Gollub, Jerry P.
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Publication The effects of polymer molecular weight on filament thinning and drop breakup in microchannels(2009-11-04) Arratia, Paulo E.; Gollub, Jerry P.; Durian, Douglas J.; Cramer, L-AWe investigate the effects of fluid elasticity on the dynamics of filament thinning and drop breakup processes in a cross-slot microchannel. Elasticity effects are examined using dilute aqueous polymeric solutions of molecular weight (MW) ranging from 1.5×103 to 1.8×107. Results for polymeric fluids are compared to those for a viscous Newtonian fluid. The shearing or continuous phase that induces breakup is mineral oil. All fluids possess similar shear-viscosity (~0.2 Pa s) so that the viscosity ratio between the oil and aqueous phases is close to unity. Measurements of filament thickness as a function of time show different thinning behavior for the different aqueous fluids. For Newtonian fluids, the thinning process shows a single exponential decay of the filament thickness. For low MW fluids (103, 104 and 105), the thinning process also shows a single exponential decay, but with a decay rate that is slower than for the Newtonian fluid. The decay time increases with polymer MW. For high MW (106 and 107) fluids, the initial exponential decay crosses over to a second exponential decay in which elastic stresses are important. We show that the decay rate of the filament thickness in this exponential decay regime can be used to measure the steady extensional viscosity of the fluids. At late times, all fluids cross over to an algebraic decay which is driven mainly by surface tension.Publication Measuring Oscillatory Velocity Fields Due to Swimming Algae(2011-09-30) Guasto, Jeffrey S.; Johnson, Karl A.; Gollub, Jerry P."Single cells exhibit a diverse array of swimming strategies at low Reynolds number to search for nutrients, light, and other organisms. The fluid flows generated by their locomotion are important to understanding biomixing and interactions between cells in suspension..."Publication Transport of Finite-Sized Particles in Chaotic Flow(2008-10-24) Ouellette, Nicholas T.; O'Malley, P.J. J.; Gollub, Jerry P.By extending traditional particle tracking techniques, we study the dynamics of neutrally buoyant finite-sized particles in a spatiotemporally chaotic flow. We simultaneously measure the flow field and the trajectories of millimeter-scale particles so that the two can be directly compared. While the single-point statistics of the particles are indistinguishable from the flow statistics, the particles often move in directions that are systematically different from the underlying flow. These differences are especially evident when Lagrangian statistics are considered.Publication Microfluidic Rheology of Soft Colloids above and below Jamming(2010-10-21) Nordstrom, Kerstin N.; Arratia, Paulo E; Basu, Anindita; Gollub, Jerry P.; Verneuil, E.; Durian, Douglas J.; Zhang, Zheng; Yodh, Arjun G.The rheology near jamming of a suspension of soft colloidal spheres is studied using a custom microfluidic rheometer that provides the stress versus strain rate over many decades. We find non-Newtonian behavior below the jamming concentration and yield-stress behavior above it. The data may be collapsed onto two branches with critical scaling exponents that agree with expectations based on Hertzian contacts and viscous drag. These results support the conclusion that jamming is similar to a critical phase transition, but with interaction-dependent exponents.Publication Dynamical Heterogeneity in Soft-Particle Suspensions Under Shear(2011-08-22) Gollub, Jerry P.; Nordstrom, Kerstin N.; Durian, Douglas J.We present experimental measurements of dynamical heterogeneities in a dense system of microgel spheres, sheared at different rates and at different packing fractions in a microfluidic channel, and visualized with high-speed digital video microscopy. A four-point dynamic susceptibility is deduced from video correlations, and is found to exhibit a peak that grows in height and shifts to longer times as the jamming transition is approached from two different directions. In particular, the time for particle-size root-mean square relative displacements is found to scale as τ*∼(γΔφ4)−1, where γ is the strain rate and Δφ=|φ−φc| is the distance from the random close-packing volume fraction. The typical number of particles in a dynamical heterogeneity is deduced from the susceptibility peak height and found to scale as n*∼(γΔφ4)−0.3. Exponent uncertainties are less than ten percent. We emphasize that the same power-law behavior is found at packing fractions above and below φc. Thus our results considerably extend a previous observation of n*∼γ−0.3 for granular heap flow at fixed packing below φc. Furthermore, the implied result n*∼(τ*)0.3 compares well with the expectation from mode-coupling theory and with prior observations for driven granular systems.Publication Oscillatory Flows Induced by Microoganisms Swimming in Two Dimensions(2010-01-01) Guasto, Jeffrey S; Johnson, Karl A; Gollub, Jerry PWe present the first time-resolved measurements of the oscillatory velocity field induced by swimming unicellular microorganisms. Confinement of the green alga C. reinhardtii in stabilized thin liquid films allows simultaneous tracking of cells and tracer particles. The measured velocity field reveals complex time-dependent flow structures, and scales inversely with distance. The instantaneous mechanical power generated by the cells is measured from the velocity fields and peaks at 15 fW. The dissipation per cycle is more than 4 times what steady swimming would require.Publication Teaching About Fluids(2008-10-01) Gollub, Jerry P.Publication Dynamic Topology in Spatiotemporal Chaos(2008-06-30) Ouellette, Nicholas T.; Gollub, Jerry P.By measuring the tracks of tracer particles in a quasi-two-dimensional spatiotemporally chaotic laboratory flow, we determine the instantaneous curvature along each trajectory and use it to construct the instantaneous curvature field. We show that this field can be used to extract the time-dependent hyperbolic and elliptic points of the flow. These important topological features are created and annihilated in pairs only above a critical Reynolds number that is largest for highly symmetric flows. We also study the statistics of curvature for different driving patterns and show that the curvature probability distribution is insensitive to the details of the flow.Publication Detecting Topological Features of Chaotic Fluid Flow(2008-12-31) Ouellette, Nicholas T; Gollub, Jerry P.