Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Q46: Rheology and Flow of Soft Materials |
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Sponsoring Units: GSOFT Chair: Roseanna Zia, Cornell University Room: 217A |
Wednesday, March 4, 2015 2:30PM - 2:42PM |
Q46.00001: Dynamic force measurement of rearrangements in a 2D network of droplets Solomon Barkley, Matilda Backholm, Kari Dalnoki-Veress The interaction between two liquid droplets in an immiscible liquid is well understood. However, the emulsions relevant to biological and industrial processes involve high concentrations of these droplets, and multi-body effects cannot be ignored. As droplets rearrange in response to a disturbance, the importance of individual pair-wise interactions between droplets changes with the geometry of neighbours. Here we report on an experimental setup consisting of a two- dimensional network of monodisperse droplets stabilized with a surfactant. The system is studied with micropipette deflection, which permits direct measurement of forces along with simultaneous imaging of the droplet network. One micropipette is used to apply a tensile or compressive force to the droplet cluster, while a second pipette acts as a force-transducing cantilever, deflecting in response to rearrangements of the droplets. [Preview Abstract] |
Wednesday, March 4, 2015 2:42PM - 2:54PM |
Q46.00002: Precise measurements of droplet-droplet contact forces in quasi-2D emulsions Janna Lowensohn, Carlos Orellana, Eric Weeks We use microscopy to visualize a quasi-2D oil-in-water emulsion confined between two parallel slides. We then use the droplet shapes to infer the forces they exert on each other. To calibrate our force law, we set up an emulsion in a tilted sample chamber so that the droplets feel a known buoyant force. By correlating radius of the droplet and length of contacts with the buoyant forces, we validate our empirical force law. We improve upon prior work in our lab by using a high-resolution camera to image each droplet multiple times, thus providing sub-pixel resolution and reducing the noise. Our new technique identifies contact forces with only a 1{\%} uncertainty, five times better than prior work. We demonstrate the utility of our technique by examining the normal modes of the droplet contact network in our samples. [Preview Abstract] |
Wednesday, March 4, 2015 2:54PM - 3:06PM |
Q46.00003: Fluidization of a bubble raft under oscillatory compression Klebert Feitosa, Nicholas Hagans, Christine O'Dea Fluidization of two-dimensional foam is characterized by rearrangement events known as T1-events where clusters of four bubbles switch neighbors. We study rearrangement events in a bubble raft subject to periodic compression by an oscillating boundary. As the amplitude of oscillation increases, T1-events transition from being mostly reversible (elastic regime), to being increasingly irreversible (plastic regime). In addition, T1-events are found to occur most frequently right before the direction of oscillation reverses, where the stress is maximum. By contrast, the velocity field of the bubble raft shows strong dynamical heterogeneity after the direction of oscillation reverses and the stress relaxes. [Preview Abstract] |
Wednesday, March 4, 2015 3:06PM - 3:18PM |
Q46.00004: Dynamical and structural transitions in periodically-driven emulsions: Reversibility loss and random hyper-unifom organization Joost H. Weijs, Rapha\"el Jeanneret, R\'emi Dreyfus, Denis Bartolo We present experiments and numerical simulations of a microfluidic echo process, in which a large number of droplets interact in a periodically driven viscous fluid [Jeanneret \& Bartolo, Nature Comm. {\bf 5}, 3474 (2013)]. Upon increasing the driving amplitude we demonstrate the collective reversibility loss of the droplet dynamics. In addition we show that this genuine dynamical phase transition is associated with a structural one: at the onset of irreversibility the droplet ensemble self-organises into a random hyperuniform state. Numerical simulations evidence that the purely reversible hydrodynamic interactions together with hard-core repulsion account for most of our experimental findings. Hyperuniformity is relevant for the production of large-band-gap materials, but are difficult to construct both numerically and experimentally. The hydrodynamic echo-process may provide a robust, fast, and simple way to produce hyper uniform structures over a wide range of packing fractions. [Preview Abstract] |
Wednesday, March 4, 2015 3:18PM - 3:30PM |
Q46.00005: The Versatile Elastohydrodynamics of a Free Particle near a Thin Soft Wall Thomas Salez, Baudouin Saintyves, L. Mahadevan We address the free motion of a buoyant particle inside a viscous fluid, in the vicinity of a thin compressible elastic wall. After discussing the main scalings, we obtain analytically the dominant drag forces within the soft lubrication approximation. By including those into the equations of motion of the particle, we establish a general governing system of three coupled nonlinear and singular differential equations, that describe the three essential motions: sedimentation, hydroplaning, and hydrospinning, through four dimensionless control parameters. Numerical integration allows us to predict a wide zoology of exotic solutions -- despite the low-Reynolds feature of the flow -- including: spontaneous oscillation, Magnus-like effect, enhanced sedimentation, and boomerang-like effect. We compare these predictions to experiments. The presented elementary approach could be of interest in the description of a broad variety of elastohydrodynamical phenomena, including: landslides, ageing of cartilaginous joints, and motion of a cell in a microfluidic channel or in a blood vessel. [Preview Abstract] |
Wednesday, March 4, 2015 3:30PM - 3:42PM |
Q46.00006: Dual-probe active microrheology Benjamin Dolata, Roseanna N. Zia Microrheology has revolutionized the study of microscopically small systems, whereby a Brownian probe particle is monitored as it travels through a complex fluid and its motion is tracked to infer properties of the embedding medium. A range of applications enables study of various materials and flow regimes: in passive microrheology probe diffusion is related to linear viscoelastic properties via a Stokes-Einstein relation, but precludes study of networked materials. Dual-probe passive microrheology overcomes this limitation in some cases.$^{\, }$\textit{But these techniques are restricted to linear-response properties}. In \textit{active }microrheology a probe is driven through the medium, and reveals \textit{strongly non-equilibrium }rheology, but is limited to dispersed systems. We have developed a new model for the microscale interrogation of general complex fluids: \textit{Dual-probe Active Microrheology. }Via a combination of asymptotic and numerical solutions to the Smoluchowski equation, we have computed the microstructural and rheological response of a colloidal dispersion to the motion of two probes driven with forces ranging from strong to weak, at arbitrary separations and orientations to their lines of centers. The interactive force between the probes and the colloids reveals a novel non-equilibrium repulsive interaction which we connect to nonlinear rheology. [Preview Abstract] |
Wednesday, March 4, 2015 3:42PM - 3:54PM |
Q46.00007: The impact of hydrodynamics on stress formation, relaxation, and memory in colloidal dispersions: Transient, nonlinear microrheology Ritesh P. Mohanty, Roseanna N. Zia In active microrheology, a probe is driven through a complex medium. Most work thus far has focused on steady behavior and established the relationship between the microstructure, probe speed, and rheology. But important information about structural development and relaxation are captured by startup and cessation of flows in the non-linear regime, where the structure is driven far from equilibrium. Here we study theoretically the rate of stress formation and relaxation under non-linear microrheological forcing of hydrodynamically interacting colloids. We study the behavior analytically in the dual limits of weak and strong probe forcing and weak and strong hydrodynamic interactions and numerically in between. To elucidate the detailed role of hydrodynamic, Brownian, and interparticle forces in stress formation and relaxation, we employ an excluded annulus model to introduce each systematically, and study the rheological and structural response for arbitrary forcing and strength of hydrodynamic interactions. Hydrodynamics introduce an additional mode of dissipation, which manifests as a reduction in the rate of stress formation during startup. While this non-equilibrium contribution vanishes instantly upon flow shutoff, a delicate interplay between Brownian and interparticle forces influences relaxation, revealing multiple relaxation modes. The recovery of entropically stored energy is studied. [Preview Abstract] |
Wednesday, March 4, 2015 3:54PM - 4:06PM |
Q46.00008: Rheology and Dynamics of Colloidal Superballs John Royer, George Burton, Daniel Blair, Steven Hudson Relatively little is known about the role particle shape plays in the dynamics of colloidal suspensions, particularly at higher packing densities where particle interactions and changes in the microstructure become increasingly important. We examine the role of particle shape by characterizing both the bulk rheology and micro-scale diffusion in a suspension of pseudo-cubic silica superballs. Varying the packing density $0 \leq \phi \leq 0.42$, we compare the high-shear viscosity and long-time self-diffusion coefficient $D_L(\phi)$ to established hard-sphere results. In dilute suspensions the superball viscosity is nearly indistinguishable from the that of hard spheres, indicating that the individual superball hydrodynamics are not dramatically different. However, there is a significant difference in the diffusion, with the superball $D_L(\phi)$ decreasing faster with increasing $\phi$. Looking at the suspension microstructure, we find that while the hard sphere pair distribution $g(r)$ jumps to a finite value at contact $r=2a$, the superball $g(r)$ is shifted to higher distances. This suggests a simple rescaling $\phi \rightarrow \phi_{eff}$ defined by the minimal sphere needed to enclose the superballs, which roughy collapses the diffusion results. [Preview Abstract] |
Wednesday, March 4, 2015 4:06PM - 4:18PM |
Q46.00009: Colloidal transport and diffusion over a tilted periodic energy landscape Xiaoguang Ma, Pik-Yin Lai, Bruce Ackerson, Penger Tong A tilted two-layer colloidal system is constructed to study force-assisted barrier-crossing dynamics over a periodic energy landscape. The energy landscape is provided by the bottom layer colloidal spheres forming a fixed crystalline pattern on a glass substrate. The corrugated surface of the bottom colloidal crystal provides a gravitational potential field for the top layer diffusing particles. By tilting the sample at an angle with respect to the direction of gravity, a tangential component of the gravitational force F is applied to the diffusing particles. The measured mean drift velocity v(F,E) and diffusion coefficient D(F,E) of the particles as a function of F and energy barrier height E agree well with the exact solution of the one-dimensional Langevin equation. From the exact solution we show analytically and verify experimentally that there exists a scaling region, in which v and D both scale as a(F)exp[-E*(F)/k$_{\mathrm{B}}$T], where the Arrhenius pre-factor a(F) and effective barrier height E*(F) are both modified by F. The experiment demonstrates the applications of this model system in evaluating different scaling forms of a(F) and E*(F) and their accuracy, in order to extract useful energetic information. [Preview Abstract] |
Wednesday, March 4, 2015 4:18PM - 4:30PM |
Q46.00010: Colloidal diffusion over a random landscape Yun Su, Xiao-guang Ma, Pik-Yin Lai, Penger Tong A two-dimensional quenched random energy landscape is generated by using a randomly packed layer of colloidal spheres of two different sizes fixed on a glass substrate. A number of monodisperse particles diffuse on the top of the first layer particles. The diffusing particles in water feel the gravitational energy landscape U(x,y) generated by the modulated surface of the first layer particles. The trajectories of the particles are obtained by optical microscopy and particle tracking. The energy landscape U(x,y) is obtained from the measured population histogram P(x,y) of the diffusing particles via the Boltzmann distribution, P(x,y) $=$exp[-U(x,y)/ k\textunderscore BT], where k\textunderscore B T is the thermal energy of the particles. The distribution of the energy barrier heights is obtained from the measured U(x,y). From the particle's trajectories, we obtain the dynamical properties of the diffusing particles over the random energy landscape, such as the mean square displacement and distribution of the escape time across the energy barriers. A quantitative relationship between the long-time diffusion coefficient and the random energy landscape is found experimentally, which is in good agreement with the theoretical prediction. *Work supported in part by the Research Grants Council of Hong Kong SAR. [Preview Abstract] |
Wednesday, March 4, 2015 4:30PM - 4:42PM |
Q46.00011: Inertial flow regimes of the suspension of finite size particles Iman Lashgari, Francesco Picano, Luca Brandt We study inertial flow regimes of the suspensions of finite size neutrally buoyant particles. These suspensions experience three different regimes by varying the Reynolds number, $Re$, and particle volume fraction, $\Phi$\footnote{I. Lashgari, F. Picano, W-P. Breugen and L. Brandt, {\emph{Arxiv}:1402.3088}}. At low values of $Re$ and $\Phi$, flow is laminar-like where viscous stress is the dominating term in the stress budget. At high $Re$ and relatively small $\Phi$, the flow is turbulent-like where Reynolds stress has the largest contribution to the total stress. At high $\Phi$, the flow regime is as a form of inertial shear-thickening characterized by a significant enhancement in the wall shear stress not due to the increment of Reynolds stress but to the particle stress. We further analyze the local behavior of the suspension in the three different regimes by studying the particle dispersion and collisions. Turbulent cases shows higher level of particle dispersion and higher values of the collision kernel (the radial distribution function times the particle relative velocity as a function of the distance between the particles) than those of the inertial shear-thickening regimes providing additional evidence of two different transport mechanisms in the Bagnoldian regime. [Preview Abstract] |
Wednesday, March 4, 2015 4:42PM - 4:54PM |
Q46.00012: Flow mechanism of colloidal solutions under shear revealed by neutron scattering and simulation Xin Li, Wei-Ren Chen, Luis Sanchez-Diaz, Yun Liu, Lionel Porcar, William Hamilton, Changwoo Do, Takuya Iwashita, Takeshi Egami, Kao-Hsiang Liu Using small angle neutron scattering technique and Brownian dynamics simulation we investigate the effect of external steady shear on the concentrated solutions of silica particles in the shear thinning region. Three dimensional anisotropic structure factors are obtained as a function of shear rate. Accordingly the evolution of local topology, defined by the colloidal connectivity, is revealed by the variation of local strain. We further determine the elastic responsive length scale of the colloidal systems via characterizing the quantitative dependence of the correlation length on the strain rate. [Preview Abstract] |
Wednesday, March 4, 2015 4:54PM - 5:06PM |
Q46.00013: Steady-state flow properties of amorphous materials Vikram Jadhao, Thomas O'Connor, Mark Robbins Molecular dynamics (MD) simulations are used to investigate the steady-state shear flow curves of a standard glass model: the bidisperse Lennard-Jones system. For a wide range of temperatures in the neighborhood of the glass transition temperature $T_\textrm{g}$ predicted by the mode coupling theory, we compute the steady-state shear stress and viscosity as a function of the shear rate $\dot{\gamma}$. At temperatures near and above $T_{\textrm{g}}$, the stress crosses over from linear Newtonian behavior at low rates to power law shear-thinning at high rates. As T decreases below $T_{\textrm{g}}$, the stress shows a plateau, becoming nearly rate-independent at low $\dot{\gamma}$. There is a weak increase in stress that is consistent with Eyring theory for activated flow of a solid. We find that when the strain rate is reduced to extremely low values, Newtonian behavior appears once more. Insights gained from these simulations are applied to the computation of flow curves of a well-established boundary lubricant: squalane. In the elastohydrodynamic regime, squalane responds like a glassy solid with an Eyring-like response, but at low rates it has a relatively small Newtonian viscosity. [Preview Abstract] |
Wednesday, March 4, 2015 5:06PM - 5:18PM |
Q46.00014: X-ray photon correlation spectroscopy studies of structural irreversibility in a colloidal gel subjected to oscillatory shear flow Mu Sung Kweon, Wesley Burghardt, Subramanian Ramakrishnan, Golda Louis, Danica Thomas, Suresh Narayanan X-ray photon correlation spectroscopy (XPCS) is used to probe the microscopic structural reversibility in a colloidal gel subjected to oscillatory shear flow. Silicon dioxide particles in decalin aggregate into a gel structure as a result of depletion interactions associated with dissolved polystyrene molecules. XPCS studies on aged quiescent gels show negligible structural dynamics on time scales of tens of seconds. Such samples were subjected to oscillatory shear with varying stain amplitude using a rheometer installed in the XPCS beam line and x-ray capable polycarbonate fixtures; this enable simultaneous rheological measurements during the XPCS experiment. In the presence of unidirectional shear flow, the decay of the XPCS autocorrelation function is dominated by the convective motion induced by the applied deformation. In oscillatory shearing of samples in the absence of significant structural relaxation, the autocorrelation function becomes periodic, returning to its initial value once every oscillation period. At higher strains, irreversible motions at the microscopic level lead to decay in the 'echos' of the autocorrelation function. Interestingly, structural irreversibility is detected by XPCS only at strains that are significantly higher than those at which nonlinearity [Preview Abstract] |
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