How Wettability Controls Nanoprinting.

Using large scale molecular dynamics simulations, we study in detail the impact of nanometer droplets of low viscosity on flat substrates versus the wettability of the solid plate. The comparison between the molecular dynamics simulations and different macroscopic models reveals that most of these models do not correspond to the simulation results at the nanoscale, in particular for the maximal contact diameter during the nanodroplet impact (D_{max}). We have developed a new model for D_{max} that is in agreement with the simulation data and also takes into account the effects of the liquid-solid wettability. We also propose a new scaling for the time required to reach the maximal contact diameter t_{max} with respect to the impact velocity, which is also in agreement with the observations. With the new model for D_{max} plus the scaling found for t_{max}, we present a master curve collapsing the evolution of the nanometer drop contact diameter during impact for different wettabilities and different impact velocities. We believe our results may help in designing better nanoprinters since they provide an estimation of the maximum impact velocities required to obtain a smooth and homogenous coverage of the surfaces without dry spots.

Pinning of a drop by a junction on an incline.

The shape of a drop pinned on an inclined substrate is a long-standing problem where the complexity of real surfaces, with heterogeneities and hysteresis, makes it complicated to understand the mechanisms behind the phenomena. Here we consider the simple case of a drop pinned on an incline at the junction between a hydrophilic half plane (the top half) and a hydrophobic one (the bottom half). Relying on the equilibrium equations deriving from the balance of forces, we exhibit three scenarios depending on the way the contact line of the drop on the substrate either simply leans against the junction or overfills (partly or fully) into the hydrophobic side. We draw some conclusions on the geometry of the overlap and the stability of these tentative equilibrium states. In the corresponding retention force factor, we find that a major role is played by the wetted length of the junction line, in the spirit of Furmidge's observations. The predictions of the theory are compared with extensive molecular dynamics simulations.

Testing the mean magnetization approximation, dimensionless and scaling numbers in magnetorheology.

The mean magnetization (MM) approximation is undoubtedly the most widely used approximation in magnetorheology both from theoretical and simulation perspectives. According to this, spherical magnetizable particles under field can be replaced by effective dipole moments m placed at their center with strength m = V(p)⟨M(p)⟩. Here V(p) and ⟨M(p)⟩ are the volume and mean (average) magnetization of the particles, respectively. In spite of being extensively used, there is not a mathematical justification to do so in most cases. In this manuscript, we test this approximation using experiments, theories and simulations, for a wide range of magnetic field strengths and particle loadings, in both conventional magnetorheological fluids (CMRFs) and inverse ferrofluids (IFFs). Results demonstrate that the MM approximation is applicable in IFFs for a very wide range of field strengths (up to external fields of 265 kA m(-1)) and particle loadings (up to 20 vol%). For CMRFs, the MM approximation is only applicable in two particular circumstances; in magnetic saturation or in infinite dilution.

Self-assembly in two-dimensions of colloidal particles at liquid mixtures.

The aim of this work is to simulate the formation of colloidal rings, circular clusters, and voids induced by oily lenses at the air-water interface. The presence of two liquids with different surface tension leads to the formation of a nonhomogeneous interface. In this case, the total interaction potential is assumed to be composed of only two terms; the first one is due to the (repulsive) pairwise dipolar force between partly immersed charged microspheres, whereas the second depends on the position of the particle at the interface and is connected to the interfacial stress caused by the difference of surface tension between both liquids. This simple potential is able to reproduce the experimental rings, circular clusters and voids found by different authors.