Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session P37: Focus Session: Microfluidic Physics III: Surface Effects and Flows |
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Sponsoring Units: DFD Chair: Lyderic Bocquet, University Lyon I Room: LACC 512 |
Wednesday, March 23, 2005 11:15AM - 11:27AM |
P37.00001: Stick or Slip ?: Measuring slip lengths with nm resolution Sung Chul Bae, Stephen Anthony, Steve Granick Fluid dynamics within small channels draws great interest due to the development of microfluidic devices, yet details about flow immediately at a solid surface remain too vague. Previous attempts to measure surface flow rate were limited to a resolution of the optical wavelength. Here, by using a fluorescence resonance energy transfer (FRET) approach, we improve the resolution by 1-2 orders of magnitude. Two different flow systems, hydrodynamic flow and electrokinetic flow, were investigated with this technique. [Preview Abstract] |
Wednesday, March 23, 2005 11:27AM - 11:39AM |
P37.00002: Large electrokinetic effects on slipping surfaces Lyderic Bocquet, Laurent Joly, Christophe Ybert, Emmanuel Trizac We show, using extensive Molecular Dynamics simulations, that the dynamics of the electric double layer (EDL) is very much dependent on the wettability of the charged surface on which the EDL develops. For a wetting surface, the dynamics, characterized by the so-called Zeta potential, is mainly controlled by the electric properties of the surface, and our work provides a clear interpretation for the traditionally introduced immobile Stern layer. In contrast, the immobile layer disappears for non-wetting surfaces and the Zeta potential deduced from electrokinetic effects is considerably amplified by the existence of a slippage at the solid substrate. Potential applications of this effect in microfluidic devices will be discussed [Preview Abstract] |
Wednesday, March 23, 2005 11:39AM - 11:51AM |
P37.00003: Models for Apparent Slip Eric Lauga, Michael Brenner, Howard Stone A number of groups have reported microfluidic experiments consistent with liquid slip on solid surfaces. We present two physical models which do not require the breakdown of the no-slip boundary condition but lead to results consistent with slip. The first model considers the dynamic response of surface-attached bubbles in drainage experiments and the second considers electrical effects in pressure-driven flow experiments. In both cases, we evaluate the apparent slip length and compare them with the appropriate experiments. [Preview Abstract] |
Wednesday, March 23, 2005 11:51AM - 12:03PM |
P37.00004: Wetting on nano-patterned surfaces observed with the Atomic Force Microscope Antonio Checco, Benjamin Ocko, Oleg Gang Recent technological advancements have allowed for precise nanometer-scale control of surface topology and chemical composition. Such surface engineering can be used to confine and manipulate tiny amounts of liquids or to enhance the liquid-repellency of a substrate. Despite the great potential of these methods, the actual behavior of liquids on the nanoscale is still to be elucidated experimentally. With this aim, we have used Atomic Force Microscopy (AFM) to investigate wetting of liquid alkanes onto chemically nanopatterned surfaces. In a first step, parallel, some tens nm-wide stripes with carboxylic acid termination (wettable) were created on the methyl-terminated surface of a self-assembled monolayer (octadecylthrichlorosilane, non wettable) through local electro-oxidation by a metallic AFM tip[1]. Noncontact mode AFM was used to image the condensation of liquids onto the nanopatterned surface. By finely controlling the amount of liquid condensed onto the striped surface we could follow morphological wetting transitions and estimate the size of the contact line and the magnitude of the line tension. [1] J. Sagiv and R. Maoz, Nano Lett. \textbf{3} 761(2003) [Preview Abstract] |
Wednesday, March 23, 2005 12:03PM - 12:15PM |
P37.00005: Manipulating Liquids on the Tunable Nanostructured Surfaces Tom Krupenkin, Ashley Taylor, Paul Kolodner, Stanley Pau, Alan Lyons, Mark Hodes Recently demonstrated electrically tunable nanostructured superhydrophobic surfaces provide a promising new way of manipulating liquids at both micro and macro scale. Dynamic control over the interaction of liquids with the solid substrate is of great interest to many research areas ranging from biology and chemistry to physics and nanotechnology. In this work the influence of the nano-scale topography on the liquid-solid interaction is further investigated. The dependence of the superhydrophobic -- wetting transition on the topography of the nanostructured layer, its electrical properties, and its surface coating is discussed. The reversibility of this transition and its dependence on the geometry of the nano-size features are addressed. Several emerging applications of these surfaces, including lab-on-a-chip, chemical microreactor, and skin drag reduction are discussed. [Preview Abstract] |
Wednesday, March 23, 2005 12:15PM - 12:27PM |
P37.00006: Effect of Korteweg Stress of Miscible Two Liquid Flow on Micro Fluidic Devices Yasuhiko Sugii, Koji Okamoto, Akihide Hibara, Manabu Tokeshi, Takehiko Kitamori In order to design the micro fluidic devices, it is important to investigate the dynamics such as mixing, molecular transformation and interfacial instability in both of miscible and immiscible multi-layer flow. In this study, miscible liquid two-layer flow, water and ethanol, in a Y-shaped microfluidic device, which consists of microchannels with 120 micro m in width and 35 micro m in depth, is experimentally investigated by particle image velocimetry (PIV) to clarify the flow characteristics. The obtained velocity distributions with a spatial resolution of 5.9 x 1.5 micro m\^{}2 around the miscible interface between water and ethanol varying flow rate and concentration of ethanol, indicate an imbalance in shear stress at interface. The difference of shear stress was compared with the Korteweg stress, which was generated by interfacial tension gradient due to a concentration gradient by diffusion in a miscible two-layer flow. The results indicate that the stress was balanced with the shear stress around the interface. [Preview Abstract] |
Wednesday, March 23, 2005 12:27PM - 12:39PM |
P37.00007: A Microfluidic Tensiometer Shelley Anna, Hans Mayer Recent theoretical predictions indicate that a shift in surfactant transport mechanism from diffusion dominated to kinetically dominated occurs at highly curved interfaces where the radius of curvature is on the same order as feature sizes in microfluidic devices (10 to 100 microns). To date experimental evidence of this shift in transport mechanism has been lacking due to limitations on the degree of interface curvature imposed by traditional methods of surface tension measurement. We show that measurement of dynamic surface tension at highly curved interfaces is possible via a microfluidic tensiometer that uses glass micropipettes to control curvature dimension. We observe a dramatic decrease in the characteristic timescale for reducing the surface tension of an initially clean interface, compared with the timescale observed using a traditional pendant bubble method. We discuss the implications of this shift in timescale toward the determination of relevant physical properties of surfactant systems. The transport of surfactant molecules to and from liquid interfaces plays an important role in the formation and motion of droplets and bubbles in microfluidic devices and the results presented here offer new insight into the relevant mass transport timescales to be considered in such applications. [Preview Abstract] |
Wednesday, March 23, 2005 12:39PM - 12:51PM |
P37.00008: Transport of nonconductive and conductive droplets in a parallel plate array Debalina Chatterjee, Boonta Hetayothin, Aaron Wheeler, Daniel King, Robin Garrell Electrowetting on dielectric technique is used to actuate conductive liquid droplets on electrodes patterned beneath a dielectric. Nonconductive liquids can be transported electrohydrodynamically inside channels. We show for the first time that it is possible to transport droplets of nonconductive liquids on dielectric surfaces, using modest voltages and frequencies ($<$100 V, $<$10 kHz). Ionic liquids, aqueous surfactants, buffers, and organic solutions can also be transported. Although conductive liquids show a significant change in liquid contact angle on application of potential, nonconductive liquids do not, suggesting a different mechanism of transport. The empirical criteria for moving droplets in a two-dimensional array are a liquid dielectric constant $\ge $ 4.3 and a molecular dipole moment $\ge $ 1.2 D. The transport mechanisms are discussed along with new microfluidic applications that these results suggest are now feasible. [Preview Abstract] |
Wednesday, March 23, 2005 12:51PM - 1:03PM |
P37.00009: Analysis of Drop Shapes during Electrowetting on a Dielectric Yousef Daneshbod, James D. Sterling, Ali Nadim Electrowetting refers to the electrostatic control of the interfacial energy of a liquid on a solid, primarily used for the transport of micro-liter volumes of drops on surfaces with embedded electrode arrays. In the present work, the drop is modeled as a two-dimensional lens-like conductor immersed in an infinite dielectric medium slightly above a planar conductor. A matched asymptotic expansion is used to approximate the electrostatic field surrounding the drop. The outer problem models the drop as a conducting circular segment resting on the conducting plane, each maintained at a separate constant potential. The inner problem corrects the region near the edge of the drop by modeling it as an infinite planar conducting wedge lying slightly above the conducting plane. By matching the inner and outer solutions, the charge density along the entire surface of the drop can be approximated, enabling the calculation of the total capacitance of the system. An energy minimization method similar to that of Shapiro \textit{et al.} [\textit{J. Appl. Phys.}, \textbf{93}, 5794 (2003)] is applied to the total energy consisting of the liquid/gas, liquid/solid and solid/gas surface energies, together with the electrostatic contribution, subject to the constraint that the drop volume remains constant. A modified form of the Young-Lippmann equation is thus derived that includes the contribution from the extra capacitance of the drop obtained via matched asymptotics. [Preview Abstract] |
Wednesday, March 23, 2005 1:03PM - 1:15PM |
P37.00010: Simulations of electrowetting dynamics in free and and confined droplet geometries Karl Glasner Electrowetting has become popular for moving small amounts of fluids in confined spaces (e.g. [ J. Lee et al. Sensors and Actuators, A95 259 (2002) ]). Models are proposed in two cases: a quasi-steady thin film approximation for a free droplet on a surface, and a Hele-Shaw type model for the two-plate geometry. In the first case, a boundary integral method is formulated which leads to a very efficient numerical algorithm. In the second case, diffuse-interface methods are utilized. The phenomenon of contact angle hysteresis and its dynamical implications are addressed. In the case of the Hele-Shaw geometry, we compare our results to experimentally observed droplet motion. [Preview Abstract] |
Wednesday, March 23, 2005 1:15PM - 1:27PM |
P37.00011: Bubble microstreaming: Transport and force actuation Sascha Hilgenfeldt, Philippe Marmottant The energy of acoustic waves can be focused onto the microscale through oscillating bubbles, setting up microfluidic flow. Rather than driving the flow directly through the bubble interface motion, we use small periodic oscillations that are rectified into a powerful, steady streaming flow. Substrate patterning allows for control of both the bubble positions and the direction of the flow. High transport speeds are obtained, while different flow patterns can be activated on the same subtrate. Directed transport is achieved by simple, localized patterning, without the need for confining microchannels. The flow field also allows for the simultaneous actuation of large forces onto transported objects, such as lipid vesicles or cells. Potential applications abound in MEMS and lab-on-a-chip systems handling biomaterials. [Preview Abstract] |
Wednesday, March 23, 2005 1:27PM - 1:39PM |
P37.00012: 3D Hydrodynamic Positioning using Micro-Steady Streaming Eddies Jian Chen, Barry Lutz, Daniel Schwartz The eddy structure of low-intensity micro-steady streaming flows is useful for positioning particles and motile cells in predictable 3D locations that are dictated by the particle size, device design, and characteristics of the primary oscillating flow field. The flow field structure responsible for this positioning is characterized using tracer imaging methods and numerical simulations. Fluid-particle interactions are studied with a combined perturbation analysis and finite element method to understand the physics of microeddy-based hydrodynamic positioning and the implications for microfluidic device designs. [Preview Abstract] |
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P37.00013: Photosensitive chemical reactions in patterned microchannels Olga Kuksenok, Anna Balazs Through computer simulations, we study the behavior of a A/B/C ternary mixtures in which two immiscible components, A and B, undergo a photosensitive chemical reaction and produce third component, C. The reverse chemical reaction, namely consumption of the A and B species from the C components, is also possible. Initially, two parallel fluid streams, A and B, are driven by an imposed pressure gradient (Poiseuille flow) through the three dimensional microchannel. The microchannel is decorated with patches that have specific interactions with different components of the mixture. The presence of the patterned substrates enhances chemical reactions in the system since it diverts two initial parallel fluid streams and creates additional interfaces between A and B components. We consider the case where chemical reactions rates can be controlled externally by the light irradiation. We show that by applying time-dependent light irradiation, we can precisely control the distribution of each component within the channel, as well as to tune dynamically the properties of the patterned substrates. [Preview Abstract] |
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