Su Jin Lim

Low internal pressure in femtoliter water capillary bridges reduces evaporation rates

Capillary bridges are usually formed by a small liquid volume in a confined space between two solid surfaces. They can have a lower internal pressure than the surrounding pressure for volumes of the order of femtoliters. Femtoliter capillary bridges with relatively rapid evaporation rates are difficult to explore experimentally. To understand in detail the evaporation of femtoliter capillary bridges, we present a feasible experimental method to directly visualize how water bridges evaporate between a microsphere and a flat substrate in still air using transmission X-ray microscopy. Precise measurements of evaporation rates for water bridges show that lower water pressure than surrounding pressure can significantly decrease evaporation through the suppression of vapor diffusion. This finding provides insight into the evaporation of ultrasmall capillary bridges.

Short time dynamics of water coalescence on a flat water pool

Coalescence is an important hydrodynamic event that frequently takes place in nature as well as in industry. Here we provide an experimental study on short time dynamics of water coalescence, particularly when a water droplet comes in contact with a flat water surface, by utilizing high-resolution high-penetration ultrafast X-ray microscopy. Our results demonstrate a possibility that an extreme curvature difference between a drop and a flat surface can significantly modify the hydrodynamics of water coalescence, which is unexpected in the existing theory. We suggest a plausible explanation for why coalescence can be modified by an extreme curvature difference.

Lattice Boltzmann simulations for water coalescence

We extensively investigate the inertial coalescence process of a water drop merging with another water drop, particularly including a water flat pool, via three-dimensional lattice Boltzmann simulations. After the two drops contact, a narrow liquid bridge is formed, which induces the coalescence process. The growth dynamics of the bridge has been a fundamental issue in coalescence study for a long time. Recently, the X-ray imaging study for the short-time dynamics of water coalescence on a flat water pool reported that asymmetry of drops affects hydrodynamics of coalescence and its neck-growth. In this report, we simulate a water drop coalescence at bottom drops with various sizes containing asymmetric conditions such as a flat water pool. We find that the thickness of the bridge grows quantitatively as the bottom drop size increases up to infinity where its surface is flat. This result shows sub-diffusive behavior with exponent of 2/5, rather than normal diffusion with exponent of 1/2 in the scaling law ...