Measurement of forces during stretching of a capillary bridge
A capillary bridge (CB) is a liquid interface formed between two rigid, solid surfaces. Here, liquid of low volume is considered, where forces due to surface tension and viscosity are taken into account. An experimental setup was developed in Force Microscopy Laboratory to study the characteristics of the CB. It comprised of a force sensor arrangement made using a fixed cantilever and a movable stage (a ball). A capillary bridge was formed between a fixed, flat surface (the cantilever) and a movable, spherical surface (the stage). The linear actuation of the stage was controlled using a stepper motor along with a lead screw arrangement to convert the rotary motion of the stepper motor to axial motion of the ball. A micro-stepper driver was used to control the motor speed. During different regimes of the CB, the cantilever deflection was measured using a fibre-optic displacement sensor. This deflection was then used to determine forces on the CB. The principles of the working and operation of every single component in the experimental setup were studied. The experiment was carried out for silicone oil, distilled water and 1-dodecanethiol at various speeds.
Different regimes of the CB: jump in contact of liquid, the formation of CB, contact of the two solid surfaces, pushing of the stage towards the cantilever, stretching of the CB and rupture of the CB were studied using the force-displacement plot. Force analysis of the CB for the liquids was done using force-displacement plots. Optical observation was done with the aid of videos recorded using a trinocular and a 20.1 Megapixel camera.
The forces experienced by each of the liquids were negative due to its hydrophilic nature. It was observed that the CB experienced negative forces during the formation of CB and stretching of the CB. The cantilever underwent oscillations after the formation of CB (due to jump in contact) and after the breakup of CB (due to rupture of CB). Viscous forces were predominant during the formation of the CB and during the rupture of the CB and forces due to surface tension were noticeable during stretching of the CB.
Keywords: capillary bridge (CB), cantilever, force-displacement