Abstract
The rate of spreading of a drop on a rigid substrate is governed by viscous dissipation in the liquid, the capillary driving force being compensated by the braking force resulting from viscous shearing in the liquid. In the case where the liquid is not newtonian but shear thinning or pseudoplastic, a deviation from the law of P.-G. de Gennes is observed, in particular a slower spreading kinetics corresponding to an increase of the liquid viscosity as the spreading speed decreases. In this study, this result is interpreted from the rheological behaviour of the non-newtonian liquid and a modified equation describing the spreading dynamics is proposed. This equation is obtained from two different calculations, either in considering an average viscosity or in expressing the distribution of speed in a shear thinning liquid wedge. The shape, slightly aspherical, of non-newtonian liquid drops having a size smaller than the capillary length, is also very simply interpreted, observing that the liquid viscosity increases from the edge to the center of drops during spreading, near the solid surface.
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