. All measurements were based on 8 readings each.
At this juncture, it is important to highlight that the liquid marble approach here does not provide a means to separate hydrophilic (those with contact angles < 90o) from hydrophobic particles (those with contact angles > 90o). Despite their low surface energies, graphite powders typically exhibit contact angles below 90o while still being able to form the shell in liquid marbles [24,25]. The ability for graphite to do this portends usefulness in separating out graphene oxide (GO), which dissolves readily in water via hydrogen bonding mechanisms [26], when produced using dry processing methods [27]. GO is a material that exhibits properties that are useful in novel applications [28,29]. Clearly, a facile method for separating unwanted wetting components from non-wetting powders for contaminant analysis has clear useful applications. The disadvantage of the method however is that
liquid marbles are not amenable to transport. This is generally not an issue as the fragile structure of most liquid marbles generally do not facilitate their handling. This approach is not restricted to the types of wetting and non-wetting materials used in this work. 4. Concluding remarks The ability to separate wetting and non-wetting particles from an initially dry mixture is achieved by liquid marble formation using pendant drop and sessile drop approaches. The method permitted the wetting particles to be separated into the liquid core which is then 4
Colloid and Interface Science Communications 35 (2020) 100237
E.S. Lin, et al.
easily withdrawn into capillary tubes for analysis without any need for actuator motive forces or careful design of specialized equipment. There is overall greater robustness in using the pendant drop over the sessile drop over a SH hole with the approach described here. Supplementary data to this article can be found online at https:// doi.org/10.1016/j.colcom.2020.100237.
[11] E. Bormashenko, R. Pogreb, R. Balter, O. Gendelman, D. Aurbach, Appl. Phys. Lett. 100 (2012) 151601. [12] D. Zang, J. Li, Z. Chen, Z. Zhai, X. Geng, B.P. Binks, Langmuir 31 (2015) 11502. [13] X.F. Jiang, C. Zhu, Y. Ma, H.Z. Li, Adv. Mater. Interfaces 4 (2017) 1700193. [14] D.C.K. Chung, S.H. Huynh, S. Wang, X. Jiang, O.W. Liew, M. Muradoglu, T.W. Ng, J. Mater. Chem. A 5 (2017) 914. [15] D.C.K. Chung, S.H. Huynh, M. Katariya, A.Y.C. Chan, S. Wang, X. Jiang, M. Muradoglu, O.W. Liew, T.W. Ng, ACS Omega 2 (2017) 6211. [16] S. Wang, Y. Zhang, N. Abidi, L. Cabrales, Langmuir 25 (2009) 11078. [17] R.F. Giese, P.M. Costanzo, C.J. van Oss, Phys. Chem. Mineral 17 (1991) 611. [18] K.K. Nanda, A. Maisels, F.E. Kruis, H. Fissan, S. Stappert, Phys. Rev. Lett. 91 (2003) 106102. [19] N. Gupta, H.P. Singh, R.K. Sharma, J. Mol. Catal. A Chem. 335 (2011) 248. [20] J.W. Ong, D.C.K. Chung, E.S. Lin, H.A. Abid, O.W. Liew, T.W. Ng, Rev. Sci. Instrum. 90 (2019) 076108. [21] D.C.K. Chung, M. Katariya, A.A. Ahmad Zahidi, C.Y. Lau, O.W. Liew, T.W. Ng, Mat. Sci. Engin. C 96 (2019) 599. [22] I.C. Nnorom, Toxicol. Environ. Chem. 93 (2011) 1135. [23] R.E. Gordon, S. Fitzgerald, J. Millette, Int. J. Occup. Environ. Health 20 (2014) 318. [24] M.D. Doganci, B.U. Sesli, H.Y. Erbil, B.P. Binks, I.E. Salama, Colloids Surf. A Physicochem. Eng. Asp. 384 (2011) 417. [25] E. Bormashenko, R. Pogreb, A. Musin, R. Balter, G. Whyman, D. Aurbach, Powder Technol. 203 (2010) 529. [26] V.V. Neklyudov, N.R. Khafizov, I.A. Sedov, A.M. Dimiev, Phys. Chem. Chem. Phys. (2017) 17000. [27] P. Dash, T. Dash, T.K. Rout, A.K. Sahu, S.K. Biswal, B.K. Mishra, RSC Adv. 6 (2016) 12657. [28] L. Zhang, J. Xia, Q. Zhao, L. Liu, Z. Zhang, Z. Small 6 (2010) 537. [29] Z. Xiong, X. Yun, L. Qiu, Y. Sun, B. Tang, Z. He, J. Xiao, D.C.K. Chung, T.W. Ng, H. Yan, R. Zhang, X. Wang, D. Li, Adv. Mat. 31 (2019) 1804434, , https://doi.org/ 10.1002/adma.201804434.
Declaration of Competing Interest The authors declare no conflicts of interest. References [1] W. Lv, X. Fan, M. Gan, Z. Ji, X. Chen, J. Yao, T. Jiang, J. Clean. Prod. 201 (2018) 382. [2] W. Guo, H.-H. Ngo, J. Li, Bioresour. Technol. 122 (2012) 27. [3] M.Y. Rezk, N.K. Allam, Colloid Surf. Sci. Com. 29 (2019) 33. [4] P. Aussillous, D. Quéré, Liquid marbles, Nature 411 (2001) 924. [5] Y. Xue, H. Wang, Y. Zhao, L. Dai, L. Feng, X. Wang, T. Lin, Adv. Mater. 22 (2010) 4814. [6] H. Gu, B. Ye, H. Ding, C. Liu, Y. Zhao, Z. Gu, J. Mater. Chem. C 3 (2015) 6607. [7] C. Li, X. Li, X. Yu, X. Peng, T. Lan, S. Fan, IEEE Sensors J. 18 (2018) 170. [8] Y.-E. Miao, H.K. Lee, W.S. Chew, I.Y. Phang, T. Liu, X.Y. Ling, Chem. Commun. 50 (2014) 5923. [9] M. Paven, H. Mayama, T. Sekido, H.J. Butt, Y. Nakamura, S. Fujii, Adv. Funct. Mater. 26 (2016) 3199. [10] D. Dupin, S.P. Armes, S. Fujii, J. Am. Chem. Soc. 131 (2009) 5386.
5
Contents lists available at ScienceDirect
Colloid and Interface Science Communications journal homepage: www.elsevier.com/locate/colcom
Rapid Communication
Liquid marble particle wetting separation a
a
a
a
b
Eric Shen Lin , Dwayne Chung Kim Chung , Jian Wern Ong , Hassan Ali Abid , Linghui Peng , ⁎ Xuchuan Jiangb, Oi Wah Liewc, Tuck Wah Nga,
T
a
Laboratory for Optics & Applied Mechanics, Department of Mechanical & Aerospace Engineering, Monash University, Clayton VIC3800, Australia Department of Chemical Engineering, Monash University, Clayton VIC3800, Australia Cardiovascular Research Institute, Yong Loo Lin School of Medicine, National University of Singapore, National University Health System, Centre for Translational Medicine, 14 Medical Drive, 117599, Singapore
b c
ABSTRACT
Liquid marbles to separate wetting (silver nanowires) from non-wetting (graphite and talcum) particles is described here. Two types of liquid marbles are formed by coating the dry mixture test material on pendant or sessile drops. The silver nanowire particles are separated from the graphite or talcum shell by entering into the liquid core of the marbles. The liquid core is extracted into capillary tubes where the presence of silver nanoparticles was confirmed by a blue to colourless change arising from silver-mediated catalytic reduction of methylene blue and sodium borohydride solution. The use of an inclined rotating surface to coat the pendant drop was found to create liquid marbles in a more controllable manner. A platform tilt angle of 15o followed by rotation was found to provide more uniform albeit only partially coating the bottom of the pendant drop.
1. Introduction Particle separation is important in a myriad of industrial and scientific endeavors. There is impetus for such processes to be conducted efficiently with minimal solid and liquid usage to limit contamination to the environment [1]. Membranes are widely used for particle separation but can be limited by fouling [2]. Coagulation-flocculation is an alternative method that circumvents fouling [3]. Coagulation destabilizes charged particles via chemical reactions while flocculation promotes agglomeration and settling. Both the membrane and coagulation-flocculation separation methods operate with the particles already present in solution. In cases where separation of particles from their anhydrous states are required, this may be achieved by exploiting their differences in wetting characteristics. Hence, a separation scheme that allows this to be achieved will be very useful. The ability of micro or nano scale particles to encapsulate a liquid drop to form a non-wetting soft solid called a liquid marble was first reported close to 20 years ago [4]. These coated droplets have been shown to be operable as miniature reactors [5,6] and micro sensors [7,8]. Liquid marble fabrication is simple, typically accomplished by first dispensing a liquid drop and moving it gently over a bed of particles. Removing the coating particles from the liquid marble to retrieve its interior contents has proven to be a challenge. Methods reported to achieve this include the use of optical radiation [9], pH-mediated disruption [10], electric fields [11], ultrasonic fields [12], and rolling the liquid marble over a film of oil [13]. The efficacies of these approaches
⁎
are dependent on some or all of a number of chemical-physical factors, such as the characteristics of the liquid (ionic, organic, etc.), the wetting properties of the liquids and particles, the delivery of motive forces through actuators, and the design of equipment necessary to provide the non-contacting fields. From a practical perspective, a method that obviates such considerations would be advantageous. This work investigates the use of liquid marbles created in the form of pendant and sessile drops to achieve particle separation on the basis of their differential wettability. The issues of usage ease and separation effectiveness are also assessed. 2. Materials and methods The schemes for particle separation with pendant and sessile drop liquid marbles are depicted in Figs. 1(A) and (B) respectively. With the former, liquid is dispensed from the proximal end of a capillary tube to form a pendant drop and lowered onto a powdered mixture of nanoparticles of different wettability. Separation is achieved with non-wetting particles coating the drop while the wetting particles are drawn into the liquid core. The liquid can then be drawn out from the liquid marble using the same capillary tube to recover the wetting particles. With the latter, a sessile drop is dispensed and maintained stably on a superhydrophobic (SH) substrate with a hole [14,15]. If the substrate is sufficiently thin, the liquid drop exhibits a sessile and an overhanging component above and below the hole respectively, and is rendered immovable by the pinning action of the solid phase at the edge of the
Corresponding author. E-mail address: [email protected] (T.W. Ng).
https://doi.org/10.1016/j.colcom.2020.100237 Received 10 December 2019; Received in revised form 14 January 2020; Accepted 21 January 2020 2215-0382/ © 2020 Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Colloid and Interface Science Communications 35 (2020) 100237
E.S. Lin, et al.
Fig. 1. The schemes of separating a dry mixture of hydrophobic and hydrophilic particles via liquid marbles formed from (A) pendant and (B) sessile drops. In both cases, the liquid core containing the hydrophilic particles are drawn out using a capillary tube. In the sessile drop approach, the dispensation of liquid onto a superhydrophobic substrate with a hole creates a liquid body with an overhanging component that is not coated by hydrophobic particles. This allows access of a capillary tube to effect drainage from the liquid core.
hole [15]. The sessile component of the drop is coated with the powder mixture to elicit the same particle separation behavior as described. The liquid core is then removed through the overhanging component using a capillary tube (wherein the thin SH substrate facilitates this). In the experiments, borosilicate capillary tubes of 0.68 mm and 1 mm inner and outer diameters were used. To create the SH substrate, holes of 2 mm diameter were drilled through a 0.1 mm thick copper plate that form the side walls of the wells. They were then ultra-sonicated in ethanol (70% v/v) and acetone for 3 min, and subsequently in deionized (DI) water for another 15 min. The substrates were oxidized in a solution containing 2 M NaOH and 1.5 M (NH4)2S2O8 for 5 min. The samples were then allowed to synthesize in an oven at 180 °C for 120 min, completing the phase transfer from hydroxides to oxides. At the end of the reaction, the substrates were removed from the solution, rinsed several times with DI water and ethanol and then dried with compressed air. In the final step, the substrates were silanized using FAS (1H,1H,2H,2H-perfluorodecyl-triethoxysilane) to obtain a lowsurface energy layer with good corrosion resistance and thermal stability. The substrates were immersed in FAS-ethanol solution for 30 min and finally dried in the oven at 150 °C for 10 min. From characterization studies done with a 20 μL drop on the surface, it was found that the equilibrium contact angle was 150.8o and the contact angle hysteresis was 20.7o. The silver nanowires were created by dissolving 1 g of Polyvinylpyrrolidone (PVP, Mw ~ 55,000 Sigma-Aldrich) and 0.014 g NaCl in 20 mL ethylene glycol (EG). 0.204 g of AgNO3 was dissolved in 10 mL EG. The PVP-NaCl-EG mixture was heated to 170 °C and then the AgNO3-EG cocktail was added in to the system at a rate of 5 mL/h with constant stirring. After that, the reaction was kept at 170 °C for 1 h. When the product was cooled down to room temperature, 70 mL acetone was added into the system. The mixture was put in the ultrasonic bath for 30 min. The product was centrifuged at a speed of 7000 rpm. Then acetone was added and the washing procedure was repeated again. The nanowires were finally dried in air under 80 °C. The non-wetting particles used were hydrous magnesium silicate (Talc) (Sigma Aldrich, 243,604, average size ≤10 μm) and graphite powder (Sigma Aldrich, 282,863, average size ≤20 μm). Previous research have indicated that the surface energies of these two types of particles were 54.8 mJ/m2 [16] and 31.5 mJ/m2 [17] respectively. The fabricated silver nanowires would act as the wetting material, since its surface energy of around 7200 mJ/m2 [18] is O(102) higher than that of Talc and graphite. To form the external shell of the liquid marble, talc or graphite powder spiked with 5% (by weight) of silver nanowires were used. Drops ranging from 10 to 30 μL by volume from a solution comprising 2 mM methylene blue (MB) and 0.1 M sodium borohydride (NaBH4) was used as the internal liquid phase of the marble. Particle separation is demonstrated when the silver nanoparticles separate from
the non-wetting shell and enters the liquid core where its presence catalyzes an NaBH4-MB redox reaction that results in a solution colour change from blue to colourless [19]. 3. Results and discussion Two methods for producing liquid marbles by sessile and pendant drop formation were investigated. With the sessile drop, the most effective way to coat the sessile drop was by depositing the particles over the surface of the drop using a plastic funnel (see Fig. 2A). Compressed air was then delivered at controlled pressures to disperse excess talc particles away (Fig. 2B). Figs. 2C to E shows the sequence with which the sessile drop was successfully encapsulated although achieving an external shell of uniform thickness was difficult. Interestingly, the powder does not move beyond the hole of the superhydrophobic powder. Hence, the “overhanging” component of the drop [15] remains uncoated (Fig. 2F) to allow access for a capillary tube to draw out the liquid core. The most efficient way to coat a pendant drop was to position it over a tilt/rotary stage covered with a thin layer of particles (Fig. 3A). The stage is tilted to a fixed angle θ, and lifted towards the drop by means of a lifting stage assembly. When the particles come into contact with the drop, the stage was rotated around to coat the drop. It can be seen that the height h1 to which the particles are able to coat the drop relative to its height h0 is dependent on θ up to 15o (see Figs. 3B – C). At higher θ angles (e.g. 30o), the hydrophobic particles tended to coat unevenly (see Fig. 3D). The ability of silver nanowires to hasten the degradation of 1 mL of the solution of methylene blue and sodium borohydride (diluted 10 times) placed in a cuvette is illustrated by colour change (see Supplementary Video). Nevertheless, in a liquid marble, the colour change of the liquid core arising from separation of the silver nanowires from the hydrophobic shell into it may not be as visually discernible especially with the particle coating is opaque. To demonstrate separation of silver nanoparticles from the shell into the liquid core, the liquid was drawn out from the pendant drop using the capillary tube and diluted 100 times with distilled water for analysis. As test controls, liquid extracted from the pendant drops without particle coverage (Fig. 4A) or coated with graphite powder alone (Fig. 4B) displayed clear blue tints indicating presence of MB in its oxidized state. On the other hand, liquid drawn out from the pendant drops coated with graphite and silver nanowire powder is colourless, showing catalytic reduction of MB by silver nanowires separated into the liquid core. Since the liquid core is withdrawn as soon as the coated pendant drop was formed, this indicates that the mechanism of wettability particle separation occurs very quickly as soon as the liquid droplet contacts the powder mixture. It is noteworthy that withdrawing the liquid core into the capillary tube at flowrates < 0.05 mL/s minimizes carryover of any 2
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Fig. 2. Generation of a liquid marble from a sessile drop created on a superhydrophobic plate with a hole. The surface of the drop is coated with the hydrophobic particles by deposition through a funnel (A) positioned over the drop. Excess particles are removed via compressed air (B) with pressure control using a valve and manometer. Side view (C) of a drop pinned over a hole on the SH substrate showing the top sessile and bottom overhanging component. The sessile component is covered with excess particles to form a liquid marble (D). Top view of the liquid marble formed from the sessile drop after excess particles are removed (E). Bottom view of the overhanging component (F) shows the absence of hydrophobic shell formation over the droplet surface.
position over the hole is determined primarily by three phase contact line pinning on the superhydrophobic surfaces [15,21]. For these reasons, the pendant drop approach for liquid marble formation is more robust even though full encapsulation of the droplet surface was not achieved to afford maximal liquid-air interface for particle separation. It should also be noted that the inability to fully encapsulate the pendant drop with the non-wetting particles does not detract from the utility of this approach. Clearly, when the stage platform was in a horizontal position, the area of contact between the powder mixture and pendant drop is minimal. At a tilt angle of up to 15o, the coverage of particles over the bottom surface of the droplet was uniform, thus allowing a quantifiable coverage metric of h1/h0 to be used (see Fig. 3C). Fig. 5 presents box plots of measurements made at 5o and 15o using graphite and Talcum powder to form the shell. The data spread is high for both types of powder. At platform tilt of 15o, an average coverage metric of 0.25 was attained. The advantage of such a low coverage metric is that propensity for carryover of talcum or graphite powder into the capillary tube during the liquid evacuation process is greatly reduced. The non-wetting particles accumulated at the distal ends of capillary tubes with the pendant drop approach after liquid evacuation (see Fig. 6A), additionally makes it amenable for their accumulation by merging (see Fig. 6B). With the onset of drying, the non-wetting particles can be collected in the anhydrous form (see Fig. 6C). This is in contrast to the case where particles would typically attach to wall surfaces following liquid evacuation from receptacles like vials (see Fig. 6D). With the proper automation incorporated, the entire separation process can take under 10 s to complete. While the volumes of each drop are limited to the 10–30 μL range, it is possible to scale up the volume (and thus particle separation) processing without any sacrifice in time by using a spatial array of drops operating simultaneously. It is pertinent to mention that powdered mixtures used in pharmaceutical and cosmetic formulations comprising dried particles of different wettability can be inadvertently contaminated with unwanted hydrophilic material. The manufacture of talcum powder, for example, has been reported to contain heavy metal particle contaminants [22]. The high surface to volume ratios of the talc powder along with the large number of MgeOe and OH– bonds present produces conditions conducive for metallic nanoparticles to latch on. The effects of prolonged exposure of these heavy metal tainted powders on human health is still being investigated. However, there is a strong likelihood that findings similar to that of talcum powder contaminated by asbestos will be uncovered [23].
Fig. 3. Generation of a liquid marble from a pendant drop held in place by a capillary tube. A lifting stage is assembled below a tilt/rotary stage lined with hydrophobic particles over its platform surface (A). The surface is tilted to an angle of θ, raised to contact the drop, and then rotated to coat the particles on the drop. Varied coating heights h1 in relation to the height of the drop h0 were achieved with platform tilt angles of (B) 5o and (C) 15o. At a platform tilt angle of (D) 30o the coverage tended to be skewed to one side.
talcum or graphite particles into the capillary tube most of the time. Greater caution should be taken especially towards the end of liquid drawing process, where the propensity of talcum or graphite entering the tube is higher. The use of an absolute syringe displacement device is advantageous in this context [20]. Although both the sessile and pendant drop approaches could be used for creating liquid marbles, achieving uniform coating with the former is more challenging. This requires optimal airflow pressure (see Fig. 2B) sufficient for distributing the particles over the liquid surface but not so high as to displace the drop. In some instances, the strong airflow resulted in distortion of the drop to one side of the SH hole, allowing the non-wetting particles to breach the neck of the hole and extend over the overhanging component of the sessile drop. When this happens, complete encapsulation of both the sessile and overhanging component of the drop made extraction of the liquid core difficult. A further challenge is that the ability of the sessile drop to be held in 3
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Fig. 4. Images of liquid extracted from pendant drops of MBNaBH4 solution using capillary tubes followed by 100 times dilution with distilled water. In the absence of any liquid marble created, the extracted solution (A) retains the distinctive blue colour of oxidized MB. The blue colour of oxidized MB is also observed from liquid extracted (B) from liquid marbles coated only with graphite powder. Liquid extracted from liquid marbles (C) coated with graphite and silver nanowire powder mixture is colourless due to reduction of MB by silver nanowire particles separating into the liquid core from the hydrophobic shell. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. When liquid is drawn out of the pendant liquid marble, the volume of the liquid core is reduced to create a smaller drop (A) at the distal end of the capillary tube covered by a thicker external shell of graphite. Two small pendant marbles can be brought into contact and merged (B) to facilitate further harvesting of the shell particles in the anhydrous state from a point source (C). When liquid is drained out of a typical receptacle (D), there is inevitable particle attachment to the wall that hampers harvesting.
Fig. 5. Box plots of h1/h0 (a metric for liquid marble coverage using the pendant drop approach) determined at platform tilt angles of 5o and 15o for liquid marbles formed from (A) graphite and (B) Talcum powder as the hydrophobic shell component. All measurements were based on 8 readings each.
At this juncture, it is important to highlight that the liquid marble approach here does not provide a means to separate hydrophilic (those with contact angles < 90o) from hydrophobic particles (those with contact angles > 90o). Despite their low surface energies, graphite powders typically exhibit contact angles below 90o while still being able to form the shell in liquid marbles [24,25]. The ability for graphite to do this portends usefulness in separating out graphene oxide (GO), which dissolves readily in water via hydrogen bonding mechanisms [26], when produced using dry processing methods [27]. GO is a material that exhibits properties that are useful in novel applications [28,29]. Clearly, a facile method for separating unwanted wetting components from non-wetting powders for contaminant analysis has clear useful applications. The disadvantage of the method however is that
liquid marbles are not amenable to transport. This is generally not an issue as the fragile structure of most liquid marbles generally do not facilitate their handling. This approach is not restricted to the types of wetting and non-wetting materials used in this work. 4. Concluding remarks The ability to separate wetting and non-wetting particles from an initially dry mixture is achieved by liquid marble formation using pendant drop and sessile drop approaches. The method permitted the wetting particles to be separated into the liquid core which is then 4
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easily withdrawn into capillary tubes for analysis without any need for actuator motive forces or careful design of specialized equipment. There is overall greater robustness in using the pendant drop over the sessile drop over a SH hole with the approach described here. Supplementary data to this article can be found online at https:// doi.org/10.1016/j.colcom.2020.100237.
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