Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session H30: Colloids and Fibers |
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Chair: Aditya Khair, Carnegie Mellon University Room: 33A |
Monday, November 19, 2012 10:30AM - 10:43AM |
H30.00001: Direct Measurements of Colloidal Hydrodynamics near Flat Boundaries Hyuk Kyu Pak, Chungil Ha, Daniel H. Ou-Yang, Dong-yun Lee We studied the hydrodynamic interaction between a colloidal particle close to flat rigid boundaries and the surrounding fluid using oscillating optical tweezers. A colloidal particle located near walls provides a model system to study the behavior of more complex systems whose boundaries can be modeled as effective walls, such as a blood tube, cell membrane, and capillary tube in bio-MEMS. In this study, we measure the hydrodynamic interaction directly without using the Stokes-Einstein relation. Two different cases are studied: a colloidal sphere near a single flat wall and a colloidal sphere located at the midplane between two flat walls. The colloidal hydrodynamics is measured as a function of the distance between the particle and the walls, and is compared with the theoretical results from well-defined hydrodynamics approximations. [Preview Abstract] |
Monday, November 19, 2012 10:43AM - 10:56AM |
H30.00002: ABSTRACT WITHDRAWN |
Monday, November 19, 2012 10:56AM - 11:09AM |
H30.00003: Phase transition in non-brownian fiber suspensions Alexandre Franceschini, Emmanouella Filippidi, Elizabeth Guazzelli, David Pine The simple shear of a suspension of fibers tends to align them with the flow direction. We previously reported that the oscillatory shear of neutrally buoyant non-Brownian fibers align them with the vorticity (Franceschini A. et al. PRL, 2011). We interpreted this phenomenon as the minimization of a ``corrected volume fraction'' defined as a function of the strain amplitude, the average orientation and the volume fraction. Below a critical value of this parameter, the system becomes fully reversible after a few periods. Above it, fluctuations remain and the fibers align with the vorticity, subsequently reducing the value of this corrected volume fraction. We present here the collective behavior of fibers constrained at the liquid-air interface. By pinning the liquid on the wall of a Couette cell, we can have a flat interface. By modifying the surface of the fibers, we get rid of most of surface tension mediated fiber-fiber interactions. In this 2D configuration we can measure spatial correlations, as well as the position and orientation of every fiber at each shear cycle. We similarly define a ``corrected surface fraction'' and see how this parameter help us understand the difference between the surface behavior and the suspension behavior. [Preview Abstract] |
Monday, November 19, 2012 11:09AM - 11:22AM |
H30.00004: Rotational motion of a thin axisymmetric disk in a low Reynolds number linear flow Vikram Singh, Donald Koch, Ganesh Subramanian, Abraham Stroock The problem of a single particle motion at low Reynolds is one of the most fundamental problems of fluid mechanics. It is rather surprising that despite our deep understanding of particle motion at large aspect ratio we know very little about particles other than spheroids at small aspect ratio. In this work, motion of thin axisymmetric rigid particles with fore-aft symmetry in simple shear flow is investigated. We determine the scaling of the effective aspect ratio for a family of shapes given by, $y(\rho) = \kappa(1-\rho^2)^{\alpha}$ where $\alpha $ is a positive parameter, $\rho $ is the radial distance in polar coordinates, $y $is the thickness of the particle, $\kappa $ is the aspect ratio of the particle, and effective aspect ratio is defined as the aspect ratio of a spheroid having the same period of rotation as that of the particle. For an axisymmetric particle, effective aspect ratio can be determined based on the torques acting on the particle in two different orientations. Starting with the integral representation of Stokes flow, matched asymptotic analysis is performed to determine the scaling of the torque acting on a stationary particle in simple shear flow with $\kappa $ as the small parameter. Using boundary element method simulations, the exact torques are obtained and the scaling of effective aspect ratio obtained from the analysis is verified. [Preview Abstract] |
Monday, November 19, 2012 11:22AM - 11:35AM |
H30.00005: Vorticity alignment of rigid fibers in oscillatory flow Jason Butler, Braden Snook Rigid fibers suspended at high concentration in a viscous, Newtonian fluid can be aligned perpendicular to the flow-gradient plane by applying an oscillatory shear flow. Direct comparisons with published experiments of Franceschini and coworkers [Phys. Rev. Letters 107, 250603 (2011)] demonstrate that a simple model, which considers only excluded volume and self-mobilities, can accurately predict the orientation distributions. However simulations reveal that this surprising alignment occurs only if the ratio of the gap width to fiber length is small (i.e. a highly confined suspension). Stresses calculated from the numerical simulations are also reported and compared to the experimentally measured rheology. [Preview Abstract] |
Monday, November 19, 2012 11:35AM - 11:48AM |
H30.00006: The onset of particle-dominated convection regime in colloidal suspensions Layachi Hadji, Mahmoud DarAssi In the experiments of B\'{e}nard convection in a suspension of microparticle by Chang {\it et al.} (2008), a parameter $\beta$ was isolated to model the interplay between the effects of thermophoresis, sedimentation and Brownian diffusion. A plot of $\beta$ as function of the particle radius, $r_p$, for a suspension of aluminum oxide particles in water shows that the function $\beta(r_p)$ has the shape of an inverted parabola with two roots so that $0 < \beta \ll 1$ for $1$ nm $\le r_p \le 5$ nm and for $r_p \approx 50$ nm where thermophoresis and sedimentation are balanced. We consider a particulate medium model to determine the threshold instability conditions. Due to the large particle size, the convection process is characterized by longer diffusion time scale, much smaller Lewis number, $\tau$, and larger separation ratio, $S$, than the binary mixture case. For $0 < \beta \ll 1$ which corresponds to two distinct particle radii, a small wavenumber expansion yields the value $R_c = 720\,\tau/S$. For $\beta = O\bigl(1\bigr)$, threshold stability conditions are depicted as function of the particle size and the height of the fluid cell.\\[4pt] Chang, B.H., Mills, A.F. and Hernandez, E., {\it Int. J. Heat Mass Transfer} {\bf 51} (2008), 1332-1341. [Preview Abstract] |
Monday, November 19, 2012 11:48AM - 12:01PM |
H30.00007: The influence of frequency dependent impedance properties on electrohydrodynamic aggregation of colloidal particles T.J. Woehl, C.S. Dutcher, N.H. Talken, W.D. Ristenpart Colloidal particles suspended in dilute electrolytes have been widely observed to aggregate laterally along electrodes in response to oscillatory electric fields, a phenomenon that has been generally attributed to electrohydrodynamic fluid flow. Fundamental aspects of the aggregation behavior, however, remain unclear. Recently, our group has observed a second order transition in the order parameter for colloidal aggregates for a variety of electrolytes over the frequency range of 100 to 500 Hz. Here we explore the frequency dependence on several parameters related to the electrochemical cell impedance, including the AC current density, an apparent DC current offset, and the current-voltage phase angle. We report the AC current density increases by at least 10\% from 100 Hz to 500 Hz for both NaCl and KCl aqueous solutions, a range commensurate with the observed second order transition. We investigate how the frequency dependent impedance properties affect the aggregation of colloidal particles and discuss the implications for controlling the crystallinity of the colloidal aggregates. [Preview Abstract] |
Monday, November 19, 2012 12:01PM - 12:14PM |
H30.00008: Richardson Dispersion in Brownian Motion Emmanuel Villermaux, Jerome Duplat Since Langevin, the Brownian motion of a microscopic particle explicitly accounts for a short-time correlated ``thermal'' force. The motion is ballistic, $\langle x^2 \rangle \sim t^2 $ at short time scales, and diffusive $\langle x^2 \rangle \sim t $ at long time scales, where $x$ is the displacement of the particle during time $t$, and the average is taken over the thermal distribution of initial conditions. High Reynolds number turbulence is known to exhibit a r\'egime called Richardson dispersion, in which the {\it relative} separation $\delta x$ of material particles grows super-diffusively. Namely, $\langle \delta x^2\rangle\sim t^3$, with the average taken over many particles released from the same initial conditions. We show that Richardson dispersion is indeed property of Brownian motion, under the condition that the initial velocity is fixed rather than distributed thermally. We analyze the motion of an optically trapped particle in air, and indeed find $t^3$ dispersion. This super-diffusive r\'egime, unveiled here, is the direct proof of the existence of the random, rapidly varying force imagined by Langevin, and reveals a profound similarity between molecular diffusion at microscopic scales and turbulent diffusion at much larger scales. [Preview Abstract] |
Monday, November 19, 2012 12:14PM - 12:27PM |
H30.00009: Advective symmetry breaking in phoretic motion of colloidal particles Aditya Khair The phoretic motion of a colloidal particle is animated by an imposed gradient of a scalar field: e.g. solute gradients cause diffusiophoresis and temperature gradients drive thermophoresis. It is customarily assumed that the scalar field evolves solely via diffusion (i.e. the Peclet number is zero). This leads to Morrison's remarkable result that the translational phoretic velocity of a colloid is independent of its size, shape, and orientation relative to the imposed gradient (the colloid does not rotate, either). Moreover, colloids comprising a dispersion are predicted to translate with identical velocities. However, intuitively, as a colloid moves it sets up a fluid flow that advects the same scalar field that instigated its motion. Here, using asymptotic analysis, we explore the first effects of advection on the phoretic motion of colloidal particles (i.e. at the experimentally relevant conditions of small but finite Peclet number). We show that advection leads to symmetry breaking in the phoretic motion of fore-aft asymmetric particles, where the particle velocity depends on the direction of the imposed gradient. We demonstrate that advection drives phoretic rotation of nonspherical colloids. Last, advection is shown to cause relative motion between colloidal particles. [Preview Abstract] |
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