### Session P21: Microfluidic Physics III

 Wednesday, March 15, 2006 11:15AM - 11:27AM P21.00001: Multi-Point Holographic Micro-Velocimetry Roberto Di Leonardo , Jonathan Leach , Hasan Mushfique , John Cooper , Giancarlo Ruocco , Miles Padgett We show how holographic optical trapping can be used for the multi-point measurement of fluid flow in microscopic geometries. An array of microprobes can be simultaneously trapped and used to map out the fluid flow in a microfluidic device. The optical traps are alternately turned on and off such that the probe particles are displaced by the flow of the surrounding fluid and then re-trapped. The particles' displacements are monitored by digital video microscopy and directly converted into velocity field values. The validity of the technique is demonstrated for the case of the flow around a spinning sphere and the flow at the outlet of a micro-channel. Wednesday, March 15, 2006 11:27AM - 11:39AM P21.00002: A Hybrid Microwave Source and Irradiator for Biological Lab On a Chip Applications. David Issadore , Tom Hunt , Kristi Adamson , Robert Westervelt , Rick Rogers Using a standard lithographic process, we have built a hybrid microwave irradiator for use in microwave enhanced chemistry and localized, rapid heating. The device combines a 100mW microwave source with a near field antenna to produce an entirely on-chip system for delivering microwave energy into a thin($<$100$\mu$m) layer above a substrate. The antenna utilizes a serpentine wire pattern to produce a thin layer of intense microwave electromagnetic field intensity that falls off exponentially in distance away from the substrate. The device, including RF electronics, was built on a standard 1'' by 3'' glass slide, and several antenna pixel sizes are tested for Biological Lab On a Chip Applications. This work is made possible by the NSEC NSF grant PHY-0117795. Wednesday, March 15, 2006 11:39AM - 11:51AM P21.00003: Simulations of Contact Line Motion in Partially Miscible Fluids Shengfeng Cheng , Colin Denniston , Mark Robbins We report on extensive molecular-dynamics simulations of contact line motion in partially miscible fluids confined between two solid walls and sheared in a Couette geometry. Our results show that diffusion alone cannot remove the stress singularities at the contact line or lead to no-slip boundary conditions on the fluid velocity. Computed velocity fields show that there is a substantial drop of the fluid velocity near the contact line, which is associated with the gradient of the fluid-solid interfacial tension in the same region. However, the fluid velocity does not fall to zero at the contact line, in contrast to the case where fluids are immiscible. The nonzero velocity leads to a net advective flux across the fluid-fluid interface, which is balanced by the diffusive flux induced by the concentration gradient. The advective and diffusive fluxes across the interface are only significant in the very first layer of fluid atoms. Wednesday, March 15, 2006 11:51AM - 12:03PM P21.00004: Macromolecular Liquids Slip Over Solid Surfaces: Experimental Studies of the Slip Length Karin Jacobs , Renate Fetzer We present a novel method to asses the slip length and viscosity of thin films of highly viscous Newtonian liquids. We quantitatively analyze dewetting fronts of low molecular weight polystyrene melts on Octadecyl- (OTS) and Dodecyltrichlorosilane (DTS) polymer brushes [1]. Using a thin film (lubrication) model derived in the limit of large slip lengths, we can extract slip length and viscosity of films with thicknesses between 50 nm and 230 nm and temperatures above the glass transition. We find slip lengths from 100 nm up to 1 micron on OTS and between 300 nm and 10 microns on DTS covered silicon wafers. The slip length decreases with temperature. The obtained values for the viscosity are consistent with independent measurements [2]. [1] R. Fetzer, K. Jacobs, A. Muench, B. Wagner, T.P. Witelski, Phys. Rev. Lett. 95, 127801 (2005) [2] R. Fetzer, K. Jacobs, M. Rauscher (to be published) Wednesday, March 15, 2006 12:03PM - 12:15PM P21.00005: Source of Shear Dependent Slip at Liquid/Solid Interfaces Nikolai Priezjev , Sandra Troian Slippage at liquid/solid interfaces can strongly influence transport behavior in micro- and nanoscale systems. Previous molecular dynamics (MD) studies of simple and polymeric fluids subject to planar shear at small Reynolds number have shown that the slip length increases as a power law in the shear rate for moderate to high values. The corresponding boundary condition provides a new generalization of the Navier slip law. In this talk, we examine what physical mechanism is responsible for the shear rate exponent by focusing on the collision events between the fluid particles in the first layer and the adjacent wall particles comprising a crystalline surface. By examining the interfacial frictional force as a function of the fluid sliding velocity, we recover similar behavior as inherent in the generalized slip condition and determine that the dominant frictional response stems from the repulsive part of the Lennard-Jones interaction potential. A reduced kinetic model describing the scattering of a single molecule with a given slip velocity along a crystalline surface helps explain the saturation in the frictional force at large sliding velocities. These results elucidate how different is the slip behavior at liquid/solid interfaces from that observed in rarefied gases. Wednesday, March 15, 2006 12:15PM - 12:27PM P21.00006: Apparent Slip at Hydrophilic Surface: Flow Profile within 1 nm from the Surface 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.~ Here, by using fluorescence energy transfer (FRET and fluorescence quenching) approaches, we measured the flow rate of fluorescence quencher molecules within 1 nm from the quartz surface within a specially-designed microfluidic device.~ In parallel, we have simulated the flow dynamics at the surface, in order to separate cleanly the actual near-surface velocity from the confounding effects of near-surface diffusion.~~ Wednesday, March 15, 2006 12:27PM - 12:39PM P21.00007: Slip versus Friction : Modifying the Navier condition Evangelos Kotsalis , Jens Walther , Petros Koumoutsakos The modeling of fluid-solid interfaces remains one of the key challenges in fluid mechanics. The prevailing model, attributed to Navier, defines the fluid slip'' velocity as proportional to the wall shear and a parameter defined as the slip length. Several works have in turn proposed models for this slip length but no universal model for the slip velocity has been accepted. We present results from large scale molecular dynamics simulations of canonical flow problems, indicating, that the inadequacy of this classic model, stems from not properly accounting for the pressure field. We propose and validate a new model, based on the fundamental observation that the finite slip'' velocity is a result of an imbalance between fluid and solid intermolecular forces. An excess force on the fluid elements will lead to their acceleration which in turn may result in a slip velocity at the interface. We formulate the slip velocity in terms of fluid-solid friction $F_f}$ and propose a generalized boundary condition: $F_{f} = F_{s} + F_{p} = \lambda_u u_{s} + \lambda_{p} p$ where $\mbox{ p}$ denotes the pressure, and $\lambda _u$and $\lambda _p$ the viscous and static friction coefficients, for which universal constants are presented. We demonstrate that the present model can overcome difficulties encountered by the classical slip model in canonical flow configurations. Wednesday, March 15, 2006 12:39PM - 12:51PM P21.00008: Slip and Air-Entrainment at Water-Solid Interfaces Yingxi Elaine Zhu , Prasad Sarangapani , Ashis Mukhopadhyay A number of recent studies performed with water flow past hydrophobic microchannels have reported the existence of slip' at wall and suggested the existence of the interfacial gas layer as the underlying mechanism for the slip motion, yet the details are much disputed. We combine microscopy and advanced laser spectroscopy to directly and non-invasively detect the interfacial gas layer in flowing water past micro/nano-channels whose surface chemistry and gap spacing are varied. We observe that the dimension of the gas layer strongly depends on surface hydrophobicity and flow rates. Surprisingly, we have also observed the slip motion of water over hydrophilic surfaces with a strong dependence on liquid-loading conditions. We propose a mechanistic theory about air-entrainment that can account for our observations to elucidate the origin of the gas formation at water-solid interface and its consequence on slip motion. Wednesday, March 15, 2006 12:51PM - 1:03PM P21.00009: Rheology of sub-nanometer thick water films Tai-De Li , Robert Szoszkiewicz , Elisa Riedo Knowing the behavior of water in small volumes is essential for the understanding of many processes in biology, tribology, and geophysics. Water under nano-confinement plays a crucial role in biological and technological systems. Here, we report an experiment in which an atomic force microscope tip approaches a flat solid surface in purified water, while small lateral oscillations are applied to the tip. The normal and lateral forces acting on the tip are measured directly and simultaneously as a function of water thickness. We find that, for hydrophilic surfaces, oscillatory solvation forces are present in the last four adjacent water layers where the dynamic viscosity is measured to grow up orders of magnitude in respect to bulk water. The same effects are present for atomically smooth surfaces and slightly rough surfaces. Oscillatory solvation forces have been detected also when the confining flat surface was hydrophobic. Wednesday, March 15, 2006 1:03PM - 1:15PM P21.00010: Wetting morphologies on surfaces nanopatterned with chemical stripes Antonio Checco , Oleg Gang , Benjamin M. Ocko Here we investigate the wetting of simple, volatile liquids on model chemical nanopatterns created using Local Oxidation Nanolithography. This technique makes use of a biased, metallic AFM tip to locally oxidize the methyl-terminations of a self-assembled monolayer (octadecylthrichlorosilane) into carboxylic acid termination[1]. With this method we have realized parallel, 50 to 500 nm wide, wettable stripes (carboxylic) embedded into a non-wettable (methyl) surface. Several organic (polar, non-polar), volatile liquids have been condensed onto the nanopatterned surface and the resulting wetting morphologies have been studied in-situ by using an environmental AFM. Initially the liquid only condenses on the wettable stripes to form a thin liquid film. Close to saturation the liquid morphology becomes drop-like. Eventually, when more and more liquid is condensed on the stripes, the liquid drops may spill over'' into the non-wettable spacer so that neighboring lines merge and undergo a morphological wetting transition''. For all of these regimes we show that long-range forces are relevant to nanoliquid shape''. Results will be compared with those of Density Functional Theory.[1] R. Maoz, S. Cohen, and J. Sagiv, Adv. Mater. \textbf{11}, 55 (1999) Wednesday, March 15, 2006 1:15PM - 1:27PM P21.00011: Self-propelled film-boiling liquids Heiner Linke , Michael Taormina , Benjamin Aleman , Laura Melling , Corey Dow-Hygelund , Richard Taylor , Matthew Francis We report that liquids perform self-propelled motion when they are placed in contact with hot surfaces with asymmetric (ratchet-like) topology. Millimeter-sized droplets or slugs accelerate at rates up to 0.1 g and reach terminal velocities of several cm/s, sustained over distances up to a meter. The pumping effect is observed when the liquid is in the film-boiling regime, for many liquids and over a wide temperature range. We propose that liquid motion is driven by a viscous force exerted by vapor flow between the solid and the liquid. This heat-driven pumping mechanism may be of interest in cooling applications, eliminating the need for an additional power source. Wednesday, March 15, 2006 1:27PM - 1:39PM P21.00012: Ratcheting motion of capsules on tailored substrates Anna C. Balazs , Kurt A. Smith , Alexander Alexeev , Rolf Verberg We study the motion of microcapsules on attractive surfaces. The capsules, modeled as fluid-filled elastic shells, represent polymeric microcapsules or biological cells. Certain periodic surface patterns give rise to directed capsule motion for a symmetric energy input, such as an oscillatory shear flow. We use a numerical model which integrates a lattice spring representation of the capsule shell and the substrate with a lattice Boltzmann representation for the fluid regions. We consider, as a surface pattern, a series of asymmetric ramps. The minimum shear necessary to drive a capsule forward'' over one ramp is less than that needed to drive the capsule `backward'' over a ramp. We show under what conditions it is possible to move the capsule forward, in a ratcheting motion, via an imposed oscillatory flow. These patterned surfaces could be used to control capsule motion precisely, based on flow and surface properties. They coud also be used to efficiently sort capsules based on their size or material properties. Wednesday, March 15, 2006 1:39PM - 1:51PM P21.00013: Electrowetting for Digital Microfluidics Tom Hunt , Kristi Adamson , David Issadore , Robert Westervelt Droplet based chemistry promises to greatly impact biomedical research, providing new avenues for high throughput, low volume assays such as drug screening. Electrowetting on Dielectric (EWOD) is an excellent technique for manipulating microscopic drops of liquid. EWOD uses buried electrodes to locally change the surface energy between a droplet and a substrate. We present microfabricated devices for moving droplets with EWOD. One example of such a device consists of a series of 16 interdigitated electrodes, decreasing in size from 1mm to 20 microns. Each electrode is addressable by an independent, computer controlled, high voltage supply. This work made possible by a gift from Phillip Morris and the NSEC NSF grant PHY-0117795. Wednesday, March 15, 2006 1:51PM - 2:03PM P21.00014: Surface mediated liquid transport on nanotube Min-Feng Yu , Kyungsuk Yum The surface mediated liquid transport on nanotubes was studied using a nanotube-based liquid transport system. Microscale liquid droplets were formed and transferred to nanotubes using the liquid transport system integrated with a nano-manipulator. If the spreading parameter $S$ is larger than a threshold value $S_{c}$, the liquid spontaneously flows out of the liquid droplet through a thin precursor film formed along the nanotube surface. The liquid transport on nanotube surfaces was studied \textit{in situ} by measuring the volume flow rate which was obtained from a direct observation of the droplet. The flow rate dependence on the size of nanotubes and surface energy were also investigated. The surface mediated liquid transport phenomenon can be exploited for the development of nanoscale liquid transport system for nanofabrication and nanoscale devices for biological and chemical applications. Reference: Kyungsuk Yum and Min-Feng Yu, Surface-mediated liquid transport through molecularly thin liquid films on nanotubes, Phys. Rev. Lett. 95, 186101 (2005)