Spreading of completely wetting or partially wetting power-law fluid on solid surface

This study investigated the drop-spreading dynamics of pseudo-plastic and dilatant fluids. Experimental results indicated that the spreading law for both fluids is related to rheological characteristics or power exponent n. For the completely wetting system, the evolution of the wetting radius over time can be expressed by the power law R = at(m), where the spreading exponent m of the dilatant fluids is > 0.1 and the spreading exponent m of pseudo-plastic fluids is < 0.1. The strength of non-Newtonian effects is positively correlated to the extent of deviation from the theoretical value 0.1 of m for Newtonian fluids. For the partially wetting system, the power law on the time dependence of the wetting radius no longer holds; therefore, an exponential power law, R = R-eq(1 - exp(-at(m)/R-eq)), is proposed, where R-eq denotes the equilibrium radius of drop and a is a coefficient. Comparing experimental data with the exponential power law revealed that both are in good agreement.