Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session A18: Microfluids: General I |
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Chair: Alexander Alexeev, Georgia Institute of Technology Room: 321 |
Sunday, November 20, 2011 8:00AM - 8:13AM |
A18.00001: Correlation-Force-Spectroscopy Rheometer Milad Radiom, Brian Robbins, Christopher D.F. Honig, John Y. Walz, Mark R. Paul, William A. Ducker We describe a new method, correlation force spectrometry, which characterizes fluids through measurement of the correlations between the thermally-stimulated vibrations of two closely spaced micrometer-scale cantilevers in fluid. We discuss applications to measurement of the rheological properties of complex fluids at high frequency and high spatial resolution. We measure a large range of frequencies (up to 1 MHz) and use very small sample volumes ($\mu $L) and demonstrate that the thermal noise in the cross correlation is much smaller than in the autocorrelation. Our experimental measurements of the equilibrium fluctuations in cantilever displacement for cantilever pairs immersed in a Newtonian fluid are described accurately using a theoretical approach based upon the fluctuation-dissipation theorem [Paul and Cross, Phys. Rev. Lett. 92, 235501 (2004)]. [Preview Abstract] |
Sunday, November 20, 2011 8:13AM - 8:26AM |
A18.00002: High-throughput rheology in a microfluidic device Eric Furst, Kelly Schultz, Hyejin Han, Chongyoup Kim High-throughput rheological measurements in a microfluidic device are demonstrated. A series of microrheology samples is generated as droplets in an immiscible spacer fluid using a microfluidic T-junction. The compositions of the sample droplets are continuously varied over a wide range. Rheology measurements are made in each droplet using multiple particle tracking microrheology. We review critical design and operating parameters, including the droplet size, flow rates and rapid fabrication methods. Validation experiments are performed by measuring the solution viscosity of glycerine and the biopolymer heparin as a function of concentration. Finally, an analysis of droplet mixing is performed in order to optimize the device performance. Overall, the combination of microrheology with microfluidics maximizes the number of rheological measurements while simultaneously minimizing the sample preparation time and amount of material, and should be particularly suited to the characterization of scarce or expensive materials. [Preview Abstract] |
Sunday, November 20, 2011 8:26AM - 8:39AM |
A18.00003: High-Frequency Microrheology using a Cantilever Pair in Fluid Brian Robbins, Milad Radiom, Chris Honig, John Walz, William Ducker, Mark Paul Measuring the rheological properties of small quantities of complex fluids at high frequencies remains an important challenge with many current technological applications. The field of two-point microrheology has made great strides yet most available approaches are subject to significant experimental and theoretical constraints regarding the accessible frequencies of measurement. In this talk, we explore the high-frequency motion of a pair of micron scale cantilevers in fluid to probe its rheological properties for a broad range of conditions. Using currently available cantilevers, it is possible to access MHz frequencies in a viscous fluid. We explore the motion of cantilever pairs that are either driven externally or by Brownian motion. We use the motion of an individual cantilever and the fluid-coupled motion of a cantilever pair to quantify the properties of the surrounding fluid. We focus here on analytical and numerical results for a range of Newtonian and power-law fluids and compare our findings directly with experimental measurements where possible. [Preview Abstract] |
Sunday, November 20, 2011 8:39AM - 8:52AM |
A18.00004: Rheology measurements using ciliated surfaces Alexander Alexeev, Brian T. Johnston, Yi Yang We employ computational modeling to examine the utility of ciliated surfaces for measuring fluid viscosity. We consider a fluid-filled channel with a wall covered by compliant synthetic cilia that are arranged in a square pattern. The cilia can be actuated by a periodical force applied to their free ends and/or can be used to measure the bending moment at the points of cilium attachment to the wall. We show that the phase difference between the applied sinusoidal force and the bending moment at cilium root can be used to estimate fluid viscosity. We probe two types of ciliated surfaces. In the first scenario, alternating sensory and actuated cilia are arranged a chessboard pattern. In the second case, each cilium in the layer is both actuated and sensory. We compare these two layer arrangements in application to rheological measurements. In particular, we show that the phase signal is insensitive to the amplitude of applied force and layer density. Our results can be useful for designing sensory surfaces for microfluidic and biomedical applications. [Preview Abstract] |
Sunday, November 20, 2011 8:52AM - 9:05AM |
A18.00005: ABSTRACT WITHDRAWN |
Sunday, November 20, 2011 9:05AM - 9:18AM |
A18.00006: Photoinduced Contact Angle Hysteresis on a Single Microsphere Samuel Rosenthal, Patricia McGuiggan An atomic force microscope (AFM) is used to measure the meniscus force on individual microspheres as they contact and are retracted from an air/liquid interface. The glass microspheres, whose radii ranged from 20 to 50 micrometers, had organic or inorganic coatings on their surfaces. By exposing the microspheres to light, the contact angle and thus the meniscus force could be dramatically altered. The measured force-distance curves are fitted to macroscopic wetting theory. From these measurements, the contact angle, contact angle hysteresis, position of the contact line pinning, and surface tension were simultaneously determined. [Preview Abstract] |
Sunday, November 20, 2011 9:18AM - 9:31AM |
A18.00007: Effect of Partially Wetted Cavities on Superhydrophobic Friction Reduction Tae Jin Kim, Carlos Hidrovo Superhydrophobicity refers to the condition of water-surface contact angle larger than 150$^{\circ}$. Such microtextured surfaces with low sliding angles($<$5$^{\circ}$) involve air pockets trapped underneath the water(Cassie state). This leads to a shear-free flow boundary condition and consequential reduction in frictional losses and pumping power requirements. We investigate the effects of partially wetted microtextured trenches on the friction reduction characteristics in microchannel flow. PDMS based superhydrophobic microchannels with sidewall trenches are used to visualize the water-air interface penetration and its effects on friction reduction. Theoretical models based on actual water layer measurements and shear free/no slip boundary conditions on the interface are used to calculate lower/upper bounds of the effective friction factor. These are compared to experimental values determined from pressure and flow rate data. Results suggest the existence of a ``start-up'' pressure required to overcome capacitance associated with surface tension effects and that friction reduction characteristics of the microchannels are unaffected by partial flooding of the trenches. [Preview Abstract] |
Sunday, November 20, 2011 9:31AM - 9:44AM |
A18.00008: Climbing of Micro-Droplets Along a Conical Fiber Erqiang Li, Sigurdur Thoroddsen When a droplet is deposited onto a conical cylinder the spatial variation of capillary pressure makes it travel up the cone away from the tip. We use high-speed video imaging to observe the motion of micro-droplets along very small conical glass fibers. The results are compared to the pioneering work of Lorenceau {\&} Qu\'er\'e (2004) who used much larger cones. Our glass cones are made using a micro-pipette glass-puller, producing cone diameters down to about 5 micron at their tips. Numerous liquids, e.g. silicone oils, water and methanol were tested, to vary the viscosity over a range of 2000 and the surface tension over a factor of 3. Translations velocities from 0.1 to over 200 mm/s have been observed and the velocity follows capillary-viscous scaling. We find that there exists an optimum aspect ratio of the droplet shape for fastest motion. [Preview Abstract] |
Sunday, November 20, 2011 9:44AM - 9:57AM |
A18.00009: Simulation of microdroplet manipulation on flat surface actuated by wettability gradient using dissipative particle dynamics Zhen Li, Guohui Hu, Zhewei Zhou A particle based mesoscopic methodology called dissipative particle dynamics (DPD) is utilized to simulate the manipulation (translation and coalescence) of micro-droplets in planar microfluidic chips with gradients of wettability. The three dimensional velocity field of a moving droplet propelled by wetting gradient is presented, which shows that the liquid spreads downwards onto substrate in the front area while it converges upwards in the back. The flow structure reveals that the drop moves in a combination of rolling and translation, and the rotation could be dominant when the contact angle is bigger than about 110$^{\circ}$. The increasing of the gradient steepness and the size of droplet, as well as the magnitude of thermal fluctuation, is capable to significantly accelerate the movement for a sub-micrometer droplet. The liquid mixing in coalescence of two droplets is investigated as well. It is found that the process of mixing consists of two stages --- a rapid active mixing through convective mass transfer and then a slow passive mixing through diffusion, which is consistent with previous experiment. Results also indicate that the thermal fluctuation is helpful to promote the coalescence and liquid mixing. [Preview Abstract] |
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