AQUEOUS DROPLETS WITHOUT SURFACE LIQUIDITY

Transcription

1 AQUEOUS DROPLETS WITHOUT SURFACE LIQUIDITY Xiaoguang Li School of Physics Science and Engineering, Tongji University, Shanghai, China ABSTRACT Liquidity that refers to the free movement of molecules is the essential feature of liquid, which ensures the smooth curved profile of a droplet and its fluid-style behaviors. According to common sense and present research, droplets with and without being manipulated maintain their liquidities at all times. Here we find that this understanding can be reversed provided rubbing nano-sized hydrophobic particles onto the droplet surface. As a typical result, a droplet can be shaped like a solid retaining its deformation stably. The mechanical pressure and the hydrophobe transfer are complementary for the liquidity elimination, which is carried out by a superhydrophobic SiO 2 coating with weak binding force. The introduced particles are very few in number so that a deformed droplet looks the same as a pristine one in terms of the transparency and can be transferred by a hydrophilic surface via adhering. This work may open the opportunity for novel application of droplet system and for new experimental study of the interaction between water and hydrophobe. KEY WORDS: water drop, nano particles, superhydrophobic surface, surface liquidity INTRODUCTION The study of superhydrophobic surfaces that feature strong water repellence has prospered since the end of 1990s (1-14). Much of the work has concentrated on the surface preparation, but recently people have paid increased attention to the spherical droplets carried by such surfaces (15-24). These are considered as promising microfluidics in the applications of biomedicine and chemistry, as well as interesting objects of physics research. Studies of aqueous droplets on superhydrophobic surfaces generally focus on aspects such as the conformation of water molecules closest to the interface (25-26), the dynamics of a bouncing droplet (15-17), the profile lengthening by electric/magnetic fields (18-20), and a few emerging fields including droplet cutting (21-22), rebounding droplet-droplet collisions (23) and a droplet reactor (24). In these studies, droplets all behaved as comprehensible fluids in terms of the liquidity that is the nature of liquid. Actually, properties and behaviors of a droplet presented in any situation are related to the liquidity; for example, in contrast to solid case in which irregular profile with edges and corners are presented, that surface tension can actualize a smooth spherical shape of a droplet is just based on the free movement of liquid molecules the liquidity. However, if there is a way to eliminate the surface liquidity, the droplet must behave differently and release the potential of novel 4-182

2 applications. We have certified that this is possible if combining hydrophobe transfer and mechanical pressure (27), and in this paper we will give more details. For the liquidity elimination, a special superhydrophobic surface should be used, with which mechanical pressure is exerted on the droplet and at the same time the droplet surface is doped with very few hydrophobic particles. Consequently, after removing the mechanical pressure, the droplet presents some unusual characteristics that reflect the elimination of surface liquidity, mainly manifesting in three aspects: 1) the droplet is deformed and retains its deformation like a solid, instead of recovering the original spherical shape; 2) the deformed droplet does not quiver with environmental disturbances such as air blowing or pedestal shocking which would subject a normal droplet to quiver; 3) if spraying some detrital materials or powders onto the deformed droplet surface, these solid substances do not move, in contrast to the free flowing that occurs on the surface of a normal droplet. Among these characteristics, the stable deformation is fundamental and most representative, so our work focuses on this issue. RESULTS AND DISCUSSION Preparation of the superhydrophobic silica coating The superhydrophobic silica coating is derived from a hexamethyldisilazane (HMDS) treated silica sol, which was synthesized by the hydrolysis and condensation of tetraethyl orthosilicate (TEOS) in ethanol (EtOH) solvent. The mole ratio was TEOS: EtOH: NH 3 :H 2 O=1:38:0.54~1.08:1.53~3.06 (All the water was from the 28% ammonia solution, and the volume ratio when the ammonia solution was least used was TEOS: EtOH: Ammonia Solution=5: 50: 0.9). First, half of EtOH was mixed with TEOS and the other half was with ammonia and water and the two solutions were stirred for 10 minutes. Afterwards, the solution with NH 3 was added, dropwise, into the TEOS solution with stirring. Lastly, the mixed solution was aged at room temperature for 2-7 days to become a silica sol suitable for coating, then a certain quantity of HMDS (0.6 TEOS) was added into the sol with stirring for 1 hour followed by 1 day standing, and then it was ready for use. Glass slide ( mm) and tube (diameter about 1 mm) were coated with this silica sol by a dip-coating method. In brief, the glasses were immersed in the sol for a short time (about 5 seconds), and then were lifted at a uniform speed of ~1.4 mm/s. It was not necessary to heat the coating, and just two minutes drying in air was enough to obtain the superhydrophobic surface. Features of the prepared superhydrophobic coating This coating is superhydrophobic with a mass of micro fluctuation (Fig. 1a) and thereby generates the Wenzel-model contact that features strong water adhesion (Fig. 1c). The surface consisted of CH 3 groups that were from HMDS, unhydrolyzed OC 2 H 5 groups that were from TEOS, and unmodified OH groups on the silica 4-183

3 particles (Fig. 1b). In particular, the hydrophobic particles on top (depth of dozens of nm) of the coating bond extremely weakly with the interior and so can be removed by contacting with water. This can be reflected, as shown in Fig. 1d, by the difference in the optical appearance as a result of local reduction of the coating thickness. In this work, glass slides and tubes were employed as substrates to form the coating, and these coated glasses together with a natural superhydrophobic material - lotus leaf - were employed to be pedestals or operating tools for the droplet manipulation. Figure 1. (a) The atomic force microscopy image of the superhydrophobic coating. (b) FTIR spectrum of the coating. (c) Water drops on (left) and hanging on (right) this kind of coating surfaces. (d) A coated glass impacted by falling water drops, with the impacted areas (denoted by the blue arrows) presenting different transmittance (left); the schematic of the particle-piled coating (right) and its surface composition: Si-CH 3, Si-OC 2 H 5, and Si-OH. Note H atoms are not shown for simplicity. The deformation of water droplets The process of deformation is very simple: the tool is operated by hand to squeeze and rub the droplet, transferring the hydrophobic particles, and then the deformation is realized. The tools in this work are a coated glass slide (CGS) and a coated capillary glass tube (CGT), and the pedestals are lotus leaf and CGS. It is easy to use the tool of CGS to deform a droplet especially when the pedestal is also a CGS. When the pedestal is a lotus leaf, relatively hard squeezing and rubbing that last several seconds are needed; besides, the shape of the deformed droplet is also affected by the leaf surface topography. (Fig. 2a). In contrast, the tool of CGT is not that handy, with the droplet always rolling away or recovering the spherical shape after removing the tube; in this case repeated squeezing and rubbing are needed, and the first stable deformation is always a quasi ellipsoid (Fig. 2b). Not just pure water, aqueous solution and suspension can also be deformed by the method introduced here

4 Figure 2. (a) Deforming a water drop (~100 l) on a lotus leaf using a CGS tool. (b) Deforming a water drop (~5 l) on the CGS pedestal using a CGT tool. Typical deformed droplets on CGS pedestals (c-d) and lotus leaf (e). Scale bars: a, 1 cm; b-e, 1 mm. Resistance from the droplet before deformation was easy to feel when squeezing it due to the mutual repulsion between water and hydrophobic surface; however, further transformation of a deformed droplet that has lost surface liquidity became much easier. Once deformed, the resistance was felt to decrease significantly, and meanwhile the viscosity of the droplet was increased, thereby the operating tool was allowed to exert force on the droplet flexibly, similar to shaping soft solid. Such manipulation of droplets brought about various deformations, and some typical ones are displayed in Fig. 2c-e. As shown, for any of these samples the curvatures at different parts are not even, which departs from the usual surface tension shape of a liquid drop. For a liquid drop the ability of surface tension γ to smooth and minimize the surface area, and the ability of Laplace pressure ΔP (ΔP = 2γ / r l, where r l is the local radius) to equalize over the whole surface (28-29), are based on the liquidity. Here, the surface liquidity is eliminated, the exact mechanism of which is complex but revolves around the interactions between solid particles, particles and water molecules, and water molecules. The deformation reveals that the interaction between water molecules varies after deformation and this variation is not homogeneous over the whole droplet surface. Thus, the traditional concept of surface tension that is determined by the homogeneous interaction of molecules at liquid surface is not appropriate here, and neither is that of Laplace pressure. The silica particles that were supposed to be transferred were not optically observed on the deformed droplets due to the very low quantity; however, they were detected in the residues after drying the droplets by SEM and EDS characterizations as discussed below, which certified their transferring. For test, a deformed water drop was settled on a clean copper sheet surface and was then dried at 150. Afterwards, the copper carrying residues left by the droplet was characterized with scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). The SEM image shows that the residues consist of 4-185

5 particles of ~25 nm (Fig. 3a) which are in accordance with those in the sol-gel silica coating. The EDS result (Fig. 3b) displaying the existence of high ratios of Si, O, and C further confirms these particles being modified silica. (The copper sheet itself and the one that carried a pure water drop after drying were also tested for comparison. The elements of Si, O, and C were also found but their ratios were very small, which were attributed to the minuscule contaminants and the reactions of Cu with environmental substances such as O 2, H 2 O and CO 2. The atom ratios of elements detected in the EDS experiment were compared in Table 1; as shown, the ratio of Si in the deformed droplet sample is as 10 times more that in the other two samples. ) Figure 3. (a) SEM image of residues of a dried deformed water drop settled on a copper sheet surface. (b) EDS spectrum of the particles area displayed in (a). Table 1. The atom ratios of O, C, Si, and Cu on the surfaces of three different samples tested by EDS. Element The copper substrate The pure water-covered The deformed water-covered copper substrate copper substrate O (orm. at%) C (orm. at%) Si (orm. at%) Cu (orm. at%) When both the operating tool and the pedestal were lotus leaves, no deformation was realized; this is because the naturally produced superhydrophobic material of the lotus leaf is quite tough, so it s not feasible to transfer the hydrophobic substances during such manipulation. The lotus leaf case implies the necessity of hydrophobe transfer for droplet deformation. Consequently, it can be stated that transferrable hydrophobe and mechanical pressure are complementary to realize the deformation. Moreover, comparing the difficulty of different manipulations reveals that the greater the contact area between the droplet and the transferrable hydrophobe, the easier it is to deform the droplet

6 Estimation of introduced hydrophobe amount The superhydrophobic sol-gel coating used in this work has particles on top interacting very weakly with the interior and thereby can be removed by contacting with water. To make certain the depth of the removable particles, part of the coated glass was soaked in water and was then lifted, resulting in different transmittance of this soaked area compared with the unsoaked one (Fig. 4a). The difference in transmittance was attributed to the reduction of coating thickness and the increase of refractive index which could be obtained by modeling the transmittance spectrum using FilmWizard32 software. The results showed that the original refractive index and thickness of the coating were about 1.32 and 250 nm respectively, and that those of the soaked area were about 1.38 and 220 nm respectively. The increase of refractive index might result from water invasion to the pores of the coating, and the ~30 nm reduction of coating thickness was assigned to the depth of the removable silica particles. Figure 4. (a) Transmittance spectra of two coating areas with different coating thicknesses. The different colors of the two areas (soaked and as-coated) in the inset are related to the shooting angle. (b) A deformed droplet hanging in the air by adhering to a military glass tube without coating. Given that the contact area between droplet and the superhydrophobic surface was about 10 mm 2 for a typical droplet of 10-2 g (10~100 l), and that the density of the coating was about 1.5 g/cm 3, the mass of the removable particles could be calculated as 30 nm 10 mm g/cm 3 = g. In other words, particles transferred to the droplet during manipulation were ~10-7 g. Thus the solid particles make up only 10-3 % of the drop mass. The ratio indicates that water molecules constitute the main body of the deformed droplet, which accordingly maintains the ability of wetting as revealed in Fig. 4b. This is distinguished from another droplet/hydrophobe system liquid marble, a product of encapsulating a liquid entirely in a hydrophobic powder, which features non-wetting 4-187

7 owing to the complete coverage of hydrophobic particles (30-33). In addition, because the ratio of the solid particles is minuscule, the deformed droplet is almost as transparent as pure water. CONCLUSIONS In conclusion, a methodology of rubbing hydrophobic nano particles onto the droplet surface was designed, by which the surface liquidity of aqueous droplet was eliminated. The droplet can be deformed stably and variously, and maintains the wetting ability due to water being the dominant subject. At present, our understanding concerning the elimination of surface liquidity represents a macro description; a molecule-level analysis regarding various interactions in the deformed droplet remains to be made. REFERENCES 1. Barthlott, W.; Neinhuis, C. Purity of the sacred lotus, or escape from contamination in biological surfaces. Planta 1997, 202, Cha, T. G.; Yi, J. W.; Moon, M. W.; Lee, K. R.; Kim, H. Y. Nanoscale Patterning of Microtextured Surfaces to Control Superhydrophobic Robustness. Langmuir. 2010, 26, Hipp, B.; Kunert, I.; Durr, M. Systematic Control of Hydrophobic and Superhydrophobic Properties Using Double-Rough Structures Based on Mixtures of Metal Oxide Nanoparticles. Langmuir. 2010, 26, Wang, S.; Feng, L.; Jiang, L. One-step solution-immersion process for the fabrication of stable bionic superhydrophobic surfaces. Adv. Mater. 2006, 18, Xie, Q.; Xu, J.; Feng, L.; Jiang, L.; Tang, W.; Luo, X.; Han, C. Facile creation of a super-amphiphobic coating surface with bionic microstructure. Adv. Mater. 2004, 16, Zhai, L.; Cebeci, F. C.; Cohen, R. E.; Rubner, M. F. Stable superhydrophobic coatings from polyelectrolyte multilayers. Nano Lett. 2004, 4, Zhang, L.; Chen, H.; Sun, J.; Shen, J. Layer-by-layer deposition of poly(diallyldimethylammonium chloride) and sodium silicate multilayers on silica-sphere-coated substrate-facile method to prepare a superhydrophobic surface. Chem. Mater. 2007, 19, Mumm, F.; Helvoort, A.; Sikorski, P. Easy Route to Superhydrophobic Copper-Based Wire-Guided Droplet Microfluidic Systems. ACS Nano. 2009, 3, Tian, Y.; Liu, H.; Deng, Z. Electrochemical growth of gold pyramidal nanostructures: Toward super-amphiphobic surfaces. Chem. Mater. 2006, 18, Shieh, J.; Hou, F.; Chen, Y.; Chen, H.; Yang, S.; Cheng, C.; Chen, H. Robust Airlike Superhydrophobic Surfaces. Adv. Mater. 2009, 22,

8 11. Han, Z.; Tay, B.; Tan, C.; Shakerzadeh, M.; Ostrikov, K. Electrowetting control of Cassie-to-Wenzel transitions in superhydrophobic carbon nanotube-based nanocomposites. ACS Nano 2009, 3, Li, X. G.; Shen, J. A facile two-step dipping process based on two silica systems for a superhydrophobic surface. Chem. Commun. 2011, 47, Jin, M. H.; Feng, X. J.; Feng, L.; Sun, T. L.; Zhai, J.; Li, T. J.; Jiang, L. Superhydrophobic aligned polystyrene nanotube films with high adhesive force. Adv. Mater. 2005, 17, Winkleman, A.; Gotesman, G.; Yoffe, A.; Naaman, R. Immobilizing a drop of water: Fabricating highly hydrophobic surfaces that pin water droplets. Nano Lett. 2008, 8, Richard, D. & Quéré, D. Bouncing water drops. Europhys. Lett. 2000, 50, Jung, Y. C.; Bhushan, B. Dynamic effects of bouncing water droplets on superhydrophobic surfaces. Langmuir 2008, 24, Malouin, B.; Jr.; Koratkar, N.; Hirsa, A.; Wang, Z. Directed rebounding of droplets by microscale surface roughness gradients. Appl. Phys. Lett. 2010, 96, Stone, H. A.; Lister, J. R.; Brenner, M. P. Drops with conical ends in electric and magnetic fields. Proc. R. Soc. Lond. A. 1999, 455, Bateni, A.; Susnar, S.; Amirfazli, A.; Neumann, A. Development of a new methodology to study drop shape and surface tension in electric fields. Langmuir 2004, 20, Bormashenko, E.; Pogreb, R.; Stein, T.; Whyman, G.; Hakham-Itzhaq, M. Electrostatically driven droplets deposited on superhydrophobic surfaces. Appl. Phys. Lett. 2009, 95, Bormashenko, E.; Bormashenko, Y. Non-stick droplet surgery with a superhydrophobic scalpel. Langmuir. 2011, 27, Yanashima, R.; Garcia, A.; Aldridge, J.; Weiss, N.; Hayes, M.; Andrews, J. Cutting a drop of water pinned by wire loops using a superhydrophobic surface and knife. Plos One. 2012, 7, Mertaniemi, H.; Forchheimer, R.; Ikkala, O.; Ras, R. Rebounding Droplet-Droplet Collisions on Superhydrophobic Surfaces: from the Phenomenon to Droplet Logic. Adv. Mater. 2012, 24, Su, B.; Wang, S.; Song, Y.; Jiang, L. A miniature droplet reactor built on nanoparticles-derived superhydrophobic pedestals. Nano Res. 2011, 4, Abraham, F. F. The interfacial density profile of a Lennard Jones fluid in contact with a (100) Lennard Jones wall and its relationship to idealized fluid/wall systems: A Monte Carlo simulation. J. Chem. Phys. 1978, 68, Matta, C. F.; Hernandez-Trujillo, J.; Tang, T. H.; Bader, R. F. W. Hydrogen hydrogen bonding: A stabilizing interaction in molecules and crystals. Chem. Eur. J. 2003, 9, Li, X. G.; Shen, J. Deforming water droplets with a superhydrophobic silica coating. Chem. Commun. 2013, 49, Chaudhury, M. K.; Whitesides, G. M. How to make water run uphill. Science. 1992, 256,

9 29. Zheng, Y.; Bai, H.;Huang, Z.; Tian, X.; Nie, F.; Zhao, Y.; Zhai, J.; Jiang, L. Directional water collection on wetted spider silk. Nature. 2010, 463, Aussillous, P.; Quéré, D. Liquid marbles. Nature. 2001, 411, Dandan, M.; Erbil, H. Evaporation Rate of Graphite Liquid Marbles: Comparison with Water Droplets. Langmuir. 2009, 25, Bormashenko, E. Liquid marbles: properties and applications. Curr. Opin. Colloid. In. 2011, 16, McHale, G.; Newton, M. I. Liquid marbles: principles and applications. Soft Matter. 2011, 7,