Flow of Deformable Droplets: Discontinuous Shear Thinning and Velocity Oscillations

Abstract

We study the rheology of a suspension of soft deformable droplets subjected to a pressure-driven flow. Through computer simulations, we measure the apparent viscosity as a function of droplet concentration and pressure gradient, and provide evidence of a discontinuous shear thinning behavior, which occurs at a concentration-dependent value of the forcing. We further show that this response is associated with a nonequilibrium transition between a "hard" (or less deformable) phase, which is nearly jammed and flows very slowly, and a "soft" (or more deformable) phase, which flows much more easily. The soft phase is characterized by flow-induced time dependent shape deformations and internal currents, which are virtually absent in the hard phase. Close to the transition, we find sustained oscillations in both the droplet and fluid velocities. Polydisperse systems show similar phenomenology but with a smoother transition, and less regular oscillations.

Discrete fluidization of dense monodisperse emulsions in neutral wetting microchannels

“…These considerations suggest that the fluidization transition occurs when the imposed pressure difference is such that σ w ∼ σ Y , i.e. when the maximum imposed stress across the material equals its yield value, lending to the idea that the origin of the discontinuity can be ascribed to local yielding events, rather than the discontinuous shear-thinning events proposed in 21 .…”

Novel nonequilibrium steady states in multiple emulsions

“…Here we illustrate the physics and the modeling of a compound emulsion made of a suspension of immiscible fluid droplets encapsulated in larger drop. Such droplets are described by using a multi-phase field approach [41,42,44], in which a set of scalar phase-field variables φ i (r, t), i = 1, ...., N (where N is the total number of droplets) accounts for the density of each droplet, while a vector field v(r, t) describes the underlying fluid velocity.…”

“…By following previous studies [41,42], an approximate mapping between simulations units and physical ones can be obtained by assuming a droplet of diameter roughly equal to 10 2 µm immersed in a background fluid of viscosity 10 −2 P a·s (assumed, for simplicity, equal to the viscosity of the fluid inside the droplet) and in which the surface tension σ, equal to 0.08 (for k = 0.1) in simulations, corresponds to ∼ 0. quantity capturing droplet deformation is the capillary number Ca = vη σ , measuring the strength of the viscous forces relative to the surface tension. If, for example, v = 0.01, Ca ∼ 0.2 (with k = 0.1).…”

“…In Table I we provide an approximate mapping between simulation parameters and real units. Parameters have been chosen in agreement with values reported in previous simulation works, such as in [8,41,42]. Realistic values can be obtained by fixing the lenght scale, the time scale and the force scale as: L = 1µm, T = 10µs and F = 100nN (in simulations units these scales are all equal to one).…”

The influence of an applied magnetic field on the self-assembly of magnetic nanogels

“…Micro-and nanogel particles are one of the most interesting approaches to create smart materials, as they can be made responsive to many different stimuli, including pH [5,6], temperature [5,7,8], electromagnetic radiation [9], ionic strength or electric fields [7,10]. Their responses include large structural changes, typically swelling/collapse transitions, and a complex rheology [11,12]. Such a plethora of responsive possibilities became available in recent years thanks to the rapid development of synthesis techniques [3,4,13,14].…”