Session F27: Suspensions: General

Phase-Field Modeling of Colloid-Polymer Mixtures in Microgravity

We present a theoretical model of colloid-polymer mixtures in a microgravity environment. Studying such mixtures gives us valuable insight into phase transition processes--insight that can be applied to various materials on earth. The addition of polymer to a colloidal suspension induces weakly attractive forces between the colloids and leads to a three-phase coexistence region, wherein a "liquid" phase coexists with a low-density gas phase and a high-density crystal phase. We construct, analyze and numerically simulate a phase-field model for phase separation in colloid-polymer mixtures. The results of our model simulations will be compared against experiments performed on the International Space Station, using image data available on the NASA Physical Sciences Informatics system.    

Sedimentation in a chiral fluid with odd viscosity

Chiral particles do not sediment in the same way as spherical ones. We ask, what if the fluid is chiral instead of the particle? In particular, we explore the modifications to Stokes flow due to coefficients of the viscosity tensor which are parity-violating (not invariant under mirror reflections of space) and non-dissipative (i.e. odd). We find that, in the Stokeslet flow, as well as in other cylindrically symmetric systems, the velocity field acquires an azimuthal component due to the additional viscosity coefficients. When treating each sedimenting particle as a Stokeslet, we show that the hydrodynamic interaction between two particles is changed as the azimuthal flow bends the particle trajectories in a manner not present in a standard fluid. For a spherical cloud of thousands of particles, the azimuthal flow impedes the transformation into a torus and the subsequent breakup that would otherwise occur. The basic mechanisms explored here are relevant for sedimentation in fluids under rotation, a magnetic field, and in fluids with internal activity, in which parity-violating viscosities have been experimentally demonstrated.   

Shear thickening beyond rheometry: The frictional transition model exposed to hydrodynamic flow configurations

Many industrial processes involve conveying, pumping or spreading shear thickening materials, however the question of how shear thickening suspensions flow has remained largely unexplored outside rheometers. Here we consider a shear thickening suspension in a wide-gap cylindrical Couette cell, in which the stress field is highly inhomogeneous. We will discuss how features of the flow (velocity field, concentration field, onset of jamming/instabilities) can be understood in terms of the recently proposed frictional transition framework and associated flow rules.   

Not Participating

Hydrodynamic slip can align plate-like particles in shear flow if the interfacial slip length (λ) exceeds a length scale comparable to the particle thickness. We investigate the effect of slip on the effective viscosity of a dilute suspension of graphene platelets in shear flow using Molecular Dynamics (MD). Our results show that for a given length over thickness aspect ratio k, the presence of slip induces a surprisingly large reduction in viscosity: the data suggest that at a high Peclet number (Pe>100) and k>10, the viscosity of the suspension is even smaller than that of the bare fluid. To get insights into this unexpected effect, we carried out boundary integral simulations of platelets for which a Navier slip boundary condition is imposed at the fluid-solid boundary. Obtaining trends similar to those in MD, we found that the effect of slip on viscosity becomes stronger for the particles with a larger aspect ratio. However, whether the viscosity is lower than that of the bare fluid or not depends on the threshold values of k, Pe and λ.   

A Wet Hourglass: silo discharge of a shear-thickening suspension

While for a Newtonian liquid the gravity-driven discharge of a vessel through a small orifice slows down as the vessel empties, we report a strikingly different picture for suspensions of small repulsive particles. Using model suspensions of polystyrene beads in water, which shear-thicken at high solid fractions, a constant discharge rate is observed as for a (dry) sandglass, albeit with a different dependance on the orifice size. We explain this law in terms of the vessel dimensions, the liquid viscosity, the critical shear stress above which the concentrated suspensions shear-thicken steeply, and the critical solid fraction above the latter stress scale. Discharges with a pressurized vessel, as well as the self-dilution of the extrudate and unsteadiness of the flow they may cause, will also be discussed.   

Sintering-free liquid metal ink for flexible electronics

Gallium-based liquid metal (LM) has been highlighted as a new material for flexible electronics since it exhibits low toxicity, excellent electrical conductivity and intrinsic stretchability. However, due to high surface tension and interface oxidation of LM, it is very difficult to fulfill the application demands unless wettability and sintering process are resolved. In this talk, we introduce a uniform coating method with sintering-free liquid metal ink. Gallium-based LM in an optimized mixture solution was emulsified under the high-power sonication environment. In the mixture, two key components are included to delaminate oxide layers and stabilize the LM ink emulsions. Owing to the low surface tension of the mixture solution, we can easily spread and uniformly coat on diverse surfaces, for instance foamed PVC, glass, Si wafer, PET, latex, and PDMS. We analyze properties of the ink and the pattern, and compare with the existing oxidized LM ink through macroscopic and microscopic observation. Furthermore, we will also present electrical and mechanical performances. We believe that the sintering-free LM ink can be sued for soft robots and wearable healthcare device.