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We investigate the rheology, microscopic structure, and dynamics of an industrially relevant dispersion made of cationic surfactant vesicles, from dilute to concentrated conditions. We find that these suspensions exhibit a shear-thinning behavior at relatively low concentrations. At the microscale, this corresponds to a well-defined transition in both the structure, marked by the appearance of a peak in the static structure factor, and the dynamics, which slow down and develop a two-step decay in the correlation functions. This low-concentration transition is particularly surprising in light of experiments showing that for surfactant vesicles of similar composition the interactions should be purely repulsive. This leads us to propose that the observed structural and dynamic transition could arise, as an entropic effect, from the large sample polydispersity coupled to crowding. The shear-thinning behavior is thus interpreted as the nonlinear response of this transient structure to the imposed flow. Our work suggests that similar effects might be a generic feature of dense, highly polydisperse charged suspensions., Comment: 14 Pages with appendix. 6 figures in the main text + 10 figures in the appendix

The study of interactions between biomimetic membranes and micron-sized particles is crucial for understanding various biological processes. Here, we control microparticle spontaneous engulfment by giant lipid vesicles by tuning particle surface charge, exploring regimes from negligible to strong adhesion. We focus our attention on dissipative phenomena at the micron- and nano- scales, occurring when a particle is wrapped by a lipid vesicle bilayer or when the particle diffuses at the lipid-monolayer interface of a droplet. For particles wrapped by membrane bilayers, we highlight the influence of the particle penetration depth and the impact of substructures on particle friction. Our work is complemented by hydrodynamic simulations that take into account the effects of the shape of the membrane wrapping the particle and the water gap separating the lipid bilayer membrane from the particle on translational particle drag. We show, however, that a purely hydrodynamic model is not suitable to describe the friction of a particle diffusing at the interface of an aqueous microdroplet in oil, stabilized by a single lipid layer. In hydrodynamic models, dissipation is solely described by the surface shear viscosity of the interface and the bulk fluid viscosity, but in this partial wetting configuration, an additional source of dissipation is required to account for fluctuations at the contact line. Hence, through experimental and numerical studies, we demonstrate that the dissipation contributions for the two geometries are fundamentally different., Comment: The final (accepted) version includes improved contextualization within the literature, as well as more detailed comparisons with experiments conducted by other authors. The discussion on how the particle surface potential affects particle-membrane adhesion has been expanded. These updates do not affect the results or conclusions of the study

The manipulation and control of microparticles through non-intrusive methods is pivotal in biomedical applications such as cell sorting and cell focusing. Although several experimental and numerical studies have been dedicated to single suspended particles or clusters of rigid spheres, analogous cases with deformable particles have not been as thoroughly studied, especially when the suspending liquid exhibits relevant viscoelastic properties. With the goal of expanding the current knowledge concerning these systems, we perform a computational study on the hydrodynamic interactions between two neutrally buoyant initially spherical elastic particles suspended in Newtonian and shear-thinning viscoelastic matrices subjected to pressure-driven flow in a cylindrical microchannel. Due to the well-known focusing mechanism induced by both particle deformability and fluid elasticity, the two particles are assumed to flow at the axis of the tube. The rheological behavior of the viscoelastic continuous phase is modelled via the Giesekus constitutive equation, whereas the particles are assumed to behave as Neo-Hookean solids. The problem is tackled by employing a mixed finite element method. The effects of particle deformability, fluid elasticity, confinement ratio, and initial interparticle separation distance on the pair dynamics are investigated. The main outcome of this study is a quantitative indication of the flow conditions and spatial configurations (initial distances) under which the particles will spontaneously form organized structures. Such results are helpful to design efficient microfluidic devices with the aim of promoting particle ordering., Comment: 28 pages including references, 17 figures. Article accepted for publication in Physics of Fluids

The sedimentation of a spherical particle in an elastoviscoplastic fluid in proximity of a flat wall is investigated by direct numerical simulations. The governing equations under inertialess conditions are solved by the finite element method with an Arbitrary Lagrangian-Eulerian formulation to manage the particle motion. The fluid is modeled with the Giesekus constitutive equation modified as proposed by Saramito (2007). The sedimentation, migration, and angular velocities are computed as a function of the particle-wall distance for various Weissenberg and Bingham numbers. The presence of a yield stress reduces the settling velocity and reverses the migration direction as compared to the purely viscoelastic case. The effect of the confining wall on the yielded and unyielded regions around the particle is investigated. The reversed particle migration phenomenon observed in the elastoviscoplastic fluid is attributed to the different shear rate distribution around the particle due to the presence of the yielded region. The negative wake behind the particle is discussed and related to the axial component of the viscoelastic stress.