Study on the wetting behavior between oil droplets and kaolinite substrate based on interaction force measurement and high-speed dynamic visualization

The basic principle of static wetting has been studied in depth, but the dynamic wetting mechanism, especially partially wetting, is still not well understood. In this work, the high-speed dynamic visualization method was used to study the partially wetting dynamics behavior of a millimeter-scale oleic acid droplet (referred to as oil droplet here) on the kaolinite substrate. On the other hand, a macroscopic force measurement technology was adopted to measure directly the wetting force of oil droplet-kaolinite surface during the interaction between a millimeter-scale oil droplet and the kaolinite substrate. The former showed that the spreading process of an oil droplet on the surface of kaolinite could be divided into three sub-processes: the early linear rapid spreading process dominated by inertial force, the intermediate exponential slow spreading process dominated by viscous force, and the long-time spreading process tending to equilibrium state. The latter indicated that the oil droplet-kaolinite wetting force measured by the microbalance could be related to the change in the system free energy during the spreading of an oil droplet on the kaolinite substrate, and theoretical analysis suggested that the change of system free energy during the spreading of droplets on the solid surface could be estimated based on experimental measurable parameters, and the surface wettability was the dominant factor determining the wetting force at a given droplet. These findings can provide fundamental understanding regarding the partial wetting mechanism and provide guidance for studying the interaction between liquid droplets and solid surfaces from the perspective of interaction force.