fabrication-visualization-capillary-bridges-slit-pore

Fabrication and Visualization of Capillary Bridges in Slit Pore Geometry

A procedure for creating and imaging capillary bridges in slit-pore geometry is presented. High aspect ratio hydrophobic pillars are fabricated and ...

Johns Hopkins University

Using the electrochemical surface forces apparatus, we investigated adhesion (from pull-off measurements) between gold and mica as the potential of the gold surface was changed externally. Measurements were performed at different concentrations of KClO(4) in a potential window where the gold electrode is ideally polarizable. At applied potentials where the gold-mica interactions are repulsive, we obtain double layer forces that are predictable by the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of colloid stability but deviate from the theory at short range. At applied potentials where the gold-mica interactions are attractive, we observed a very strong dependence of adhesion on the applied potential, a result that cannot be directly related to DLVO theory. We show, however, that an approach based on electrocapillary thermodynamics can be employed to model the potential dependence of adhesion seen in our measurements. This electrocapillary approach presents evidence of charging at the gold-mica interface and stresses the relation between the charge within and outside of the contact area.

Electrocapillary at contact: potential-dependent adhesion between a gold electrode and a mica surface.

The thermal stability of perfluoralkylsiloxane monolayers in a vacuum is investigated via X-ray photoelectron spectroscopy (XPS) for temperatures up to 600 degrees C. 1H,1H,2H,2H,-perfluorodecyltrichlorosilane (FDTS) monolayers are deposited on oxidized Si(100) surfaces from the vapor phase with various degrees of surface coverage. Significant monolayer desorption is observed to occur at temperatures below 300 degrees C regardless of the initial monolayer coverage. The desorption mechanism follows first-order kinetics and is independent of the initial coverage. Removal of FDTS is found to occur by the loss of the entire molecular chain, as evidenced by the fact that the CF(3)/CF(2) peak area ratios remain unaffected by the annealing process although CF(n)()/Si peak ratio declines with annealing. This is in sharp contrast to the behavior observed for octadecyltrichlorosilane monolayer for which elevated temperature leads to C-C bond breakage and successive shortening of the alkyl chain. It is also shown that the binding energy and the shape of the F 1s line are good indicators of the degree of disorder in the chain, as well as a measure of the interaction of the chain with the silicon surface.

Thermal behavior of perfluoroalkylsiloxane monolayers on the oxidized Si(100) surface.

The orientation and extent of adsorption of pyridine on a gold electrode is known to depend on applied potential and is well characterized. By use of the electrochemical surface forces apparatus, we measured the potential dependence of the double-layer interactions and adhesive forces between a gold electrode and a mica surface for different pyridine concentrations. We observed that, unlike mica-mica interactions, the gold-mica interactions were strongly affected by the presence of small concentrations of pyridine. We are able to reach high negative surface potentials (as determined by applying Derjaguin-Landau-Verway-Overbeek theory to our force measurements), which is similar to what is observed in the absence of pyridine. This demonstrates the electronic nature of the forces measured and shows that pyridine does not displace potential-determining ions on the surface. At positive potentials, where the interaction between gold and mica is attractive, pull-off measurements are a strong function of applied potential. The major effect of the presence of pyridine is on the observed shift in the potential of zero force (PZF), moving it to more negative potentials. This effect is caused by the strong dipole of the pyridine molecule. When the applied potential is cast as a deviation from the PZF, the effect of pyridine is to reduce adhesion between gold and mica. We modeled the potential-dependent adhesion of this system using an electrocapillary framework developed previously, and in doing so, we establish the relationship between the gold-liquid and gold-mica surface energies. In addition, we show that pyridine adsorption affects the capacitance of the gold-mica interface.

Control of adhesion and surface forces via potential-dependent adsorption of pyridine.

Silicon carbide nanowires are being actively pursued as components for nanoelectromechanical sensors, nanocatalytic elements, and nano-optical circuits able to operate in harsh environment, high temperature, and high power applications. The effect of geometric confinement and polarization anisotropy in confocal Raman spectroscopy has been employed to detect axial and radial composition information in individual nanowires. Polarization anisotropy causes a significant increase in signal from the surface layer (relative to bulk), and combined with the increased surface-to-volume ratio at the nanoscale, it allows for the direct characterization of bulk and surface defects.

Resolving radial composition gradients in polarized confocal raman spectra of individual 3C-SiC nanowires.

We demonstrate that noncovalent ion-pair interactions in solution can be employed to control the molecular spacing of thiols in a self-assembled monolayer (SAM) on gold. Ion-pairs formed between the carboxylate tail-group of 16-mercaptohexadecanoic acid (MHA) and tetraalkylammonium (TAA+) hydroxide salts of various alkyl side-chain lengths remain intact during chemisorption of the thiol on gold. The resulting ion-pair SAMs exhibit a 1:1 molar ratio of MHA:TAA+ on the surface and are covalently bound to the gold surface through the thiol headgroup of MHA. We hypothesize that the incorporation of the bulky TAA+ group competes with the strong tendency of the thiols to organize into an ordered monolayer, which highlights the strength of the ion-pair complexes. The ion-pair films can be converted into a loosely packed MHA monolayer by rinsing the SAM with a solution of potassium perchlorate, which releases the TAA+ from the surface. Contact angle measurements and X-ray spectroscopy (XPS) confirm the stoichiometry and covalent attachment of the monolayers. XPS analysis and contact angle measurements indicate that the surface density of bound MHA decreases with increasing size of the TAA+ cation. These results suggest that steric hindrance created by the bulky side-chains of the TAA+ cation dictates the lateral spacing of MHA chains on the surface.

Supramolecular ion-pair interactions to control monolayer assembly.

We performed macroscopic experiments on the motion of a sphere through an array of obstacles that highlight the deterministic nature of the lateral displacements that lead to particle separation in microfluidic systems. The motion of the spheres is irreversible and displays directional locking. The locking directions can be predicted with a single parameter that distinguishes between reversible and irreversible particle-obstacle collisions. These results stress the need to incorporate irreversible interactions to predict the movement of a non-Brownian sphere passing through a periodic array.

Directional locking and the role of irreversible interactions in deterministic hydrodynamics separations in microfluidic devices.

Capillary condensation is employed to probe the solid-liquid interfacial energy in electrowetting on dielectric. The height of an annular water meniscus formed via capillary condensation inside the surface force apparatus is measured as a function of the potential applied across the meniscus and the dielectric stack where the meniscus is formed. According to the Kelvin equation, a decrease in the solid-liquid interfacial energy at constant temperature and relative humidity should lead to an increase in the meniscus height. Our experimental results on nanometer-sized meniscus are in agreement with the work of Mugele [J. Phys.: Condens. Matter 2007, 19, 375112] and unequivocally demonstrate that the real contact angle (or the solid-liquid interfacial energy) remains unaltered in electrowetting on dielectric.

Invariance of the solid-liquid interfacial energy in electrowetting probed via capillary condensation.

Pinning of the triple contact line adversely affects electrowetting on dielectric. Electrowetting response of substrates with contact angle hysteresis ranging from 1° to 30° has been characterized, and the results are interpreted within the framework of electromechanics corrected for pinning. The relationship between contact angle hysteresis, threshold potential for liquid actuation, and electrowetting hysteresis is quantified. Our results demonstrate that a modified electrowetting equation, based on balance of forces (including the pinning forces) acting on the triple contact line and on the drop, describes the electrowetting response of substrates with significant contact angle hysteresis. Finally, the surface properties of PDMS Sylgard 184 were found to be influenced by the electric field.

Impact of pinning of the triple contact line on electrowetting performance.

The shape and motion of drops on surfaces is governed by the balance between the driving and the pinning forces. Here we demonstrate control over the motion of droplets on an inclined surface by exerting control over the contact angle hysteresis. The external modulation of contact angle hysteresis is achieved through a voltage-induced local molecular reorganization within the surface film at the solid-liquid interface. We show that tuning contact angle hysteresis alone is sufficient to direct and deform drops when subjected to a constant external driving force, here gravity, in the absence of a pre-defined surface energy gradient or pattern. We also show that the observed stretching and contraction of the drops mimic the motion of an inchworm. Such reversible manipulation of the pinning forces could be an attractive means to direct drops, especially with the dominance of surface forces at micro-/nanoscale.

Modulating contact angle hysteresis to direct fluid droplets along a homogenous surface.

We investigate the separation of drops in force-driven deterministic lateral displacement (f-DLD), a promising high-throughput continuous separation method in microfluidics. We perform scaled-up macroscopic experiments in which drops settle through a square array of cylindrical obstacles. These experiments demonstrate the separation capabilities-and provide insight for the design-of f-DLD for drops of multiple sizes, including drops that are larger than the gaps between cylinders and exhibit substantial deformation as they move through the array. We show that for any orientation of the driving force relative to the array of obstacles, the trajectories of the drops follow selected locking directions in the lattice. We also found that a simple collision model accurately describes the average migration angles of the drops for the entire range of sizes investigated here, and for all forcing directions. In addition, we found a difference of approximately 20° between the critical angles at which the smallest and largest drops first move across a line of obstacles (column) in the array, a promising result in terms of potential size resolution of this method. Finally, we demonstrate that a single line of cylindrical obstacles rotated with respect to the driving force is capable of performing binary separations. The critical angles obtained in such single line experiments, moreover, agree with those obtained using the full array, thus validating the assumption in which the trajectory (and average migration angle) of the drops is calculated from individual obstacle-drop collisions.

Force driven separation of drops by deterministic lateral displacement.

Central to the adhesion and locomotion of tree frogs are their structured toe pads, which consist of an array of 10 μm hexagonal epithelial cells separated by interconnected channels that are 1 μm wide and 10 μm deep. It has been proposed that the channels facilitate the drainage of excess fluid trapped between the toe pads and the contacting surface, and thus reduce the hydrodynamic repulsion during approach. We performed direct force measurement of the normal hydrodynamic interactions during the drainage of fluid from the gap between a structured and a smooth surface using surface force apparatus. The structured surface consisted of a hexagonal array of cylindrical posts to represent the network of interconnected channels. The measured hydrodynamic drainage forces agree with the predictions from Reynolds' theory for smooth surfaces at large separations. Deviations from theory, characterized by a reduction in the hydrodynamic repulsion, are observed below some critical separation (h(c)), which is independent of drive velocity. We employ a scaling analysis to establish the relationship between structural features (channel depth, width, and post diameter) and the critical separation for the onset of deviations. We find agreement between our experiments and the scaling analysis, which allows us to estimate a characteristic length scale that corresponds to the transition from the fluid being radially squeezed out of the nominal contact area to being squeezed out through the network of interconnected channels.

Measurement and scaling of hydrodynamic interactions in the presence of draining channels.

We investigate the morphological evolution of nonaxisymmetric capillary bridges in slit-pore geometry as the height of the pore and aspect ratio of the bridge are varied. The liquid bridges are formed between two hydrophobic surfaces patterned with hydrophilic strips. The aspect ratio of the capillary bridges (length/width) is varied from 2.5 to 120 by changing the separation between the surfaces, the width of the strips, or the fluid volume. As the bridge height is increased, the aspect ratio decreases and we observe a large increase in the mean curvature of the bridge. More specifically, the following counterintuitive result is observed: the mean curvature of the bridges changes sign and goes from negative (concave bridge) to positive (convex bridge) when the height is increased at constant volume. These experimental observations are in quantitative agreement with Surface Evolver simulations. Scaling shows a collapse of the data indicating that this transition in the sign of the Laplace pressure is universal for capillary bridges with high aspect ratios. Finally, we show that the morphology diagrams obtained from our 3D analysis are considerably different from those expected from a 2D analysis.

From concave to convex: capillary bridges in slit pore geometry.

We studied the multiple beam interferometry of surfaces with well-defined microscale features in the surface force apparatus (SFA). The structures investigated consist of hexagonal arrays of microscale cylindrical posts made out of the photoresist SU-8. The ability of the SFA to visualize the profile and topography of the interacting surfaces leads to the observation of discontinuities in primary fringes of equal chromatic order that are caused by the microscale structural features. The shift in wavelengths has been analyzed to extract the post height and compared to independent profilometry measurements. The analysis based on the shift in wavelength is shown to be viable only when the order of the fringe and the position of the discontinuity is precisely known and within the field of view. Analysis of the full profile of the interacting surface for two orthogonal 2-dimensional slices can be used to determine how the surface lattice is oriented within the point of closest approach. Finally, we discuss cases in which the structural features detrimentally affect the spatial resolution of the SFA.

Interferometry of surfaces with well-defined topography in the surface force apparatus.

We consider the shape evolution of non-axisymmetric capillary bridges in slit pore geometry as the pore height is increased at constant volume. Experiments and finite element simulations using Surface Evolver have shown that as the height of the pore is increased the mean curvature of the bridge, and hence Laplace pressure, changes its sign from negative to positive. Here we propose an intuitive explanation of this surprising phenomenon. We suggest that it is the balance between the confinement and the wetting properties of the supporting strips that causes the change in sign of the Laplace pressure. The theory proposed relies on three simple approximations, which are tested individually, and is in good agreement with experiments and simulations in the regime where the curvature transition from negative to positive takes place. Theoretical arguments take into account only the wetting properties and geometry of the system (the width and height of the pore). Along with the formula for the curvature, we derive also a relation for the pinning angle of the capillary bridge, which is also verified experimentally.

Curvature of Capillary Bridges as a Competition between Wetting and Confinement.

The ability to separate and analyze chemical species with high resolution, sensitivity, and throughput is central to the development of microfluidics systems. Deterministic lateral displacement (DLD) is a continuous separation method based on the transport of species through an array of obstacles. In the case of force-driven DLD (f-DLD), size-based separation can be modelled effectively using a simple particle-obstacle collision model. We use a macroscopic model to study f-DLD and demonstrate, via a simple scaling, that the method is indeed predominantly a size-based phenomenon at low Reynolds numbers. More importantly, we demonstrate that inertia effects provide the additional capability to separate same size particles but of different densities and could enhance separation at high throughput conditions. We also show that a direct conversion of macroscopic results to microfluidic settings is possible with a simple scaling based on the size of the obstacles that results in a universal curve.

Article	Year
Electrocapillary at contact: potential-dependent adhesion between a gold electrode and a mica surface. Langmuir : the ACS journal of surfaces and colloids\| PubMed ID: 15667179	2005
Thermal behavior of perfluoroalkylsiloxane monolayers on the oxidized Si(100) surface. Langmuir : the ACS journal of surfaces and colloids\| PubMed ID: 16519475	2006
Control of adhesion and surface forces via potential-dependent adsorption of pyridine. The journal of physical chemistry. B\| PubMed ID: 16851457	2005
Resolving radial composition gradients in polarized confocal raman spectra of individual 3C-SiC nanowires. Journal of the American Chemical Society\| PubMed ID: 17105265	2006
Supramolecular ion-pair interactions to control monolayer assembly. Langmuir : the ACS journal of surfaces and colloids\| PubMed ID: 19199724	2009
Directional locking and the role of irreversible interactions in deterministic hydrodynamics separations in microfluidic devices. Physical review letters\| PubMed ID: 19792691	2009
Invariance of the solid-liquid interfacial energy in electrowetting probed via capillary condensation. Langmuir : the ACS journal of surfaces and colloids\| PubMed ID: 20552998	2010
Impact of pinning of the triple contact line on electrowetting performance. Langmuir : the ACS journal of surfaces and colloids\| PubMed ID: 22050215	2011
Modulating contact angle hysteresis to direct fluid droplets along a homogenous surface. ACS applied materials & interfaces\| PubMed ID: 22220753	2012
Force driven separation of drops by deterministic lateral displacement. Lab on a chip\| PubMed ID: 22699242	2012
Measurement and scaling of hydrodynamic interactions in the presence of draining channels. Langmuir : the ACS journal of surfaces and colloids\| PubMed ID: 23009050	2012
From concave to convex: capillary bridges in slit pore geometry. Langmuir : the ACS journal of surfaces and colloids\| PubMed ID: 23061424	2012
Interferometry of surfaces with well-defined topography in the surface force apparatus. Journal of colloid and interface science\| PubMed ID: 24144377	2013
Curvature of Capillary Bridges as a Competition between Wetting and Confinement. Langmuir : the ACS journal of surfaces and colloids\| PubMed ID: 24274673	2013
Inertia and scaling in deterministic lateral displacement. Biomicrofluidics\| PubMed ID: 24396545	2013

Joelle Frechette

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