Dense suspensions of hard particles in a liquid can exhibit strikingly counter-intuitive behavior, such as discontinuous shear thickening (DST) [1,2,3,4,5,6,7,8] and reversible shear jamming (SJ) into a state with finite yield stress [9,10,11,12,13]. Recent studies identified a stress-activated crossover from hydrodynamic interactions to frictional particle contacts to be key for these behaviors [2,3,4,6,7,8,10,14]. However, many suspensions exhibit only DST and not SJ. Here we show that particle surface chemistry can play a central role in creating conditions that allow for SJ. We find the system's ability to form interparticle hydrogen bonds when sheared into contact elicits SJ. We demonstrate this with charge-stabilized polymer microspheres and non-spherical cornstarch particles, controlling hydrogen bond formation with solvents. The propensity for SJ is quantified by tensile tests [13] and linked to an enhanced friction by atomic force microscopy. Our results extend the fundamental understanding of the SJ mechanism and open new avenues for designing strongly non-Newtonian fluids.

A critical complication in handling nanoparticles is the formation of large aggregates when particles are dried e.g. when they need to be transferred from one liquid to another. The particles in these aggregates need to disperse into the destined liquid medium, which has been proven difficult due to the relatively large interfacial interaction forces between nanoparticles. We present a simple method to capture, move and release nanoparticles without the formation of large aggregates. To do so, we employ the co-non-solvency effect of poly(N-isopropylacrylamide) (PNIPAM) brushes in water-ethanol mixtures. In pure water or ethanol, the densely end-anchored macromolecules in the PNIPAM brush stretch and absorb the solvent. We show that under these conditions, the adherence between the PNIPAM brush and a silicon oxide, gold, polystyrene or poly(methyl methacrylate) colloid attached to an atomic force microscopy cantilever is low. In contrast, when the PNIPAM brushes are in a collapsed state in a 30-70 vol% ethanol-water mixture, the adhesion between the brush and the different counter surfaces is high. For potential application, we demonstrate that this difference in adhesion can be utilized to pick up, move and release 900 silicon oxide nanoparticles of diameter 80 nm using only 10 × 10 μm PNIPAM brush.

Abstract

For the first time, we used computer simulations to study lift forces on two static disks placed side-by-side within a two-dimensional granular flow and found them to be either repulsive or attractive depending on the flow velocity and separation between the disks. Our simulations results reveal that differences in the flow velocity between the disks and outside of that region are closely correlated with the lift force. We propose an empirical function for the lift force based on this correlation and our dimensional analysis. The specific region where the measured velocity exhibits this correlation suggests that attractive lift is not a Bernoulli-like effect. Instead, we speculate that it might be explained by a force balance based on Coulomb’s theory of passive failure in a Mohr–Coulomb material. Our results confirm that repulsive lift is due to the jamming of particles flowing between the disks.

No abstract

In the present work, we investigate the dynamic phenomena induced by solvent evaporation from ternary solutions confined in a Hele-Shaw cell. The model solutions consist of ethanol, water, and oil, and with the decrease in ethanol concentration by selective evaporation, they may undergo microdroplet formation via the ouzo effect or macroscopic liquid–liquid phase separation. We varied the initial concentration of the three components of the solutions. For all ternary solutions, evaporation of the good solvent ethanol from the gas–liquid interface, aligned with one side of the cell, leads to a Marangoni instability at the early stage of the evaporation process. The presence of the Marangoni instability is in agreement with our recent predictions based on linear stability analysis of binary systems. However, the location and onset of subsequent microdroplet formation and phase separation are the result of the interplay between the Marangoni instability and the initial composition of the ternary mixtures. We classified the ternary solutions into different groups according to the initial concentration of oil. For each group, based on the ternary diagram of the mixture, we offer a rationale for the way phase separation takes place and discuss how the instability influences droplet nucleation. Our work helps us to understand under what conditions and where droplet nucleation can take place when advection is present during phase separation inside a microfluidic device.

No abstract

No abstract