Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session L53: Flow of Complex Fluids, Polymers, Gels |
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Sponsoring Units: DFD GSOFT DPOLY Room: Hilton Inner Harbor Holiday Ballroom 4 |
Wednesday, March 16, 2016 11:15AM - 11:27AM |
L53.00001: Anomalous response of nematic platelets under LAOStress and Strain revealed by 3D RheoSAXS O. Korculanin, H. Hirsemann, B. Struth, G. Portale, M. P. Lettinga Dispersions of colloidal Gibbsite platelets in the nematic phase display a complex response to Large Amplitude Oscillatory Shear (LAOS) flow that strongly depends on the strain amplitude. [1] In this work we applied LAOStress and LAOStrain to the nematic dispersion and probed the structure with time-resolved SAXS measurements. By using plate-plate and couette geometry, we had access to both the flow-vorticity and flow-gradient plane, respectively, thus obtain full 3D rotational motion of the director. For LAOStress, we observe strong asymmetrical behavior both in the rheological and the microscopic response. This asymmetry is connected to the yielding behavior of the platelets. By increasing the stress amplitude we observed that the response becomes more symmetric; however, this strongly depends on the frequency, hence the time necessary for the system to yield. Softening of the response towards the center of the gap was observed by scanning the gap while performing LAOStrain. The structural response at low strain amplitude does not propagate throughout the gap, where as at high strain amplitudes the response in the bulk emerges as erratic. [1] M. P. Lettinga, et al., Non-linear behavior of colloidal platelets in shear flow. Phys. Rev. Lett.(2012) Vol. 109, 246001 [Preview Abstract] |
Wednesday, March 16, 2016 11:27AM - 11:39AM |
L53.00002: Distinctive viscoelastic and viscoplastic nanomechanics of ionically cross-linked polyelectrolyte complexes under intermittent relaxation and creep Biao Han, Tianzhu Ma, Daeyeon Lee, Vivek Shenoy, Lin Han This study aims to reveal unique nanoscale viscoelastic and viscoplastic properties of ionically linked polyelectrolyte networks. Layer-by-layer PAH/PAA complexes were tested by four continuous loading cycles in aqueous solutions. In each cycle, AFM-nanoindentation via a microspherical tip (R$=$5$\mu $m) was applied up to 1$\mu $N force, followed by a 30-60 sec hold at either a constant indentation depth to measure relaxation, or a constant force to measure creep. At a highly cross-linked, net neutral state (0.01M, pH 5.5), instantaneous modulus increased by 2.7-fold from first to last cycle, while the degree of relaxation (\textgreater 95{\%}) remain consistent. These results indicate repeated loading increases local cross-link density, while relaxation is consistently dominated by cross-link breaking and re-formation. In contrast, under creep, modulus increased by a similar 3.5-fold, and degree of creep is significantly attenuated from $\approx $50{\%} to 45{\%} from first to last cycle. Results from creep suggest constant viscous flow of polymer chains in the absence of permanent anchorage. As a result, an irreversible deformation ($\approx $370nm) was observed after multiple creep cycles, suggesting the presence of viscoplasticity. [Preview Abstract] |
Wednesday, March 16, 2016 11:39AM - 11:51AM |
L53.00003: Non-equilibrium Stokes-Einstein relation via active microrheology of hydrodynamically interacting suspensions Henry Chu, Roseanna Zia In our recently developed non-equilibrium Stokes-Einstein relation, we showed that, in the absence of hydrodynamic interactions, the stress in a suspension is given by a balance between fluctuation and dissipation. Here, we generalize our theory for systems of hydrodynamically interacting colloids, via active microrheology, where motion of a Brownian probe through the medium reveals rheological properties. The strength of probe forcing compared to the entropic restoring force defines a Peclet number, \textit{Pe}. In the absence of hydrodynamics, the first normal stress difference and the osmotic pressure scale as \textit{Pe}$^{\mathrm{4}}$ and \textit{Pe}$^{\mathrm{2}}$ respectively when probe forcing is weak, and uniformly as \textit{Pe} for strong probe forcing. As hydrodynamics become important, interparticle forces give way to lubrication interactions. Hydrodynamic coupling leads to a new low-\textit{Pe} scaling of the first normal stress difference and the osmotic pressure as \textit{Pe}$^{\mathrm{2}}$, and high-\textit{Pe} scaling as \textit{Pe}$^{\delta }$, where 0.799$\le \delta \le $1 as hydrodynamics vary from strong to weak. For the entire range of the strength of hydrodynamic interactions and probe forcing, the new phenomenological theory is shown to agree with standard micromechanical definitions of the stress. We further draw a connection between the stress and the energy storage in a suspension, and the entropic nature of such storage is identified. [Preview Abstract] |
Wednesday, March 16, 2016 11:51AM - 12:03PM |
L53.00004: Sticky-probe active microrheology Derek Huang, Roseanna Zia We study the strongly nonlinear flow behavior of a sticky colloidal dispersion via active microrheology, where the motion of a Brownian probe driven by external forces through the suspension is tracked to infer material properties. Most prior work focused on repulsive hard spheres and the influence of Brownian and hydrodynamic forces on rheological behavior, but in many biological suspensions, particles exert attractive forces on one another. Previous attempts to model the effects of particle attractions on sheared suspensions show that interparticle attractions increase suspension stress and viscosity, but these results are limited to weak shearing flows in macroscopic systems. In our microrheological model, probe motion through the suspension distorts the configuration of particles; the P\'{e}clet number, probe forcing compared to thermal forces, gives the extent of this distortion. The equilibrium microstructure and its distortion under probe forcing are also influenced by the strength of interparticle attractions relative to thermal forces. We determine the equilibrium and non-equilibrium microstructure and examine the forcing and attraction contributions to particle motion and suspension stress. [Preview Abstract] |
Wednesday, March 16, 2016 12:03PM - 12:15PM |
L53.00005: Stress diffusion in models for shear banding Elian Masnada, Peter Olmsted Understanding shear banding is of utmost importance from both theoretical and experimental point of view and consequently it has been studied for several decades [1]. Despite this study numerous aspects of shear banding remains poorly understood. Because of the intrinsic inhomogeneity in the shear banded state, applicable constitutive models must be include spatial inhomogeneities, leading to a so-called 'diffusive' term in the equation of motion for the slow variables that carry stress [2,3]. Such terms are also vital in describing the interaction of bulk shear banding flows with walls and incorporation of wall slip. In this work, we consider different sources of 'diffusion' in polymer models in which concentration degrees of freedom are negligible. The simplest models used are consistent with diffusive terms whose origin is intrinsically dissipative, such as due to hydrodynamic interactions. By contrast, models in which elastic effects such as finite chain stiffness contribute to stress diffusion are inconsistent with simple diffusive models, and we propose alternative consistent models. [1] P. D. Olmsted, Rheol. Acta, 47, 283-300 (2008). [2] C.-Y. D. Lu et al, Phys. Rev. Lett., 84, 642 (2000). [3] A. W. El-Kareh and L. G. Leal, J. Non-Newton. Fluid Mech., 33, 257 (1989). [Preview Abstract] |
Wednesday, March 16, 2016 12:15PM - 12:27PM |
L53.00006: Strength of self-pinning in coffee drops. Andrzej Latka, Kimberly Kawczinski, Sidney Nagel The equilibrium contact angle $\theta_e$ of a liquid drop placed on a solid surface is uniquely determined by a balance of surface tension forces according to Young's Equation, yet is rarely observed in real systems. Due to contact angle hysteresis, liquids can make contact with a surface at any angle between the receding and advancing contact angle: $\theta_R<\theta_e<\theta_A$. A particularly striking example of this phenomenon is the familiar coffee stain. For coffee $\theta_R=0$, thus as the drop evaporates the contact line remains pinned at its initial location. This results in the majority of the coffee being deposited in a characteristic ring at the drop's original boundary. We investigate how solid particles suspended in a liquid could so strongly influence contact angle hysteresis, by measuring the receding contact angle of a drop at various times during the evaporation process. For low solute concentrations, $\theta_R$ slowly decreases as the drop evaporates, but remains positive. Surprisingly, we find that increasing the solute concentration results in $\theta_R=0$ and a fully pinned contact line almost immediately after the drop is deposited. [Preview Abstract] |
Wednesday, March 16, 2016 12:27PM - 12:39PM |
L53.00007: Microfluidics of soft granular gels Ryan Nixon, Tapomoy Bhattacharjee, W. Gregory Sawyer, Thomas E. Angelini Microfluidic methods for encapsulating cells and particles typically involve drop making with two immiscible fluids. The main materials constraint in this approach is surface tension, creating inherent instability between the two fluids. We can eliminate this instability by using miscible inner and outer phases. This is achieved by using granular micro gels which are chemically miscible but physically do not mix. These microgels are yield stress materials, so they flow as solid plugs far from shear gradients, and fluidize where gradients are generated -- near an injection nozzle for example. We have found that tuning the yield stress of the material by varying polymer concentration, device performance can be controlled. ~The solid like behavior of the gel allows us to produces infinitely stable jets that maintain their integrity and configuration over long distances and times. These properties can be combined and manipulated to produce discrete particulate bunches of an inner phase, flowing inside of an outer phase, well enough even to print a Morse code message suspended within flow chambers about a millimeter in diameter moving at millimeters a second. [Preview Abstract] |
Wednesday, March 16, 2016 12:39PM - 12:51PM |
L53.00008: Lift-enhanced Electrical Pinched Flow Fractionation for Particle and Cell Separation. Cory Thomas, Andrew Todd, Xinyu Lu, Xiangchun Xuan Pinched flow fractionation (PFF) is a microfluidic technique that utilizes the laminar flow profile in microchannels to continuously separate particles or cells by size. The flow can be either pressure-driven or electric field-driven. We demonstrate in this work that the wall-induced electrical lift force can be exploited to significantly increase the particle or cell displacement in electrical PFF due to its strong size dependence. This enhanced particle and cell separation is implemented by a simple elongation of the pinched segment in electrical PFF. It is demonstrated through both a binary and a ternary separation of polymer particles and biological cells based on surface charge and/or size. We also develop a numerical model to predict and understand this lift-enhanced electrical PFF. [Preview Abstract] |
Wednesday, March 16, 2016 12:51PM - 1:03PM |
L53.00009: Particle Size Effect on Wetting Kinetics of a Nanosuspension Drop: MD Simulations Baiou Shi, Edmund Webb The behavior of nano-fluids, or fluid suspensions containing nanoparticles, has garnered tremendous attention recently for applications in advanced manufacturing. In our previous results from MD simulations, for a wetting system with different advancing contact angles, cases where self-pinning was observed were compared to cases where it was not and relevant forces on particles at the contact line were computed. To advance this work, the roles of particle size and particle loading are examined. Results presented illustrate how particle size affects spreading kinetics and how this connects to dynamic droplet morphology and relevant forces that exist nearby the contact line region. Furthermore, increased particle size in simulations permits a more detailed investigation of particle/substrate interfacial contributions to behavior observed at the advancing contact line. Based on changes in spreading kinetics with particle size, forces between the particle and liquid front are predicted and compared to those computed from simulations. At high loading, particle/particle interactions become relevant and forces computed between particles entrained to an advancing contact line will be presented. [Preview Abstract] |
Wednesday, March 16, 2016 1:03PM - 1:15PM |
L53.00010: Numerical Computation of Mass Transport in Low Reynolds Number Flows and the Concentration Boundary Layer Nicholas A. Licata, Nathaniel J. Fuller Understanding the physical mechanisms by which an individual cell interacts with its environment often requires detailed information about the fluid in which the cell is immersed. Mass transport between the interior of the cell and the external environment is influenced by the flow of the extracellular fluid and the molecular diffusivity. Analytical calculations of the flow field are challenging in simple geometries, and not generally available in more realistic cases with irregular domain boundaries. Motivated by these problems, we discuss the numerical solution of Stokes equation by implementing a Gauss-Seidel algorithm on a staggered computational grid. The computed velocity profile is used as input to numerically solve the advection-diffusion equation for mass transport. Special attention is paid to the case of two-dimensional flows at large P\'eclet number. The numerical results are compared with a perturbative analytical treatment of the concentration boundary layer. [Preview Abstract] |
Wednesday, March 16, 2016 1:15PM - 1:27PM |
L53.00011: Cell mechanics through analysis of cell trajectories in microfluidic channel Samuel Bowie, Alexander Alexeev, Todd Sulchek The understanding of dynamic cell behavior can aid in research ranging from the mechanistic causes of diseases to the development of microfluidic devices for cancer detection. Through analysis of trajectories captured from video of the cells moving in a specially designed microfluidic device, insight into the dynamic viscoelastic nature of cells can be found. The microfluidic device distinguishes cells viscoelastic properties through the use of angled ridges causing a series of compressions, resulting in differences in trajectories based on cell stiffness. Trajectories of cell passing through the device are collected using image processing methods and data mining techniques are used to relate the trajectories to cell properties obtained from experiments. Furthermore, numerical simulation of the cell and microfluidic device are used to match the experimental results from the trajectory analysis. Combination of the modeling and experimental data help to uncover how changes in cellular structures result in changes in mechanical properties. [Preview Abstract] |
Wednesday, March 16, 2016 1:27PM - 1:39PM |
L53.00012: New analysis method for passive microrheology Kengo Nishi, Christoph Schmidt, Fred MacKintosh Passive microrheology is an experimental technique used to measure the mechanical response of materials from the fluctuations of micron-sized beads embedded in the medium. Microrheology is well suited to study rheological properties of materials that are difficult to obtain in larger amounts and also of materials inside of single cells. In one common approach, one uses the fluctuation-dissipation theorem to obtain the imaginary part of the material response function from the power spectral density of bead displacement fluctuations, while the real part of the response function is calculated using a Kramers-Kronig integral. The high-frequency cut-off of this integral strongly affects the real part of the response function in the high frequency region. Here, we discuss how to obtain more accurate values of the real part of the response function by an alternative method using autocorrelation functions. [Preview Abstract] |
Wednesday, March 16, 2016 1:39PM - 1:51PM |
L53.00013: Effect of droplet shape on ring stains from dried liquid Melvin Santiago, Katherine Brown, Harsh Mathur A landmark experimental paper on coffee stains by Deegan et al included a simple theoretical analysis of circular droplets [1]. The analysis was based on a model informally called the Maxwell House equations. It describes the evolving height profile of the droplet, the evaporation of the solvent and the outflow of solute to the rim of the droplet. Since typical droplets are not circles, here we extend the analysis to more general shapes. We find that for thin droplets the height profile may be determined by solving Poisson's equation in a domain corresponding to the footprint of the droplet. Evaporation is treated in a simple approximation via an electrostatic analogy and is dominated by the sharp edges of the droplet. Assuming zero vorticity allows us to analyze the solvent flow in droplets of arbitrary shape. We compare circular droplets to other shapes including long linear droplets, ring shaped droplets and droplets with an elliptical footprint. [1] R.D. Deegan et al, Nature 389, 827 (1997). [Preview Abstract] |
Wednesday, March 16, 2016 1:51PM - 2:03PM |
L53.00014: Lie Algebraic Analysis of Thin Film Marangoni Flows: Multiplicity of Self-Similar Solutions Zachary Nicolaou, Sandra Troian The rapid advance of an insoluble surfactant monolayer on a thin liquid film of higher surface tension is controlled by distinct flow regimes characterized by the relative strength of viscous, Marangoni and capillary forces. Such flows play a critical role in human pulmonary and ocular systems. During the past quarter century, researchers have focused exclusively on self-similar solutions to the governing pair of nonlinear PDEs for the film thickness, $H(r/t^a)$, and surface concentration, $\Gamma(r/t^a)/t^b$, in the limit where the Marangoni or capillary terms vanish, where $r$ denotes the spatial variable, $t$ is time, and $a$ and $b$ are fractional exponents. Using Lie algebraic techniques, we demonstrate for the first time the existence of several embedded symmetries in this system of equations which yield multiple self-similar solutions describing more complex scaling behavior, even when all three forces are incorporated. A special and previously unrecognized subset of these solutions reveals the dynamical behavior of film thinning and surfactant distribution near the origin, which ultimately meters the downstream flow. Finite element simulations confirm the suite of scaling exponents obtained analytically. [Preview Abstract] |
Wednesday, March 16, 2016 2:03PM - 2:15PM |
L53.00015: Convective flows generated by evaporation: experiments, linear stability analysis and numerical simulations Jocelyn Dunstan, Kyoung Jin Lee, Simon Park, Raymond E. Goldstein A novel form of convection was observed in a suspension of non-motile \emph{Photobacterium phosphoreum} bacteria. The pattern resembles classical bioconvection, however this strain has limited if any motility, which excludes this possible explanation. After performing a series of control experiments we found that the convection was actually driven by the evaporation of the salty bacterial medium, and the same kind of plumes were observed using polystyrene beads suspended in water with salt added. A mathematical model was formulated for the process and studied using a linear stability analysis and finite element method simulations, reproducing most of the observed experimental features. From the linear stability analysis, a threshold in salt concentration to observe convective motion was obtained, as well as the wavelength of the pattern at the onset of the instability. This was complemented by finite element simulations, which produced plume dynamics remarkably similar to the experimental observations. Evaporation-driven convection on the millimeter scale has not been studied extensively, and its effect may have been underestimated in other experiments. [Preview Abstract] |
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