Session P11: Surface Tension Effects: General (3:10pm - 3:55pm CST)

Surface Tension Effects in Oscillatory Squeeze Flow Rheometry

We report experiments, numerical simulations, and analytical models of the time-dependent force exerted by low- and high-viscosity fluids undergoing oscillatory squeezing between two parallel plates in a commercial extensional rheometer. At high oscillation frequencies and high fluid viscosities, the force is driven primarily by viscous shear stresses according to standard predictions of lubrication theory. We show that surface tension modifies the force response at low frequencies and viscosities, resulting in an ''apparent elasticity'' akin to a Kelvin-Voigt solid. The latter response is highly sensitive to the film aspect ratio and strain amplitude of oscillation. Using a combination of semi-analytical theory and finite element simulations, we are able to reproduce the force response seen in experiments over six decades in the modified capillary number (ratio of oscillatory viscous stresses to surface-tension stresses). The overall impact of this work is twofold. First, surface-tension effects oftentimes emerge as an unwanted artifact in linear and nonlinear viscoelastic measurements of bulk material properties. Our analysis precisely isolates and quantifies this undesired contribution and, therefore, may be used to properly interpret measurements of low-viscosity fluids at low frequencies. Second, the agreement between our experimental measurements and models suggest the use of oscillatory squeezing as a method of measuring the surface tension or, more generally, the dynamic surface properties of liquids.   

Particle deposition in a capillary tube during the translation of a suspension slug

The translation of a liquid slug in a capillary tube leads to coating a thin film on the tube's inner wall. When particles or contaminants are present in the liquid, they can be deposited and contaminate the tube if the liquid film is thick enough. We investigated experimentally the condition under which particles are deposited by initially filling a capillary tube with a particulate suspension, and then expelling it at a prescribed velocity. We observed that the entrainment of particles in the liquid film is controlled by the ratio of the particle, the capillary tube radii and the capillary number associated with the liquid/air interface velocity. A model that accounts for these observations is developed and suggests optimal operating conditions to avoid contamination during withdrawal of a particulate suspension. This deposition mechanism can also be leveraged to developing new coating methods by depositing particles on the inner walls of channels.   

Microgel Interactions over Full Temperature Range

An effective interaction potential unifying the behavior of thermoresponsive microgel particles across the phase transition temperature is a fundamental yet unsolved question in soft matter physics. Using coarse-grained simulations, we quantitatively probed the pair interactions between microgels having a lower critical solution temperature. We find that the polymer-solvent interfacial tension is indispensable for capturing microgel interactions at the critical temperature and higher. Once contacting each other, microgels act like soft repulsive particles at low temperatures. In contrast, an effective attraction and the formation of a connecting gel bridge were observed at contact between shrunken microgels at high temperatures. We propose an analytical model that couples an inverse power law repulsion and temperature-dependent surface energy to unify microgel interactions over the full range of temperatures and overlapping distances. The effective interaction represents a competition between the elastic repulsion from polymer deformation and the surface energy induced attraction. The new elastocapillary model agrees well with the simulation results and overcomes limitations of traditional models that consider only elastic response of soft particles.   

Theoretical modeling of surfers on a vibrating bath

We present a theoretical investigation into the dynamics of capillary-scale objects surfing on their own self-generated wave field. Our study is motivated by a newly discovered system consisting of superhydrophobic objects floating on a vertically vibrating fluid bath. Experiments have demonstrated that such ``surfers" propagate along the fluid interface, and that multiple surfers may self-organize through their wave-induced interactions. Our theoretical model consists of coupled equations for the surfers' positional and orientational dynamics, in which a surfer is modeled as a source of capillary waves. The model predictions exhibit good agreement with experimentally observed interaction modes between two surfers. Generally, this work shows that the surfer system is amenable to quantitative theoretical modeling, and thus constitutes a promising platform for constructing and validating new theories of interfacial active matter.   

Collapse of a Granular Raft -- Experiments and Modeling

Particle-laden fluid interfaces exhibit an interplay between interfacial energy, which minimizes the surface area, and steric repulsion between particles, which provides a constraint on the minimum area. We experimentally and theoretically investigate the behavior of an initially flat fluid-fluid interface covered in rigid passive particles under isotropic compression. Specifically, we investigate spherical glass particles with a diameter in the range of 0.1 mm to 2 mm deposited on a water-air or water-oil interface inside a conical funnel. We impose axisymmetric compression of the raft by lowering the water level inside the funnel. For small particles or large density differences between the two fluids, single particles are expelled from the raft under compression. However, for larger particles or small density differences between the two fluids, the raft forms a collective crease under compression. Based on these observations, we construct a model for the deformation of the interface and the redistribution of particles along the interface, and use this model to predict these two failure modes.