Experiments in fluid spreading in the partial wetting regime

Abstract

The spread of a fluid on a flat surface has been the subject of intense research for a number of decades, having importance in surface treatment technologies, porous media flow, and common naturally ocurring phenomena. In the complete wetting case, a fluid will spread until it can cover the entire surface with a thin film of molecular thickness; the dynamics of these fluids are well understood. In the partial wetting case, the fluid first spreads and then stops, forming a puddle. Research on partial wetting flows has revealed that the macroscopic behavior is impacted by microscopic behavior, but there is no broadly accepted model that connects these two regions. Some experimental data exist for a small volume of partial wetting fluid spreading, but there is a distinct gap in data for larger volumes. Here, we first review currently established understanding of the spreading problem for the complete wetting and partial wetting cases. We then present the results of laboratory experiments for large volume fluid spreading in the partial wetting regime. We observe an early time spreading regime, which is followed by a relaxation to steady state. Comparisons to similarly large volumes of a complete wetting fluid spreading show differences even in the early time behavior. We compare the results of these experiments with a macroscopic model in development and find agreement between experiment and model, with the model revealing an early spreading regime of the form A ~ t1/4. The results of these experiments and model of simple fluid spreading lay the foundation for future work investigating more complex multiphase flow phenomena in porous media under the partial wetting regime.