Direct Evidence of a Radius of Collection Area for Thin Film Flow in Liquid Bridge Formation by Repeated Contacts Using AFM.

A liquid bridge in a nanoscale gap is of considerable significance in lots of scientific and industrial fields. However, the formation mechanism is not well understood, leading to many contradictory experimental results. In this work, contact experiments were carried out between tipless cantilevers coated with potassium hydroxide and a silica surface on an atomic force microscope under different relative humidities (RHs). Results show that capillary condensation is dominant and thin film flow is difficult or even impossible at low RHs (31-37%). However, at high RHs (62-82%), thin film flow is dominant and materials were collected with a domed three-dimensional feature in the contact zone. There was a circle centered at the feature with a radius of collection area (can be as large as ∼23.6 μm), inside which all of the liquid seems to flow into the water bridge. The radius of collection area is used as direct evidence and as a parameter to reflect the efficiency of thin film flow. This fabrication technique of a domed feature may be viewed as a promising additive manufacturing in the microscale, and this work may also shed some light on the study of the controversial RH dependence of capillary force and other related research works.