The swimming behaviour of microorganisms can be strongly influenced by the rheology of their fluid environment. In this manuscript, we experimentally investigate the effects of shear-thinning viscosity on the swimming behaviour of an undulatory swimmer, the nematode Caenorhabditis elegans. Tracking methods are used to measure the swimmer's kinematic data (including propulsion speed) and velocity fields. We find that shear-thinning viscosity modifies the velocity fields produced by the swimming nematode but does not modify the nematode's speed and beating kinematics. Velocimetry data show significant enhancement in local vorticity and circulation, and an increase in fluid velocity near the nematode's tail, compared to Newtonian fluids of similar effective viscosity. These findings are compared to recent theoretical and numerical results. * parratia@seas.upenn.edu arXiv:1407.5854v2 [physics.flu-dyn]

Motivated by the observed coordination of nearby beating cilia, we use a scale model experiment to show that hydrodynamic interactions can cause synchronization between rotating paddles driven at constant torque in a very viscous fluid. Synchronization is only observed when the shafts supporting the paddles have some flexibility. The phase difference in the synchronized state depends on the symmetry of the paddles. We use the method of regularized Stokeslets to model the paddles and find excellent agreement with the experimental observations. We also use a simple analytic theory based on far-field approximations to derive scaling laws for the synchronization time as a function of paddle separation.

Numerous natural processes are contingent on microorganisms' ability to swim through fluids with non-Newtonian rheology. Here, we use the model organism Caenorhabditis elegans and tracking methods to experimentally investigate the dynamics of undulatory swimming in shear-thinning fluids. Theory and simulation have proposed that the cost of swimming, or mechanical power, should be lower in a shear-thinning fluid compared to a Newtonian fluid of the same zero-shear viscosity. We aim to provide an experimental investigation into the cost of swimming in a shear-thinning fluid from (i) an estimate of the mechanical power of the swimmer and (ii) the viscous dissipation rate of the flow field, which should yield equivalent results for a self-propelled low Reynolds number swimmer. We find the cost of swimming in shear-thinning fluids is less than or equal to the cost of swimming in Newtonian fluids of the same zero-shear viscosity; furthermore, the cost of swimming in shear-thinning fluids scales with a fluid's effective viscosity and can be predicted using fluid rheology and simple swimming kinematics. Our results agree reasonably well with previous theoretical predictions and provide a framework for understanding the cost of swimming in generalized Newtonian fluids. * parratia@seas.upenn.edu arXiv:1610.05811v1 [physics.flu-dyn]

Droplet deposition onto a hydrophobic surface is studied experimentally and numerically. A wide range of droplet sizes can result from the same syringe, depending strongly on the needle retraction speed. Three regimes are identified according to the motion of the contact line. In Region I, at slow retraction speeds, the contact line expands and large droplets can be achieved. In Region II, at moderate needle speeds, a quasi-cylindrical liquid bridge forms resulting in drops approximately the size of the needle. Finally, at high speeds (Region III), the contact line retracts and droplets much smaller than the syringe diameter are observed. Scaling arguments are presented identifying the dominant mechanisms in each regime. Results from nonlinear numerical simulations agree well with the experiments, although the accuracy of the predictions is limited by inadequate models for the behavior of the dynamic contact angle. [5,6]. The process is, at first glance, straightforward and is initiated by the formation of a liquid bridge between the substrate and a dispensing syringe. As the syringe retreats, the liquid bridge stretches, grows and breaks, leaving a drop on the substrate. A seemingly simple question can be askedhow does the drop size depend on the syringe geometry, speed and the fluid properties? A comprehensive answer must consider the stability of the liquid bridge and the physics of the moving contact line at the liquid-air-solid interface -both difficult problems. Theoretical studies of liquid bridge stability date back to Rayleigh [7], and have been extended to include gravity and non-cylindrical geometries [8,9]. In addition, the nonlinear dynamics have been solved numerically, using both 2-D (axisymmetric) [10] and 1-D (slender-jet) [11,12] models. Previous work has concentrated on geometries in which the contact line is pinned at both ends of the liquid bridge [12,13], and there are only a few results that couple the liquid bridge with a moving contact line [14,15]. A possible reason for this is the difficulty in solving the flow near the contact line where the continuum equations are invalid [16,17] and a microscopic description must be imposed (e.g. [18]). In this letter, we focus on the physics of drop dispensing on a flat, smooth, hydrophobic substrate in which the contact line is free to move and is inherently coupled with the liquid bridge stability. Experiments and numerical simulations are used to identify a range of complex flow phenomena which enable the deposited drop size to vary by two orders of magnitude as the syringe retraction speed is changed.In our experiment, a stainless steel syringe (typical radius, R = 200µm) is mounted vertically on a computercontrolled stage. The syringe is connected by a small tube to a 10cc barrel mounted on the same stage. This configuration maintains a constant hydrostatic head, H, at the syringe tip (H ∼ 4cm). The fluid (a 85-15 mixture by volume of glycerol and water) has viscosity µ = 84 cP and surface tension γ = 0.063 N/m. The fluid exhibits a static co...