The role of water capillary forces in electro-rheological fluids

In many types of electro-rheological fluids (ERFs) it is observed that under the same electric field, the viscosity of the suspension passes through a maximum as the water content of the particles is increased. In this paper the authors explain this phenomenon assuming that the particles are held together by the capillary force of water trapped in their contact area. They propose a model where the cohesive force between particles is mainly due to numerous microscopic 'water bridges' created by the electric field. The number of water bridges depends on the strength of the applied field and the amount of water in the particle. The authors estimate this number using a crude model, and show that: (i) the water content Wmax which gives the maximum viscosity depends on the electric field E and the particle radius a as Wmax varies as E-2/3a-1; (ii) for W<Wmax, the yield stress is written as sigma y varies as EW32/a1/2 gamma ws, where gamma ws is the surface tension between water and the solvent liquid. These features qualitatively agree with experiments for hydrous ERF particles. The authors have also estimated the timescales for the formation and relaxation of the water bridges, and the results are comparable to experimental measurements.