ABSTRACT
We consider the displacement, in a rectangular channel, of a Newtonian oil pushed by different types of liquids (Newtonian, shear-thinning, viscoelastic) of slightly higher apparent viscosity. In the absence of viscoelastic effects the interface between the two fluids becomes sharper at larger velocities, so that the thickness of the lateral film left behind increases with the flow rate. On the contrary, with a viscoelastic fluid, the shape of the interface is almost independent of the velocity so that the thickness of the lateral film is approximately constant. Moreover this thickness decreases when the ratio of normal to tangential stresses increases, suggesting that this effect can be attributed to normal stress differences. A heuristic theoretical approach tends to confirm this statement.
ABSTRACT
We have computed interfacial tension in oil-water-surfactant model systems using dissipative particle dynamics (DPD) simulations. Oil and water molecules are modelled as single DPD beads, whereas surfactant molecules are composed of head and tail beads linked together by a harmonic potential to form a chain molecule. We have investigated the influence of the harmonic potential parameters, namely, the force constant K and the equilibrium distance r0, on the interfacial tension values. For both parameters, the range investigated has been chosen in agreement with typical values in the literature. Surprisingly, we observe a large effect on interfacial tension values, especially at large surfactant concentration. We demonstrate that, due to a subtle balance between intra-molecular and inter-molecular interactions, the local structure of surfactants at the oil-water interface is modified, the interfacial tension is changed and the interface stability is affected.
ABSTRACT
We analyse the consequences of Quincke rotation on the conductivity of a suspension. Quincke rotation refers to the spontaneous rotation of insulating particles dispersed in a slightly conducting liquid and subject to a high DC electric field: above a critical field, each particle rotates continuously around itself with an axis pointing in any direction perpendicular to the DC field. When the suspension is subject to an electric field lower than the threshold one, the presence of insulating particles in the host liquid decreases the bulk conductivity since the particles form obstacles to ion migration. But for electric fields higher than the critical one, the particles rotate and facilitate ion migration: the effective conductivity of the suspension is increased. We provide a theoretical analysis of the impact of Quincke rotation on the apparent conductivity of a suspension and we present experimental results obtained with a suspension of PMMA particles dispersed in weakly conducting liquids.
ABSTRACT
Nonconducting particles suspended in a liquid usually decreases the bulk conductivity since they form obstacles to the ions' migration. However, for sufficiently high dc electric fields, these particles rotate spontaneously (Quincke rotation) and facilitate the ions migration: the effective conductivity of the suspension is thus increased. We present a theoretical analysis and show experimental results which demonstrate that the apparent conductivity of the whole suspension can be higher than that of the suspending liquid.
