Wall fluidization in two acts: from stiff to soft roughness

Abstract

Fluidization of soft glassy materials (SGMs) in microfluidic channels is affected by the wall roughness in the form of microtexturing. When SGMs flow across microgrooves, their constituents are likely trapped within the grooves' gap, and the way they are released locally modifies the fluidization close to the walls. By leveraging a suitable combination of experiments and numerical simulations on concentrated emulsions (a model SGM), we quantitatively report the existence of two physically different scenarios. When the gap is large compared to the droplets in the emulsion, the droplets hit the solid obstacles and easily escape scrambling with their neighbors. Conversely, as the gap spacing is reduced, droplets get trapped inside, creating a "soft roughness" layer, i.e. a complementary series of deformable posts from which overlying droplets are in turn released. In both cases, the induced fluidization scales with the grooves' density, although with a reduced prefactor for narrow gaps, accounting for the softness of the roughness. Both scenarios are also well distinguished via the statistics of the droplets displacement field close to the walls, with large deviations induced by the surface roughness, depending on its stiffness.

Metastability at the Yield-Stress Transition in Soft Glasses

“…The values of the cooperativity scale ξ extracted from the model [41,47,48] are in agreement with experimental observations [7,49]. Recently, the model has been used in synergy with experiments on real emulsions in order to quantify the impact of the fluidization induced by the roughness of microchannels on the flow behavior of the emulsion [47,48]. As shown in the reminder of the paper, for this "model emulsion" the static and dynamic yield-stress values are found to differ.…”

Wall slip and bulk yielding in soft particle suspensions

“…The steady state normalised profiles (Fig. 4, left) reveal two separate contributions to the slip: one from a very thin solvent layer within about ∆y = 0.0025 of the wall (inset), and another over about ∆y = 0.1, corresponding to an increase in fluidity over the first few particle layers near the wall 26,44,55 . Importantly, we find the first contribution to dominate the total slip at stresses below yield, whereas above yield both are important.…”

“…It is thus capable of properly capturing rheological wall slip. (Most existing methods instead simply assume a spherical interparticle potential and an effective solvent drag, although more advanced methods also exist 1,[43][44][45] .) Second, we quantify the effects of slip on steady state flow behaviour, confirming that it radically changes a material's flow curve σ( γ) by conferring a branch of slip-induced apparent flow even for σ < σ y .…”

Mesoscale Modelling of the Tolman Length in Multi-component Systems

“…If one sets 𝐺 = 0 the two components are completely decoupled and one effectively simulates two parallel multi-phase systems which can display phase separation whenever 𝐺 AA , 𝐺 BB < 𝐺 𝑐 where 𝐺 𝑐 is the critical coupling constant whose value depends on the choice of 𝜓 J (x, 𝑡) [15,31]. A similar approach has been used in [40,41] for the simulation of emulsions and comparison with experimental results [42,43]. The vectors 𝝃 are the discrete forcing directions such that their squared lengths are |𝝃 𝑎 | 2 = 1, 2, and 𝑊 (1) = 1/6 and 𝑊 (2) = 1/12 are the weights ensuring 4-th order lattice force isotropy [44,45].…”

“…The SC model has been widely used to model complex fluids with a non-trivial impact on the study of the interface physics, one may cite heterogeneous cavitation [50] and emulsion rheology physics [51], also in presence of complex boundary conditions [43].…”