Discrete fluidization of dense monodisperse emulsions in neutral wetting microchannels

Abstract

The rheology of pressure-driven flows of two-dimensional dense monodisperse emulsions in neutral wetting microchannels is investigated by means of mesoscopic lattice simulations, capable of handling large collections of droplets, in the order of several hundreds. The simulations reveal that the fluidization of the emulsion proceeds through a sequence of discrete steps, characterized by yielding events whereby layers of droplets start rolling over each other, thus leading to sudden drops of the relative effective viscosity. It is shown that such discrete fluidization is robust against loss of confinement, namely it persists also in the regime of small ratios of the droplet diameter over the microchannel width. We also develop a simple phenomenological model which predicts a linear relation between the relative effective viscosity of the emulsion and the product of the confinement parameter (global size of the device over droplet radius) and the viscosity ratio between the disperse and continuous phases. The model shows excellent agreement with the numerical simulations. The present work offers new insights to enable the design of microfluidic scaffolds for tissue engineering applications and paves the way to detailed rheological studies of soft-glassy materials in complex geometries.The shear rheology of a wide class of SGMs 10-12 can be described by the Herschel-Bulkley relation 13 between the applied stress σ and the responsive shear rateγ,In Eq. (1), σ Y is the yield stress, below which the material J o u r n a l N a me , [ y e a r ] , [ v o l . ] , 1-8 | 1 arXiv:1910.01978v2 [cond-mat.soft] 30 Nov 2019 2 | 1-8 J o u r n a l N a me , [ y e a r ] , [ v o l . ] ,

Effect of particle deformability on shear thinning in a 3D channel

“…Simulations were performed using a 2D hybrid LBM. A more recent study 29 investigated and confirmed the robustness of this discontinuous behavior. This study reported additional discontinuous jumps in viscosity over a larger parameter range, showing how the discontinuous shear thinning is preserved at lower values of the confinement, defined as the ratio between channel height and droplet radius.…”

“…The simulations were performed for a fixed value of the ratio between the channel diameter and the droplet radius. In 2D 29 , it is reported that the effective relative viscosity µ r increases with this ratio. We did not vary this ratio but, we do not expect that the effect of confinement is qualitatively different in 3D.…”

A unified lattice Boltzmann model and application to multiphase flows

“…The present model is able to achieve a positive disjoining pressure, independent of the viscosity ratio between components 1 and 2. This enables the simulation of highly packed droplet emulsions under pressure-driven flows [56,58].…”

Lattice Boltzmann simulations capture the multiscale physics of soft flowing crystals

“…Microfluidic crystals consist of a highly ordered and uniform mesoscale porous matrix, made by dispersion of a dense emulsion of droplets (air bubbles or fluid droplets, such as water) stabilized by a surfactant, embedded in a continuous phase, such as oil [21–24]. This soft material is usually produced within microchannels, structures a few dozen micrometres in width and manufactured by soft-lithography techniques [25].…”