Modes of capillary bridge evolution during evaporation
The aim of the study is to identify the mechanisms and critical variables that control the evolution of the water body within capillary bridges during evaporation process. The initial series of experiments has been focused on the changes of the liquid/gas interface forming during desiccation between small clusters of two or three grains, constructed of precision glass spheres with diameter of 8 mm, and linked by a single multi-branch liquid bridge. The beads are fixed in a single plane, the space between them is filled with deionized water and exposed to room temperature evaporation in controlled conditions. Different distances between the spheres are considered. The capillary forces and the mass of the evaporating water are measured in time. The evolving configuration of the liquid bridge is recorded with digital cameras. It is hence possible to correlate bridge evolution with changes of capillary forces and to determine precisely geometrical properties of bridge (contact angle, meniscus radii, areas). A significant change in bridge geometry and its rupture occurs in several different modes. To observe the moment of rupture, a highspeed digital camera with 27000 fps was used. For two-spheres configuration the rupture of the liquid bridge resembles breaking of a ductile/brittle rod (for example steel rod) during axial extension tests, when the strength of material is exceeded. The capillary bridge evolves from a form of a thinning anvil with receding lateral meniscus surfaces, into an extremely thin water-wire, which is eventually ruptured. The capillary forces decrease monotonically during the process until the rapture, associated with a force jump to zero. Fig.1 The moment of reconfiguration of the capillary bridge between three spheres (time in milliseconds) For three in-plane spheres two possible rupture modes arise depending on the intergranular distance. In one, water drains from a single intergranular contact area bond, which progressively disappears. In another scenario, two external opposite boundary surfaces converge near a concavity between the grains to evolve into a thin film, which within about 5 miliseconds ruptures by forming a spherical gas bubble or a cylindrical channel (Fig. 1). Then water mass drains into three separate toroidal bonds. A significant jump in capillary forces is seen at the point of rupture.