Session X50: Focus Session: Micro and Nano Fluidics II: Structured or Active Surfaces and Electrotransport

Dynamics of self-oscillating cilia designed from active polymer gels

Using theory and simulations, we design active synthetic surfaces which are capable of replicating functionalities of biological cilia. In order to design such exquisite biomimetic systems we harness unique properties of polymer gels that undergo photosensitive Belousov-Zhabotinsky (BZ) reaction. Powered by internalized BZ reaction these polymer gels swell and de-swell autonomously by chemo-mechanical transduction and therefore are ideal materials for designing our system. In order to simulate the dynamics of the BZ cilia in surrounding fluid we have developed a nonlinear hybrid 3D model which captures elasto-dynamics of polymer gel and diffusive exchange of BZ reagents between the gel and the fluid. Here we show that the geometrical arrangement of cilia and the distribution of BZ activator in the fluid determine the dynamic response of the cilia. We further show that using light as an external stimulus we can sequentially modulate height of individual cilium and thereby create the ``piano effect''. Finally, we demonstrate that synchronized oscillations in the cilia result from the distribution of BZ-activator in the surrounding fluid. Our findings can be used to design active surfaces which can be remotely tuned depending upon the magnitude of external stimuli.   

Using actuated synthetic cilia to enhance microscale heat transport

We used three dimensional computer simulations to examine heat transport in a microchannel that encompasses a periodic array of actuated synthetic cilia. The channel was filled with a viscous fluid and its walls were maintained at different temperatures. Elastic synthetic cilia were attached to the bottom channel wall and were actuated by a periodic external force applied horizontally to their free ends. To model this multi-component system, we employed a thermal lattice Boltzmann model coupled with the lattice spring model. We probed how the beating cilia affect the heat transfer between channel walls, and how the thermal transport coefficient changes depending on the oscillating frequency and the relative distance between actuated filaments. Our findings could be useful for developing new methods for temperature control in microscale devices.   

Transport of Micro-particles by Active Cilia Arrays

Biological organisms are known to use hair-like filaments called cilia to manipulate and transport particles. The coordinated motion of cilia is known to be effective at propelling surrounding fluid. In this work, we show that adhesive interaction between the actuated cilia and particulates can be crucial towards controlling particle transport. We model transport of a microscopic particle via a regular array of beating elastic cilium, whose tips experience an adhesive interaction with the particle's surface. At optimal adhesion strength, the average particle velocity is maximized. Using simulations spanning a range of cilia stiffness, particle radius, and cilia-particle adhesion strength, we explore the parameter space over which the particle can be ``released,'' ``propelled'' or ``trapped'' by the cilia. We use a low-order model to predict parameters for which the cilia are able to attach themselves to the particle. We also study the effect of varying the particle size and stiffness on its transport properties. This is the first study that shows how both stiffness and adhesion strength are crucial for manipulation of particles by active cilia arrays.   

Fluid flows around nanoelectromechanical resonators

To explore properties of fluids on a nanosize scale, we fabricated by a standard top down technique a series of nanoelectromechanical resonators (cantilevers and bridges) with widths w and thicknesses t from 100 to 500 nm; lengths l from 0.5 to 12 micron; and resonant frequencies f from 10 to 400 MHz. For the sake of purity of the experiment, the undercut in the widest (w=500 nm) devices was eliminated using the focused ion beam. To model the fluidic environment the devices were placed in the atmosphere of compressed gases (He, N$_{2}$, CO$_{2}$, Ar, H$_{2}$) at pressures from vacuum up to 20 MPa, and in liquid CO$_{2}$; their properties were studied by the real time stroboscopic optical interferometry. Thus, we fully explored the Newtonian and non-Newtonian flow damping models. Observing free molecular flow extending above atmospheric pressure, we find the fluid relaxation time model to be the best approximation throughout, but not beyond, the non-Newtonian regime, and both, vibrating spheres model and the model based on Knudsen number, to be valid in the viscous limit.   

Dynamic Similarity Principle for Nanoscale Resonant Devices in Gaseous Environments

The mechanical performance of cantilevers on the nanoscale operating in atmosphere is dominated by gas damping. However, theoretical modeling of gas-solid interactions on the nanoscale is non-trivial due to the non-continuum nature of the gas flow. In addition, these gas-structure interactions can significantly affect the sensitivity of these devices. Instead of using numerical simulations to determine the gas flow and consequently, gas damping, of a nanoscale device, we used a general dynamic similarity principle to determine the gas damping of a nanoscale device by measuring the gas damping of a scaled up prototype device.   

Computational modeling of traveling wave electrophoresis

Traveling wave electrophoresis (TWE) is a microfluidic separation technique in which electrodes flanking a microchannel apply a traveling potential wave along the channel. Charged particles, including small molecules, proteins, and nanoparticles are differentially transported along the channel at a rate dependent on their mobility. TWE is ideally suited for application in lab-on-a-chip and field deployed sensor systems. In order to fully exploit this technology, a series of computational models have been developed, including 1-dimensional and 2-dimensional models. These models allow for testable predictions of single-particle motion, and the effects of factors such as Ph and concentration upon separation efficiency. Efforts to include diffusive components within the model, and to consider the motion of bands, rather than single particles will be discussed.   

Activated wetting dynamics in the presence of mesoscopic surface disorder

Although disorder is commonly used to explain contact angle hysteresis, it is often neglected when considering wetting dynamics. When viscous forces are negligible, contact-line velocity is modelled by the Molecular Kinetic Theory [1], which predicts an activated motion driven by molecular jumps on preferential adsorption sites. We believe that in the presence of mesoscopic disorder, this model can be reinterpreted and that the activation length is no longer molecular-sized but is related to depinning events on the surface. This hypothesis is supported by a study of the wetting of cesium by liquid hydrogen in which it was shown that the activation length is of the order of the expected roughness [2]. However, no systematic study between the activation area and the length scale of the disorder has previously been made. We study wetting dynamics on metal films evaporated under different conditions, allowing us to obtain films with lateral grain sizes ranging from 10 to 200 nm. We find that the activation area deduced from wetting experiments is coherent with these sizes; however, its precise relation to the scale of disorder is not clear.\newline [1] T.D. Blake and J.M. Haynes, J. Colloid Interface Sci. 30, 421 (1969)\newline [2] E. Rolley and C. Guthmann, PRL 98, 166105 (2007)   

Experimental test of Schrodinger's first-passage-time theory using colloids in micro-channels

We report an experimental study of the first-passage-time problem of driven diffusion in micro-channels. Fluorescent microspheres of 190nm diameter are confined in channels of 1.0 micron in width and 1.0 micron depth and driven by an applied longitudinal electric field. The images are acquired by a fluorescent microscope. The time dependence of the particle positions is tracked using particle tracking algorithms. The first passage times at different electric field values are extracted from the real-time data and compared with the exact solution given by Schrodinger for the 1D biased diffusion equation with one absorbing boundary condition.   

Liquid-impregnated surfaces: overcoming the limitations of superhydrophobic surfaces for robust non-wetting and anti-icing surface

In this work we address fundamental limitations of superhydrophobic surfaces for non-wetting and anti-icing applications by impregnating them with a hydrophobic liquid. The impregnating liquid serves as a barrier to the penetration of impinging water droplets and forces preferential condensation and frost formation on texture tops. We predict the thermodynamically stable wetting states based on a free energy analysis, and model the behavior of rolling droplets on liquid-impregnating surfaces. We conducted droplet impact and roll-off experiments to assess the robustness of liquid-impregnateding micro- and nano-scale textured surfaces and found that their ability to shed droplets was improved dramatically. Furthermore, environmental scanning electron microscope experiments demonstrated that frost formation as well as condensation occurs preferentially on these surfaces thereby limiting ice contact to texture tops only. Ice adhesion strength was quantified using a custom-built adhesion testing apparatus to demonstrate greatly enhanced anti-icing performance of the liquid-impregnating surfaces compared to superhydrophobic surfaces.   

Using Superhydrophobic Surfaces and Optical Caustics to Detect Nanoparticle Aggregation

A 3-D envelope of refracted light known as an optical caustic, can be formed by shaping an aqueous drop on a superhydrophobic surface which is used to generate a signal that is very sensitive to changes in particle size. When the sample being detected is suspended in the drop, slow evaporation induces movement that segregates smaller from larger particles, enhancing the speed of detection via induced aggregation. While the unique properties of optical caustics have been used in engineering science to evaluate stress distributions and contact between material components, they have not been widely used in diagnostics or biological analyses. This paper demonstrates how this method can track aggregation of gold nanoparticles for rapid detection of molecular disease markers using immunoassays.   

Droplet condensation and growth on nanotextured surfaces impregnated with an immiscible liquid

For effective dropwise condensation, a surface that sheds droplets easily is desirable due to the enhancement in accompanying heat transfer. Incorporating nano-textures on the surface can enhance the droplet shedding or spreading. We demonstrate that droplet shedding can be further influenced by impregnating the nano-textured surface with a liquid which is immiscible with respect to the droplet. In this study, the dynamics of dropwise condensation on such immiscible liquid impregnated nano-textured surfaces have been investigated in pure quiescent water vapor conditions. Condensation experiments were conducted using an Environmental Scanning Electron Microscope by controlling the chamber water vapor pressure and substrate temperature. We show preferential sites for condensation and different modes under which droplets grow, depending upon the surface chemistry, surface texture, and the impregnating liquid properties. Concurrently, we show an evolution of apparent contact angles during the condensation process on the impregnated surfaces.   

Molecular diffusion and tensorial slip at surfaces with periodic and random nanoscale textures

The influence of periodic and random surface textures on the flow structure and effective slip length in Newtonian fluids is investigated by molecular dynamics (MD) simulations. This study is motivated by the possibility to generate transverse flows in microfluidics devices to enhance mixing and separation processes. We consider a situation where the typical pattern size is smaller than the channel height and the local boundary conditions at wetting and nonwetting regions are characterized by finite slip lengths. In case of anisotropic textures, the interfacial diffusion coefficient of fluid molecules near heterogeneous surfaces correlates well with the effective slip length as a function of the shear flow direction with respect to the texture orientation. In addition, it was found that the angular dependence of the effective slip length obtained from MD simulations is in good agreement with hydrodynamic predictions provided that the pattern size is larger than several molecular diameters. These findings lend support for the microscopic justification of recently introduced tensor formulation of the effective slip boundary conditions in the case of noninertial flows of Newtonian fluids over smooth surfaces with nanoscale anisotropic textures.   

Wetting as a basis for a highly selective colorimetric indicator for organic liquids

We present a colorimetric indicator for organic liquids that couples distinct macroscopic color patterns to minute differences in liquids' intrinsic wettability to a surface. We find that when a liquid percolates through the pores of large-area, defect-free silica inverse-opal films, a highly consistent re-entrant geometry leads to sharply defined threshold wettability for liquid infiltration, occurring at intrinsic contact angles near 20\r{ }. The structure also acts as a 3D photonic crystal, producing bright iridescent color that disappears when infiltrated with liquid, coupling the highly selective wetting observed to an easy-to-visualize colorimetric response. Combining a percolation model and FDTD optical simulations, we estimate the selectivity of the colorimetruic response. In addition, we present a technique to generate precisely controlled spatial patterns of surface chemistry throughout the porous network. This lets us tailor the wettability threshold to specific liquids across a continuous range. Using these techniques, we demonstrate the applicability of this indicator to colorimetrically distinguish: i) ethanol-water mixtures varying by only 2.5{\%} in concentration; ii) hexane, heptane, octane, nonane, and decane; and iii) samples of gasoline (regular unleaded) and diesel.   

Drag Measurements in Laminar Flows over Superhydrophobic Porous Membranes

An anomalous hydrodynamic response has recently been observed in oscillating flows on mesh-like porous superhydrophobic membranes.\footnote{S. Rajauria, O. Ozsun, J. Lawall, V. Yakhot, and K. L. Ekinci, Phys. Rev. Lett. 107, 174501 (2011)} This effect was attributed to a stable Knudsen layer of gas at the solid-liquid interface. In this study, we investigate laminar channel flow over these porous superhydrophobic membranes. We have fabricated surfaces with solid area fraction $\phi_{s}$, which can maintain intimate contact with both air and water reservoirs on either side. Typical structures have linear dimensions of 1.5~mm $\times$ 15~mm $\times$ 1~$\mu$m and pore area of 10~$\mu$m $\times$ 10~$\mu$m. The surfaces are enclosed with precisely machined plastic microchannels, where pressure driven flow of DI water is generated. Pressure drop across the microchannels is measured as a function of flow rate. Slip lengths are inferred from the Poiseuille relation as a function of $\phi_{s}$ and compared to that of similar standard superhydrophobic surfaces, which lack intimate contact with an air reservoir.   

Thermo-super-hydrophobic effect

Super-hydrophobic effect involves capture of gas bubbles in pores of solid wall. These bubbles separate moving liquid from the solid surface resulting in a substantial reduction of shear drag experienced by the liquid. The super-hydrophobic effect requires presence of two phases and thus drag reduction can be accomplished only for liquids. Thermo-super-hydrophobic effect takes advantage of the localized heating to create separation bubbles and thus can work with single phase flow systems. Analysis of a simple model problem shows that this effect is very strong in the case of small Re flows such as those found in micro-channels and can reduce pressure drop down to 50{\%} of the reference value if the heating pattern as well as the heating intensity are suitable chosen. The thermo-super-hydrophobic effect becomes marginal when Re increases above a certain critical value.