- Email: [email protected]
Manipulating microobject by using liquid droplet as a transporting vehicle
Journal of Colloid and Interface Science 329 (2009) 196–201
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Manipulating microobject by using liquid droplet as a transporting vehicle Bharat Bhushan ∗ , Xing Ling Nanoprobe Laboratory for Bio- and Nanotechnology and Biomimetics (NLB2 ), The Oxchio State University, 201 W. 19th Avenue, Columbus, OH 43210-1142, USA
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 1 August 2008 Accepted 3 October 2008 Available online 9 October 2008
Liquid droplets are proposed as transport vehicles for manipulating microobjects. An atomic force microscope (AFM) cantilever is used as a gripper to pick-up a droplet and then a particle from a hydrophobic substrate. The droplet and the particle are then released to a hydrophilic substrate from the cantilever. During the pick-up and release, the liquid bridge that formed between the gripper and the substrate is studied. The efficiency of micromanipulation is quantified by the term “volumetric distribution ratio,” which is the liquid volume retained by the substrate divided by the whole volume of the droplet during the rupture of the liquid bridge. Based on the theoretical analysis, an optimized micromanipulation is suggested which could achieve 100% efficiency by carefully controlling the wetting properties of the gripper and the substrate. © 2008 Elsevier Inc. All rights reserved.
Keywords: Micromanipulation Atomic force microscope Liquid bridge Particle
1. Introduction The ability to handle, convey, and assemble preformed components is fundamental for fabricating functional devices. With the continuous development of miniature tools such as optical tweezers [1], micromanipulators [2], and nanomanipulators based on the atomic force microscope (AFM) [3], which allows both observation and interaction with the objects, the ability to handle microand nanoobjects has been significantly enhanced. Various objects such as micro- and nanoparticles [4], rods [5], tubes [6], and biomolecules [7] have been successfully manipulated to explore their individual and collective properties and functions. Manipulations typically involve large forces, as relocating objects among different environments is required. Except for a few applications which demand decomposition of the objects [7], it is usually necessary to avoid damage to the objects due to concentrated forces. Use of rigid tools can easily cause this kind of problem. In the continuous discovery of tools flexible enough while being capable of delivering required forces, the liquid droplet has recently been explored [8–11]. While the droplet can easily deform itself to conform to the geometry of the objects to avoid concentrations of forces, it can deliver a controllable capillary force which is usually larger than most other physical constraints by adjusting the surface tension and volume of the droplet. For a typical manipulation cycle based on capillary force, a micro-sized tool or gripper with a known volume of tiny droplet attached to its head is used to pick-up the object from its original location, transport and then release it to the desired location [8].
*
Corresponding author. E-mail address: [email protected] (B. Bhushan).
0021-9797/$ – see front matter doi:10.1016/j.jcis.2008.10.006
© 2008
Elsevier Inc. All rights reserved.
During the pick-up and release, the capillary force as generated from the liquid bridge between the gripper and the object needs to overcome the adhesive force between the object and the substrate on which the object sits. However, a large gripping force, as favored during pick-up, might cause a problem during release when the picked-up object needs to be removed from the gripper. Various strategies of release have been proposed, namely aids from a droplet sitting on the destination location or an auxiliary sharpened tool, the use of vibrational energy, rolling the gripper, and the evaporation of the liquid bridge [8,9]. They all suffer from the shortcoming that the adhesive force between the object and the substrate is difficult to control. Here, we propose to use a droplet which is large enough to encapsulate the object to be manipulated. By manipulating the droplet, the object encapsulated inside the droplet would be manipulated simultaneously. Thus the droplet acts as a transporting vehicle rather than as glue between the gripper and the object. It is of advantage since the adhesive force between the droplet and the substrate can be controlled by modulating the surface energy of the substrate using such techniques as electrowetting [12–14]. Surface energy modification is used to create a loading/unloading cycle for various purposes such as energy conversion in a capillary engine [15], so it is natural to apply the same mechanism for the droplet pick-up/release. It is desirable to transfer the whole droplet from the substrate to the gripper during pick-up and from the gripper to the substrate during release. Usually the liquid transferred is only a small amount of the droplet. Thus the term “volumetric distribution ratio V r ,” which is defined as the liquid volume retained by the substrate divided by the whole volume of the droplet, can be used to describe the efficiency of transfer. V r strongly depends on the
B. Bhushan, X. Ling / Journal of Colloid and Interface Science 329 (2009) 196–201
197
wetting properties of the gripper and the substrate, and their dependences will be studied, from which optimized manipulation conditions are obtained and verified experimentally using an AFM cantilever as the gripper. 2. Experimental Substrates were prepared from a 300-nm-thick silicon oxide wafer thermally grown on a p-type boron doped conductive silicon (1–20 cm, Silicon Quest Int’l). Fresh silicon oxide is hydrophilic (with a water contact angle measured to be smaller than 10◦ ) and used as the substrate to receive a particle released from an AFM cantilever. A hydrophobic substrate was prepared by immersing a silicon oxide substrate into a hexane solution of 1H,1H,2H,2H-perfluorooctyldimethylchlorosilane (PFODCS) (97%, Alfa Aesar) (1:10) for 1 h. The contact angle of the PFODCS substrate with water was measured to be 101◦ . The particles were dry soda lime glass microspheres with a diameter of 30 ± 2 μm (Duke Scientific Corporation). They were directly spread on a PFODCS substrate. On another PFODCS substrate, microdroplets of a mixture of water and glycerol were deposited from an aerosol, which was created by rapidly pushing a syringe containing a large percent of air and a small percent of the water–glycerol liquid. The glycerol is added to reduce the evaporation of the droplet under an open ambient environment. The cantilever used to pick-up and release the particle was prepared from a commercially available AFM probe (VL300, Veeco) which has a high nominal spring constant of 40 N/m to overcome the large capillary force during interaction with a droplet. The probe has a tip of 15–20 μm height, which prevents the cantilever from getting closer to the substrate than this distance. The cantilever was hydrophobized with a hexane solution of octadecyltrichlorosilane (OTCS) (95%, Acros Organics) (1:10, 1 h) to increase its contact angle with water to 99◦ . The contact angle was measured from a flat silicon oxide substrate immersed in the same solution. Before hydrophobization, the cantilever was treated with a piranha solution (H2 O2 :H2 SO4 = 3:7) at room temperature for 3 h to remove possible contaminates and to increase its hydrophilicity. The manipulation of droplets and particles with an AFM cantilever was performed inside a commercial AFM (Dimension 3000, Veeco). The built-in vision system of the AFM was used to guide the positioning of the cantilever on top of droplets and particles and to collect optical images during the manipulation. To pick-up a particle from the PFODCS substrate where the particle was spread, a droplet was first picked up by the cantilever from the PFODCS substrate on which the droplet was deposited. After that, the particle was picked up by bringing the cantilever into contact with the particle with the droplet between them (Fig. 1a). To release the particle from the cantilever, a SiO2 substrate was used, and the droplet, along with the picked-up particle, was brought into contact with the SiO2 substrate (Fig. 1b). After pick-up and release, the cantilever was taken out of the AFM and examined with an optical microscope (Optiphot-2, Nikon). A few particles were manipulated to form an array on the SiO2 substrate. To remove the droplets surrounding the particles, the substrate was heated in an oven at 90 ◦ C for 10 min. The temperature and relative humidity were 21 ± 1 ◦ C and 30 ± 5%, respectively. 3. Results and discussion 3.1. Theory We first explore the dependence of volumetric distribution ratio, V r , on the wetting properties of the gripper and the substrate. The liquid bridge confined between a flat gripper and a flat substrate assumes an axisymmetric shape with a constant mean
Fig. 1. Schematic drawing of the manipulation of microobject by using liquid droplet as a transporting vehicle.
Fig. 2. Schematic drawing of the configuration of the liquid bridge confined between a flat gripper and a flat substrate.
curvature κ¯ everywhere on its surface. In principle, all axisymmetric bridging shapes are parameterized by Delaunay (1841) surfaces [16]. Delaunay proved that the meridian profile r ( z) (Fig. 2) for such a surface could be obtained by tracing the focus of an arbitrary conic section rolling along the z axis. If the rolling curve is an ellipse, then the surface is called unduloid. If the rolling curve is a hyperbola, then surface is called nodoid. In the case of a parabola, κ¯ = 0 and the surface is called catenoid. A graphic illustration of the meridian profile for Delaunay surfaces was given by Orr et al. [17]. For a stable surface, the number of stationary points (r = 0) in r ( z) should not be larger than one even for an unduloid [18]. Therefore only when both contact angles, θ1 or θ2 (Fig. 2), are smaller than 90◦ is a neck with a minimum radius rn is formed in r ( z) at r = 0 (Fig. 2). Otherwise the thinnest point appears on one of the ends of the bridge whose contact angle is larger. Therefore, using an experimentally verified criterion that the bridge breaks apart at the thinnest point [19], V r can be determined to be either 0 or 100% if one of the angles is larger than 90◦ . Only when both angles are smaller than 90◦ does the bridge split, and V r takes a value between 0 and 100%, which is calculated by considering the bridge profile at the rupture distance as follows. To precisely calculate the profile for an axisymmetric bridge with a constant mean curvature κ¯ , the Young–Laplace equation needs to be solved numerically [14,17] 2κ¯ = −
r
(1 + r 2 )3/2
+
1 r (1 + r 2 )1/2
.
(1)
198
B. Bhushan, X. Ling / Journal of Colloid and Interface Science 329 (2009) 196–201
By substituting Eq. (9) into Eq. (6), the profile can be fully determined for a given liquid volume of V 0 , which leads to
For given boundary conditions r1 = tan(−π /2 + θ1 ),
Vr =
r2 = tan(−π /2 − θ2 ),
z2
π r 2 dz = V 0 , z1
z2 − z1 = D .
(2)
To determine the distance at which the bridge ruptures, a widely accepted method is to search solutions for increasing steps of separation distance D (Fig. 2) [20–22]. It is usually found that two solutions coexist and then converge into one solution when D increases to a maximum distance D m . D m is treated as the rupture distance. Stability analysis of the bridge using calculus of variations yields similar results [23] as the bridge ruptures when the inflection point (at r = 0) first appears in r ( z) if solutions are possible, which corresponds to the situation that two solutions converge. Here, since the splitting of the bridge only happens under a limited case when both contact angles are smaller than 90◦ , which we avoided experimentally by purposely hydrophobizing the surface of the gripper and substrate for an optimized efficiency of manipulation, we will only briefly touch the calculation by using a widely adopted toroidal approximation [17,19,24,25]. The meridian profile of the bridge is assumed to be a circular arc. The arc is conveniently described using the parameters z0 , r0 , R, and α z = z0 + R cos α , r = r0 + R sin α ,
(3)
where z0 and r0 represent the coordinates of the circular center of the arc, R is the circular radius of the arc, and α is the angle with respect to the z axis clockwise (Fig. 2). α1 and α2 can be directly determined from the contact angles; for the case θ1 + θ2 < π , they are given by
α1 = π + θ1 , α2 = 2π − θ2 .
(4)
The volume of the liquid bridge encapsulated by revolving its profile around the z axis is calculated by
z2
π r 2 dz.
V =
(5)
z1
Substituting Eq. (3) into Eq. (5) yields V = F (α2 ) − F (α1 ),
(6) 3
where F (α ) = π Rr02 cos α − π R 2 r0 (α − sin α cos α ) + π 3R (sin2 α × cos α + 2 cos α ). For a given separation distance of D = z2 − z1 , the circular radius of the arc can be directly calculated from Eq. (3) as R=
D cos α2 − cos α1
(7)
.
Since z1 ≡ 0, substituting Eq. (7) into Eq. (3) yields z0 = −
D cos α1 cos α2 − cos α1
.
(8)
r0 and thus the profile of the liquid bridge can be fully determined by combining Eqs. (6) and (7) for given a droplet volume V 0 and separation distance D. To estimate the rupture distance, we simply assume the bridge ruptures when the bridge neck eventually reaches the z axis, which leads to a condition R = r0 .
(9)
F (3π /2) − F (α1 ) V0
.
(10)
A more accurate estimation of rupture distance for toroidal approximation can be found through the comparison of free energy between connected and ruptured bridge [19]. However, a case study using Eq. (9) for a condition of θ1 = 30◦ , θ2 = 60◦ shows an error of only 5% compared to a precise numerical method [14]. 3.2. Manipulation of particle Based on the above analysis, optimized experimental conditions for manipulation of a droplet with particle would be: (1) for the gripper to pick-up the droplet, it needs to be more hydrophilic than the substrate; (2) to release the droplet from the gripper to the substrate, a substrate more hydrophilic than the gripper needs to be used since the wetting property of the substrate is controlled here; and (3) the contact angle between the gripper and the liquid needs to be larger than 90◦ to achieve 100% efficiency. In short
θ2 > 90◦ , θ1 (pick-up) > θ2 , θ1 (release) < θ2 .
(11)
Therefore an OTCS-hydrophobized cantilever as the gripper, a PFODCS-hydrophobized silicon oxide as the substrate for pick-up, and then a natural silicon oxide as the substrate for release are used to verify this concept. 3.2.1. With a large droplet A droplet is first picked up from the PFODCS substrate using the cantilever. The sequence of pick-up is shown in Fig. 3. All scale bars in the figures indicate a length of 100 μm. In Fig. 3a, to the left is a spherical droplet with a diameter of 70 μm (Fig. 3a) to be picked up. To its right is the rectangular cantilever with a triangular end pointing toward the droplet. The cantilever is initially higher than the droplet and out of the focus of optical microscope, leaving a fuzzy image in Fig. 3a. Its reflection on the substrate is also visible. Using the measured contact angle of 101◦ , its height is calculated to be 42 μm, which is significantly larger than the height of the AFM tip. Thus, after the AFM tip was lowered and brought into contact with the droplet and the substrate in Fig. 3b, the cantilever was able to contact the droplet. In Fig. 3b, the cantilever is within the focus of optical microscope, and its shape is clearly identified. It is also clear that the droplet is squeezed out below the cantilever. In Fig. 3c after retraction of the cantilever, the droplet is mostly picked up by the cantilever with only a tiny residual droplet of 20 μm in diameter left behind. V r is calculated be ∼2%, as expected for the combination of OTCS cantilever and PFODCS substrate. The presence of tiny residual droplets is repeatable, indicating the dynamic effect during the rupturing of the bridge. The sequence of manipulating a particle on a PFODCS substrate with the droplet on the cantilever is shown in Fig. 4. In Fig. 4a, to the left is the glass particle to be picked up. To its right is the cantilever with a pre-picked droplet (in Fig. 3) attached to its bottom side (the droplet is not visible from above). In Fig. 4b, the particle was easily picked up after it contacted with the droplet and the left image shows the residual droplet of 10 μm in diameter left over on the substrate after the cantilever retracted from the substrate. After retraction, the cantilever was removed from the AFM and examined ex situ using a different optical microscope. Its side and bottom views are shown in the right column of Fig. 4b. It is
B. Bhushan, X. Ling / Journal of Colloid and Interface Science 329 (2009) 196–201
Fig. 3. Sequence of picking-up a droplet deposited on a PFODCS substrate using an OTCS cantilever.
(a) Particle to be picked-up from PFODCS substrate
(b) Residual droplet left over on PFODCS substrate and picked-up particle on cantilever
(c) Particle released to SiO2 substrate and cantilever after releasing Fig. 4. Sequence of manipulating a particle from a PFODCS substrate to a SiO2 substrate using an OTCS cantilever with a large droplet.
199
200
B. Bhushan, X. Ling / Journal of Colloid and Interface Science 329 (2009) 196–201
(a) Particle to be picked up from PFODCS substrate
(b) No residual droplet left over on PFODCS substrate and picked-up particle on cantilever
(c) Particle released to SiO2 substrate and cantilever after releasing Fig. 5. Sequence of manipulating a particle from a PFODCS substrate to a SiO2 substrate using an OTCS cantilever with a small droplet.
clear that the 30-μm-diameter particle was fully absorbed into the droplet with a measured diameter of ∼65 μm. The position of the particle inside the droplet is slightly off-center possibly due to the presence of the AFM tip. Using a SiO2 substrate with a contact angle smaller than 10◦ , the particle is easily released to the substrate with the droplet spreading around the particle as a ring with a diameter of 140 μm (left image of Fig. 4c). The particle outside the ring was already there before releasing (the particle in Fig. 5c). From the ex situ side- and bottom-views of the cantilever shown in the right column of Fig. 4c after release, it is clear that no liquid was left over on the cantilever as expected for the combination of OTCS cantilever and SiO2 substrate. We notice that although the
presence of an AFM tip prevented pick-up of small droplets, it did help to confine the position of the droplet on the cantilever as the droplet tends to adhere to the tip to minimize its surface energy. This finding suggests that by carefully designing anchor points on the gripper, the position of the droplet on the gripper may be precisely controlled, which is important for manipulation. The anchor points may be of flat chemical patterns instead of physical AFM tip to facilitate operation. 3.2.2. With a small droplet Although small droplets lower than the tip height cannot be directly picked up by the cantilever, a small droplet as the trans-
B. Bhushan, X. Ling / Journal of Colloid and Interface Science 329 (2009) 196–201
201
Fig. 6. Array of particles manipulated to a SiO2 substrate.
porting vehicle for the particle can be attached to the cantilever by allowing a picked-up large droplet to evaporate at room temperature for a certain duration. Even with such a small droplet, the manipulation of a particle can be successfully performed. In Fig. 5a, the particle to be picked up is visible in the center. Another particle located in the top right region served as a reference. In the left image of Fig. 5b, the particle was picked up by the cantilever with the small droplet attached to its bottom side. The droplet is so small that it is barely visible from the side-view of the cantilever, shown in the right image of Fig. 5b. The perfect sphere is the 30-μm-diameter particle. In the left image of Fig. 5c, the particle was easily released to the SiO2 substrate from the cantilever after its contact with the substrate. It is of relevance to point out that since the glass particle is hydrophilic, the small droplet may be able to spread to the bottom of the particle, which is important for the particle to be released back to the SiO2 substrate. Otherwise we would expect the particle to remain on the cantilever as the adhesive force between particle and SiO2 substrate should be smaller than the capillary force between particle and cantilever. Evidence can be found from the ex situ side-view of the cantilever shown in Fig. 5c. It is clear that after release the droplet was completely transferred to the substrate. The effectiveness of the droplet as the transporting vehicle for manipulating particles was proved from the experiments directly using dry cantilevers. Various trials with dry cantilevers failed without exception.
3.2.3. Array of particles Five particles were manipulated and arranged closely to form an array shown in Fig. 6a. All particles have apparent liquid rings surrounding them except the one manipulated with the smallest droplet (the particle in Fig. 5c). The sample was stored at room temperature for one day. Except for the particle which was too close to the liquid ring surrounding an adjacent particle to be attracted inside, all the other particles maintain their positions (Fig. 6b). Heating the sample at 90 ◦ C for 10 min effectively removed all liquid rings surrounding the particles. All particles still remain at their original positions (Fig. 6c). For a clean, uniform substrate such as a SiO2 used here, the droplets spread on the substrate as perfect circles (Fig. 6b), where the capillary force ensures the particle to be at the center of the circle even during the heating. Thus by precisely controlling the position of the droplet on the gripper using carefully engineered anchor points, a very high manipulation precision is expected to be achievable, regardless of the size of the droplet used for transporting the particle.
4. Conclusions The results show that the manipulation of microobjects by using liquid droplets as transport vehicles is practically possible. The method is non-destructive. The droplets can be easily removed after manipulation while the positions of manipulated microobjects remain intact. We notice that the coalescence of particles shown in Fig. 6b can be exploited for fabricating a closely packed structure of well-defined numbers of microobjects which may be practically interesting for studying coupling effects at microscale. In addition, as electrowetting can be used to modulate the wetting property of surfaces, it is possible to realize the pick-up and release on the same substrate simply by modulating the voltage. Nevertheless, the saturation problem of electrowetting should be tackled before the conditions described in Eq. (11) can be met. References [1] K. Dholakia, P. Reece, M. Gu, Chem. Soc. Rev. 37 (2008) 42–55. [2] J. Cecil, D. Vasquez, D. Powell, Int. J. Prod. Res. 43 (2005) 819–828. [3] S. Fahlbusch, S. Mazerolle, J.M. Breguet, A. Steinecker, J. Agnus, R. Perez, J. Michler, J. Mater. Process. Technol. 167 (2005) 371–382. [4] T. Junno, K. Deppert, L. Montelius, L. Samuelson, Appl. Phys. Lett. 66 (1995) 3627–3629. [5] S.C. Hsieh, S. Meltzer, C.R.C. Wang, A.A.G. Requicha, M.E. Thompson, B.E. Koel, J. Phys. Chem. B 106 (2002) 231–234. [6] M.R. Falvo, G.J. Clary, R.M. Taylor, V. Chi, F.P. Brooks, S. Washburn, R. Superfine, Nature 389 (1997) 582–584. [7] D. Fotiadis, S. Scheuring, S.A. Muller, A. Engel, D.J. Muller, Micron 33 (2002) 385–397. [8] K.J. Obata, T. Motokado, S. Saito, K. Takahashi, J. Fluid Mech. 498 (2004) 113– 121. [9] P. Lambert, A. Delchambre, Assem. Autom. 25 (2005) 275–283. [10] S. Saito, T. Motokado, K.J. Obata, K. Takahashi, Appl. Phys. Lett. 87 (2005) 3. [11] P. Lambert, F. Seigneur, S. Koelemeijer, J. Jacot, J. Micromech. Microeng. 16 (2006) 1267–1276. [12] C. Quilliet, B. Berge, Curr. Opin. Colloid Interface Sci. 6 (2001) 34–39. [13] F. Mugele, J.C. Baret, J. Phys. Condens. Matter 17 (2005) R705–R774. [14] B. Bhushan, X. Ling, 2008, submitted for publication. [15] M. Nosonovsky, B. Bhushan, J. Adhes. Sci. Technol. (2008) in press. [16] C. Delaunay, J. Math. Pure Appl. 6 (1841) 309–320. [17] F.M. Orr, L.E. Scriven, A.P. Rivas, J. Fluid Mech. 67 (1975) 723–742. [18] M.A. Fortes, J. Colloid Interface Sci. 88 (1982) 338–352. [19] X. Pepin, D. Rossetti, S.M. Iveson, S.J.R. Simons, J. Colloid Interface Sci. 232 (2000) 289–297. [20] G.P. Lian, C. Thornton, M.J. Adams, J. Colloid Interface Sci. 161 (1993) 138–147. [21] C.D. Willett, M.J. Adams, S.A. Johnson, J.P.K. Seville, Langmuir 16 (2000) 9396– 9405. [22] J.C. Baret, M. Brinkmann, Phys. Rev. Lett. 96 (2006) 4. [23] A.M. Alencar, E. Wolfe, S.V. Buldyrev, Phys. Rev. E 74 (2006) 13. [24] M. Farshchi-Tabrizi, M. Kappl, Y.J. Cheng, J. Gutmann, H.J. Butt, Langmuir 22 (2006) 2171–2184. [25] S. Cai, B. Bhushan, Mater. Sci. Eng. R 61 (2008) 78–106.
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Manipulating microobject by using liquid droplet as a transporting vehicle Bharat Bhushan ∗ , Xing Ling Nanoprobe Laboratory for Bio- and Nanotechnology and Biomimetics (NLB2 ), The Oxchio State University, 201 W. 19th Avenue, Columbus, OH 43210-1142, USA
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 1 August 2008 Accepted 3 October 2008 Available online 9 October 2008
Liquid droplets are proposed as transport vehicles for manipulating microobjects. An atomic force microscope (AFM) cantilever is used as a gripper to pick-up a droplet and then a particle from a hydrophobic substrate. The droplet and the particle are then released to a hydrophilic substrate from the cantilever. During the pick-up and release, the liquid bridge that formed between the gripper and the substrate is studied. The efficiency of micromanipulation is quantified by the term “volumetric distribution ratio,” which is the liquid volume retained by the substrate divided by the whole volume of the droplet during the rupture of the liquid bridge. Based on the theoretical analysis, an optimized micromanipulation is suggested which could achieve 100% efficiency by carefully controlling the wetting properties of the gripper and the substrate. © 2008 Elsevier Inc. All rights reserved.
Keywords: Micromanipulation Atomic force microscope Liquid bridge Particle
1. Introduction The ability to handle, convey, and assemble preformed components is fundamental for fabricating functional devices. With the continuous development of miniature tools such as optical tweezers [1], micromanipulators [2], and nanomanipulators based on the atomic force microscope (AFM) [3], which allows both observation and interaction with the objects, the ability to handle microand nanoobjects has been significantly enhanced. Various objects such as micro- and nanoparticles [4], rods [5], tubes [6], and biomolecules [7] have been successfully manipulated to explore their individual and collective properties and functions. Manipulations typically involve large forces, as relocating objects among different environments is required. Except for a few applications which demand decomposition of the objects [7], it is usually necessary to avoid damage to the objects due to concentrated forces. Use of rigid tools can easily cause this kind of problem. In the continuous discovery of tools flexible enough while being capable of delivering required forces, the liquid droplet has recently been explored [8–11]. While the droplet can easily deform itself to conform to the geometry of the objects to avoid concentrations of forces, it can deliver a controllable capillary force which is usually larger than most other physical constraints by adjusting the surface tension and volume of the droplet. For a typical manipulation cycle based on capillary force, a micro-sized tool or gripper with a known volume of tiny droplet attached to its head is used to pick-up the object from its original location, transport and then release it to the desired location [8].
*
Corresponding author. E-mail address: [email protected] (B. Bhushan).
0021-9797/$ – see front matter doi:10.1016/j.jcis.2008.10.006
© 2008
Elsevier Inc. All rights reserved.
During the pick-up and release, the capillary force as generated from the liquid bridge between the gripper and the object needs to overcome the adhesive force between the object and the substrate on which the object sits. However, a large gripping force, as favored during pick-up, might cause a problem during release when the picked-up object needs to be removed from the gripper. Various strategies of release have been proposed, namely aids from a droplet sitting on the destination location or an auxiliary sharpened tool, the use of vibrational energy, rolling the gripper, and the evaporation of the liquid bridge [8,9]. They all suffer from the shortcoming that the adhesive force between the object and the substrate is difficult to control. Here, we propose to use a droplet which is large enough to encapsulate the object to be manipulated. By manipulating the droplet, the object encapsulated inside the droplet would be manipulated simultaneously. Thus the droplet acts as a transporting vehicle rather than as glue between the gripper and the object. It is of advantage since the adhesive force between the droplet and the substrate can be controlled by modulating the surface energy of the substrate using such techniques as electrowetting [12–14]. Surface energy modification is used to create a loading/unloading cycle for various purposes such as energy conversion in a capillary engine [15], so it is natural to apply the same mechanism for the droplet pick-up/release. It is desirable to transfer the whole droplet from the substrate to the gripper during pick-up and from the gripper to the substrate during release. Usually the liquid transferred is only a small amount of the droplet. Thus the term “volumetric distribution ratio V r ,” which is defined as the liquid volume retained by the substrate divided by the whole volume of the droplet, can be used to describe the efficiency of transfer. V r strongly depends on the
B. Bhushan, X. Ling / Journal of Colloid and Interface Science 329 (2009) 196–201
197
wetting properties of the gripper and the substrate, and their dependences will be studied, from which optimized manipulation conditions are obtained and verified experimentally using an AFM cantilever as the gripper. 2. Experimental Substrates were prepared from a 300-nm-thick silicon oxide wafer thermally grown on a p-type boron doped conductive silicon (1–20 cm, Silicon Quest Int’l). Fresh silicon oxide is hydrophilic (with a water contact angle measured to be smaller than 10◦ ) and used as the substrate to receive a particle released from an AFM cantilever. A hydrophobic substrate was prepared by immersing a silicon oxide substrate into a hexane solution of 1H,1H,2H,2H-perfluorooctyldimethylchlorosilane (PFODCS) (97%, Alfa Aesar) (1:10) for 1 h. The contact angle of the PFODCS substrate with water was measured to be 101◦ . The particles were dry soda lime glass microspheres with a diameter of 30 ± 2 μm (Duke Scientific Corporation). They were directly spread on a PFODCS substrate. On another PFODCS substrate, microdroplets of a mixture of water and glycerol were deposited from an aerosol, which was created by rapidly pushing a syringe containing a large percent of air and a small percent of the water–glycerol liquid. The glycerol is added to reduce the evaporation of the droplet under an open ambient environment. The cantilever used to pick-up and release the particle was prepared from a commercially available AFM probe (VL300, Veeco) which has a high nominal spring constant of 40 N/m to overcome the large capillary force during interaction with a droplet. The probe has a tip of 15–20 μm height, which prevents the cantilever from getting closer to the substrate than this distance. The cantilever was hydrophobized with a hexane solution of octadecyltrichlorosilane (OTCS) (95%, Acros Organics) (1:10, 1 h) to increase its contact angle with water to 99◦ . The contact angle was measured from a flat silicon oxide substrate immersed in the same solution. Before hydrophobization, the cantilever was treated with a piranha solution (H2 O2 :H2 SO4 = 3:7) at room temperature for 3 h to remove possible contaminates and to increase its hydrophilicity. The manipulation of droplets and particles with an AFM cantilever was performed inside a commercial AFM (Dimension 3000, Veeco). The built-in vision system of the AFM was used to guide the positioning of the cantilever on top of droplets and particles and to collect optical images during the manipulation. To pick-up a particle from the PFODCS substrate where the particle was spread, a droplet was first picked up by the cantilever from the PFODCS substrate on which the droplet was deposited. After that, the particle was picked up by bringing the cantilever into contact with the particle with the droplet between them (Fig. 1a). To release the particle from the cantilever, a SiO2 substrate was used, and the droplet, along with the picked-up particle, was brought into contact with the SiO2 substrate (Fig. 1b). After pick-up and release, the cantilever was taken out of the AFM and examined with an optical microscope (Optiphot-2, Nikon). A few particles were manipulated to form an array on the SiO2 substrate. To remove the droplets surrounding the particles, the substrate was heated in an oven at 90 ◦ C for 10 min. The temperature and relative humidity were 21 ± 1 ◦ C and 30 ± 5%, respectively. 3. Results and discussion 3.1. Theory We first explore the dependence of volumetric distribution ratio, V r , on the wetting properties of the gripper and the substrate. The liquid bridge confined between a flat gripper and a flat substrate assumes an axisymmetric shape with a constant mean
Fig. 1. Schematic drawing of the manipulation of microobject by using liquid droplet as a transporting vehicle.
Fig. 2. Schematic drawing of the configuration of the liquid bridge confined between a flat gripper and a flat substrate.
curvature κ¯ everywhere on its surface. In principle, all axisymmetric bridging shapes are parameterized by Delaunay (1841) surfaces [16]. Delaunay proved that the meridian profile r ( z) (Fig. 2) for such a surface could be obtained by tracing the focus of an arbitrary conic section rolling along the z axis. If the rolling curve is an ellipse, then the surface is called unduloid. If the rolling curve is a hyperbola, then surface is called nodoid. In the case of a parabola, κ¯ = 0 and the surface is called catenoid. A graphic illustration of the meridian profile for Delaunay surfaces was given by Orr et al. [17]. For a stable surface, the number of stationary points (r = 0) in r ( z) should not be larger than one even for an unduloid [18]. Therefore only when both contact angles, θ1 or θ2 (Fig. 2), are smaller than 90◦ is a neck with a minimum radius rn is formed in r ( z) at r = 0 (Fig. 2). Otherwise the thinnest point appears on one of the ends of the bridge whose contact angle is larger. Therefore, using an experimentally verified criterion that the bridge breaks apart at the thinnest point [19], V r can be determined to be either 0 or 100% if one of the angles is larger than 90◦ . Only when both angles are smaller than 90◦ does the bridge split, and V r takes a value between 0 and 100%, which is calculated by considering the bridge profile at the rupture distance as follows. To precisely calculate the profile for an axisymmetric bridge with a constant mean curvature κ¯ , the Young–Laplace equation needs to be solved numerically [14,17] 2κ¯ = −
r
(1 + r 2 )3/2
+
1 r (1 + r 2 )1/2
.
(1)
198
B. Bhushan, X. Ling / Journal of Colloid and Interface Science 329 (2009) 196–201
By substituting Eq. (9) into Eq. (6), the profile can be fully determined for a given liquid volume of V 0 , which leads to
For given boundary conditions r1 = tan(−π /2 + θ1 ),
Vr =
r2 = tan(−π /2 − θ2 ),
z2
π r 2 dz = V 0 , z1
z2 − z1 = D .
(2)
To determine the distance at which the bridge ruptures, a widely accepted method is to search solutions for increasing steps of separation distance D (Fig. 2) [20–22]. It is usually found that two solutions coexist and then converge into one solution when D increases to a maximum distance D m . D m is treated as the rupture distance. Stability analysis of the bridge using calculus of variations yields similar results [23] as the bridge ruptures when the inflection point (at r = 0) first appears in r ( z) if solutions are possible, which corresponds to the situation that two solutions converge. Here, since the splitting of the bridge only happens under a limited case when both contact angles are smaller than 90◦ , which we avoided experimentally by purposely hydrophobizing the surface of the gripper and substrate for an optimized efficiency of manipulation, we will only briefly touch the calculation by using a widely adopted toroidal approximation [17,19,24,25]. The meridian profile of the bridge is assumed to be a circular arc. The arc is conveniently described using the parameters z0 , r0 , R, and α z = z0 + R cos α , r = r0 + R sin α ,
(3)
where z0 and r0 represent the coordinates of the circular center of the arc, R is the circular radius of the arc, and α is the angle with respect to the z axis clockwise (Fig. 2). α1 and α2 can be directly determined from the contact angles; for the case θ1 + θ2 < π , they are given by
α1 = π + θ1 , α2 = 2π − θ2 .
(4)
The volume of the liquid bridge encapsulated by revolving its profile around the z axis is calculated by
z2
π r 2 dz.
V =
(5)
z1
Substituting Eq. (3) into Eq. (5) yields V = F (α2 ) − F (α1 ),
(6) 3
where F (α ) = π Rr02 cos α − π R 2 r0 (α − sin α cos α ) + π 3R (sin2 α × cos α + 2 cos α ). For a given separation distance of D = z2 − z1 , the circular radius of the arc can be directly calculated from Eq. (3) as R=
D cos α2 − cos α1
(7)
.
Since z1 ≡ 0, substituting Eq. (7) into Eq. (3) yields z0 = −
D cos α1 cos α2 − cos α1
.
(8)
r0 and thus the profile of the liquid bridge can be fully determined by combining Eqs. (6) and (7) for given a droplet volume V 0 and separation distance D. To estimate the rupture distance, we simply assume the bridge ruptures when the bridge neck eventually reaches the z axis, which leads to a condition R = r0 .
(9)
F (3π /2) − F (α1 ) V0
.
(10)
A more accurate estimation of rupture distance for toroidal approximation can be found through the comparison of free energy between connected and ruptured bridge [19]. However, a case study using Eq. (9) for a condition of θ1 = 30◦ , θ2 = 60◦ shows an error of only 5% compared to a precise numerical method [14]. 3.2. Manipulation of particle Based on the above analysis, optimized experimental conditions for manipulation of a droplet with particle would be: (1) for the gripper to pick-up the droplet, it needs to be more hydrophilic than the substrate; (2) to release the droplet from the gripper to the substrate, a substrate more hydrophilic than the gripper needs to be used since the wetting property of the substrate is controlled here; and (3) the contact angle between the gripper and the liquid needs to be larger than 90◦ to achieve 100% efficiency. In short
θ2 > 90◦ , θ1 (pick-up) > θ2 , θ1 (release) < θ2 .
(11)
Therefore an OTCS-hydrophobized cantilever as the gripper, a PFODCS-hydrophobized silicon oxide as the substrate for pick-up, and then a natural silicon oxide as the substrate for release are used to verify this concept. 3.2.1. With a large droplet A droplet is first picked up from the PFODCS substrate using the cantilever. The sequence of pick-up is shown in Fig. 3. All scale bars in the figures indicate a length of 100 μm. In Fig. 3a, to the left is a spherical droplet with a diameter of 70 μm (Fig. 3a) to be picked up. To its right is the rectangular cantilever with a triangular end pointing toward the droplet. The cantilever is initially higher than the droplet and out of the focus of optical microscope, leaving a fuzzy image in Fig. 3a. Its reflection on the substrate is also visible. Using the measured contact angle of 101◦ , its height is calculated to be 42 μm, which is significantly larger than the height of the AFM tip. Thus, after the AFM tip was lowered and brought into contact with the droplet and the substrate in Fig. 3b, the cantilever was able to contact the droplet. In Fig. 3b, the cantilever is within the focus of optical microscope, and its shape is clearly identified. It is also clear that the droplet is squeezed out below the cantilever. In Fig. 3c after retraction of the cantilever, the droplet is mostly picked up by the cantilever with only a tiny residual droplet of 20 μm in diameter left behind. V r is calculated be ∼2%, as expected for the combination of OTCS cantilever and PFODCS substrate. The presence of tiny residual droplets is repeatable, indicating the dynamic effect during the rupturing of the bridge. The sequence of manipulating a particle on a PFODCS substrate with the droplet on the cantilever is shown in Fig. 4. In Fig. 4a, to the left is the glass particle to be picked up. To its right is the cantilever with a pre-picked droplet (in Fig. 3) attached to its bottom side (the droplet is not visible from above). In Fig. 4b, the particle was easily picked up after it contacted with the droplet and the left image shows the residual droplet of 10 μm in diameter left over on the substrate after the cantilever retracted from the substrate. After retraction, the cantilever was removed from the AFM and examined ex situ using a different optical microscope. Its side and bottom views are shown in the right column of Fig. 4b. It is
B. Bhushan, X. Ling / Journal of Colloid and Interface Science 329 (2009) 196–201
Fig. 3. Sequence of picking-up a droplet deposited on a PFODCS substrate using an OTCS cantilever.
(a) Particle to be picked-up from PFODCS substrate
(b) Residual droplet left over on PFODCS substrate and picked-up particle on cantilever
(c) Particle released to SiO2 substrate and cantilever after releasing Fig. 4. Sequence of manipulating a particle from a PFODCS substrate to a SiO2 substrate using an OTCS cantilever with a large droplet.
199
200
B. Bhushan, X. Ling / Journal of Colloid and Interface Science 329 (2009) 196–201
(a) Particle to be picked up from PFODCS substrate
(b) No residual droplet left over on PFODCS substrate and picked-up particle on cantilever
(c) Particle released to SiO2 substrate and cantilever after releasing Fig. 5. Sequence of manipulating a particle from a PFODCS substrate to a SiO2 substrate using an OTCS cantilever with a small droplet.
clear that the 30-μm-diameter particle was fully absorbed into the droplet with a measured diameter of ∼65 μm. The position of the particle inside the droplet is slightly off-center possibly due to the presence of the AFM tip. Using a SiO2 substrate with a contact angle smaller than 10◦ , the particle is easily released to the substrate with the droplet spreading around the particle as a ring with a diameter of 140 μm (left image of Fig. 4c). The particle outside the ring was already there before releasing (the particle in Fig. 5c). From the ex situ side- and bottom-views of the cantilever shown in the right column of Fig. 4c after release, it is clear that no liquid was left over on the cantilever as expected for the combination of OTCS cantilever and SiO2 substrate. We notice that although the
presence of an AFM tip prevented pick-up of small droplets, it did help to confine the position of the droplet on the cantilever as the droplet tends to adhere to the tip to minimize its surface energy. This finding suggests that by carefully designing anchor points on the gripper, the position of the droplet on the gripper may be precisely controlled, which is important for manipulation. The anchor points may be of flat chemical patterns instead of physical AFM tip to facilitate operation. 3.2.2. With a small droplet Although small droplets lower than the tip height cannot be directly picked up by the cantilever, a small droplet as the trans-
B. Bhushan, X. Ling / Journal of Colloid and Interface Science 329 (2009) 196–201
201
Fig. 6. Array of particles manipulated to a SiO2 substrate.
porting vehicle for the particle can be attached to the cantilever by allowing a picked-up large droplet to evaporate at room temperature for a certain duration. Even with such a small droplet, the manipulation of a particle can be successfully performed. In Fig. 5a, the particle to be picked up is visible in the center. Another particle located in the top right region served as a reference. In the left image of Fig. 5b, the particle was picked up by the cantilever with the small droplet attached to its bottom side. The droplet is so small that it is barely visible from the side-view of the cantilever, shown in the right image of Fig. 5b. The perfect sphere is the 30-μm-diameter particle. In the left image of Fig. 5c, the particle was easily released to the SiO2 substrate from the cantilever after its contact with the substrate. It is of relevance to point out that since the glass particle is hydrophilic, the small droplet may be able to spread to the bottom of the particle, which is important for the particle to be released back to the SiO2 substrate. Otherwise we would expect the particle to remain on the cantilever as the adhesive force between particle and SiO2 substrate should be smaller than the capillary force between particle and cantilever. Evidence can be found from the ex situ side-view of the cantilever shown in Fig. 5c. It is clear that after release the droplet was completely transferred to the substrate. The effectiveness of the droplet as the transporting vehicle for manipulating particles was proved from the experiments directly using dry cantilevers. Various trials with dry cantilevers failed without exception.
3.2.3. Array of particles Five particles were manipulated and arranged closely to form an array shown in Fig. 6a. All particles have apparent liquid rings surrounding them except the one manipulated with the smallest droplet (the particle in Fig. 5c). The sample was stored at room temperature for one day. Except for the particle which was too close to the liquid ring surrounding an adjacent particle to be attracted inside, all the other particles maintain their positions (Fig. 6b). Heating the sample at 90 ◦ C for 10 min effectively removed all liquid rings surrounding the particles. All particles still remain at their original positions (Fig. 6c). For a clean, uniform substrate such as a SiO2 used here, the droplets spread on the substrate as perfect circles (Fig. 6b), where the capillary force ensures the particle to be at the center of the circle even during the heating. Thus by precisely controlling the position of the droplet on the gripper using carefully engineered anchor points, a very high manipulation precision is expected to be achievable, regardless of the size of the droplet used for transporting the particle.
4. Conclusions The results show that the manipulation of microobjects by using liquid droplets as transport vehicles is practically possible. The method is non-destructive. The droplets can be easily removed after manipulation while the positions of manipulated microobjects remain intact. We notice that the coalescence of particles shown in Fig. 6b can be exploited for fabricating a closely packed structure of well-defined numbers of microobjects which may be practically interesting for studying coupling effects at microscale. In addition, as electrowetting can be used to modulate the wetting property of surfaces, it is possible to realize the pick-up and release on the same substrate simply by modulating the voltage. Nevertheless, the saturation problem of electrowetting should be tackled before the conditions described in Eq. (11) can be met. References [1] K. Dholakia, P. Reece, M. Gu, Chem. Soc. Rev. 37 (2008) 42–55. [2] J. Cecil, D. Vasquez, D. Powell, Int. J. Prod. Res. 43 (2005) 819–828. [3] S. Fahlbusch, S. Mazerolle, J.M. Breguet, A. Steinecker, J. Agnus, R. Perez, J. Michler, J. Mater. Process. Technol. 167 (2005) 371–382. [4] T. Junno, K. Deppert, L. Montelius, L. Samuelson, Appl. Phys. Lett. 66 (1995) 3627–3629. [5] S.C. Hsieh, S. Meltzer, C.R.C. Wang, A.A.G. Requicha, M.E. Thompson, B.E. Koel, J. Phys. Chem. B 106 (2002) 231–234. [6] M.R. Falvo, G.J. Clary, R.M. Taylor, V. Chi, F.P. Brooks, S. Washburn, R. Superfine, Nature 389 (1997) 582–584. [7] D. Fotiadis, S. Scheuring, S.A. Muller, A. Engel, D.J. Muller, Micron 33 (2002) 385–397. [8] K.J. Obata, T. Motokado, S. Saito, K. Takahashi, J. Fluid Mech. 498 (2004) 113– 121. [9] P. Lambert, A. Delchambre, Assem. Autom. 25 (2005) 275–283. [10] S. Saito, T. Motokado, K.J. Obata, K. Takahashi, Appl. Phys. Lett. 87 (2005) 3. [11] P. Lambert, F. Seigneur, S. Koelemeijer, J. Jacot, J. Micromech. Microeng. 16 (2006) 1267–1276. [12] C. Quilliet, B. Berge, Curr. Opin. Colloid Interface Sci. 6 (2001) 34–39. [13] F. Mugele, J.C. Baret, J. Phys. Condens. Matter 17 (2005) R705–R774. [14] B. Bhushan, X. Ling, 2008, submitted for publication. [15] M. Nosonovsky, B. Bhushan, J. Adhes. Sci. Technol. (2008) in press. [16] C. Delaunay, J. Math. Pure Appl. 6 (1841) 309–320. [17] F.M. Orr, L.E. Scriven, A.P. Rivas, J. Fluid Mech. 67 (1975) 723–742. [18] M.A. Fortes, J. Colloid Interface Sci. 88 (1982) 338–352. [19] X. Pepin, D. Rossetti, S.M. Iveson, S.J.R. Simons, J. Colloid Interface Sci. 232 (2000) 289–297. [20] G.P. Lian, C. Thornton, M.J. Adams, J. Colloid Interface Sci. 161 (1993) 138–147. [21] C.D. Willett, M.J. Adams, S.A. Johnson, J.P.K. Seville, Langmuir 16 (2000) 9396– 9405. [22] J.C. Baret, M. Brinkmann, Phys. Rev. Lett. 96 (2006) 4. [23] A.M. Alencar, E. Wolfe, S.V. Buldyrev, Phys. Rev. E 74 (2006) 13. [24] M. Farshchi-Tabrizi, M. Kappl, Y.J. Cheng, J. Gutmann, H.J. Butt, Langmuir 22 (2006) 2171–2184. [25] S. Cai, B. Bhushan, Mater. Sci. Eng. R 61 (2008) 78–106.