Fluid droplet spreading and adhesion studied by a microbalance: a review

A contact angle observed for a liquid-solid system is not necessarily a unique value and a few contact angles need to be carefully considered in relation to liquid spreading, adhesion and phase separation. Understanding of the significance of different contact angles has improved in the last few years through direct measurements of interactive forces between droplets/bubbles and solids together with the simultaneous visualization of the changes in their shapes. A microelectronic balance system is employed to measure the force of spreading after either liquid droplet or gas bubble attachment to a substrate surface, and the droplet/bubble-substrate adhesion forces after droplet/bubble compression, retraction, and detachment. Equipped with a camera in flank and data acquisition software, the instrument measures directly the forces, monitors droplet/bubble-surface separation with respect to distances over which the droplet/bubble stretches and collects optical images simultaneously. The images are used to analyze capillary pressure and surface tension forces based on the measured droplet/bubble dimensions, shapes of surfaces and values of contact angles. These measurements allow researchers to correlate the advancing, receding and most-stable contact angles with liquid-solid interactive forces and analyze their scientific meaning. This review summarizes the very recent literature reports on measurements and interpretation of liquid droplet/gas bubble interactive forces and associated contact angles.