Leader : Lemaire Élisabeth
The electroration of a particle induced by a DC electric field is known as Quincke rotation and occurs when an insulating particle immersed into a low conducting liquid is subjected to a sufficiently high DC field.. A simple physical interpretation can be given : during their migration, the free negative and positive charges of the liquid meet the insulating particle and accumulate at its surface. This results in a charge distribution with the particle polarised in a direction opposite to the field. This equilibrium is unstable and when the particle begins to rotate in order to flip the orientation of the induced dipole, P, it undergoes a driving torque which increases the angular tilt further.
We have shown theoretically and experimentally that the equations that govern the dynamics of the dipole, P, are exactly the same as the Lorenz equations. Thus the Quincke khas been shown to be an exact model of the Lorenz attractor.
The Quinke rotation is also responsible for unexpected orientations of dielectric fibres in the presence of a DC electric field. In particular, the orientation of an insulating fibre perpendicular to the direction of the electric field is stabilised by Quincke rotation.
Concerning the behaviour of an ensemble of particles, namely a suspension, we have shown that the Quincke rotation led to an increase of the effective conductivity of a suspension, or, more interestingly to a drastic decrease of its viscosity (more than one order of magnitude). We have studied this viscosity decrease both in a simple shear flow and in a Poiseuille flow. In this last case, thanks to the Pulsed Ultrasound Doppler Velocimetry technique, we have been able to show that the flow increase which is observed when a DC electric field is applied in the velocity gradient direction, was accompanied by a modification of the shape of the velocity profile.
At last we have carried out a numerical study using the finite elements method to show that when the particle size was not using the finite elements method to show that when the particle size was not very large compared to the ion layer thickness built by the action of the electric field, the Quincke rotation was not as efficient as for larger particles.
Our present studies focuss on the collective motion of particles, either with or without superimposed shear flow, induced by Quincke rotation.
Fluides & Matériaux Complexes, Rhéologie des Suspensions
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