It has been known for more than 200 years that the maximum static friction force between two solid surfaces is usually greater than the kinetic friction force—the force that is required to maintain the relative motion of the surfaces once the static force has been overcome. But the forces that impede the lateral motion of a drop of liquid on a solid surface are not as well characterized, and there is a lack of understanding about liquid–solid friction in general.

Using a laser deflection system we investigate lateral adhesion forces at liquid-solid interfaces. Our set-up consists of a laser, a deflectable capillary, and a position sensitive detector (PSD). Drops of liquid are moved laterally against solid substrates using the deflectable capillary. A laser beam incident on the capillary is reflected to the PSD, which instantly generates electric signals according to the lateral adhesion forces. With assistance of optical imaging, we have been able to resolve the drop motion synchronised to the force measurement.

Our results have demonstrated that the instantaneous lateral adhesion forces at the liquid-solid interfaces are determined by the front and rear contact angles as well as the contact width. More importantly, we report that the lateral adhesion force between a liquid drop and a solid can also be divided into a static and a kinetic regime. This striking analogy with solid–solid friction is a generic phenomenon that holds for liquids of different polarities and surface tensions on smooth, rough and structured surfaces.

References:

1. Butt, H.-J., et al., Energy Dissipation of Moving Drops on Superhydrophobic and Superoleophobic Surfaces. Langmuir, 2017. 33(1): p. 107-116.

2. Gao, N., et al., How drops start sliding over solid surfaces. Nature Physics, 2018. 14: p. 191.