When a liquid drop is gently deposited on a wetting solid surface, it spreads due to capillary forces until it reaches a thermodynamical equilibrium set by the relative surface energies of the system. We investigate here experimentally the spreading ability of drops made of yield stress fluids, which flow only if the applied stress is above a finite value. We observe that in this case, after a spreading phase, the motion stops and a well-defined contact angle can be measured. This contact angle depends on the rheological properties of the fluid and in particular on its yield stress, on the drop radius and on the hydrodynamic boundary condition at the surface. These results are quantitatively compared to an analysis showing that, due to the yield stress of the fluid, a mechanical equilibrium is indeed reached which does not correspond to the thermodynamical equilibrium.

中文翻译：

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屈服应力液滴分布过程中的动态阻滞

当液滴缓慢沉积在湿润的固体表面上时，由于毛细管作用力，液滴会扩散，直到达到系统相对表面能所设定的热力学平衡。我们在这里通过实验研究由屈服应力流体制成的液滴的扩散能力，只有当施加的应力高于有限值时，液滴才能流动。我们观察到，在这种情况下，在扩展阶段之后，运动停止，并且可以测量明确定义的接触角。该接触角取决于流体的流变性质，尤其取决于其屈服应力，液滴半径和表面处的流体动力学边界条件。将这些结果与分析结果进行定量比较，结果表明，由于流体的屈服应力，