演讲人：Dr. Stephan Weiss

题  目：Moist convection - studying clouds in the lab

时  间：2018年9月20日（周四） 上午10:00

地  点：哈工大（深圳）A406

讲座内容：

My talk presents results from moist convection experiments, i.e., convection with a fluid that undergoes a phase transition from the gas to the liquid. In our experiments we use sulfur-hexafluoride (SF6) as the working fluid that is confined between a cold horizontal plate from the top and a warm horizontal plate from the bottom. If the bottom plate temperature is warmer and the top plate temperature colder than the phase transition temperature, a liquid layer forms close to the bottom from which gaseous SF6 evaporates. The SF6 vapor rises to the top plate, where it condenses and forms a liquid layer underneath it. This layer undergoes a Rayleigh-Taylor like instability and finally liquid drips down back into the liquid pool at the bottom.

The first part of my talk focuses on the nucleation of small droplets that occur in the wake of larger cold drops falling from the top liquid layer through the supersaturated SF6-atmosphere. I will discuss whether such a mechanism is important in clouds, where it might enhance the formation of cloud droplets.

In the second part, I will focus on the instability patterns that occur in the thin liquid layer underneath the top plate. These patterns are surprisingly steady and form a hexagonal lattice, which is not expected for a conventional Rayleigh-Taylor instability. We investigate the order and dynamics of these pattern as a function of pressure and temperature and discuss how evaporation can stabilize the pattern and prevent coarsening.

个人简介：

Dr. Stephan Weiss received his doctorate in experimental physics from the University of Goettingen in 2009. He was postdoctoral researcher at the University of California, Santa Barbara and the University of Michigan, Ann Arbor in 2009-2015. Since 2015 he became group leader at the Max Planck Institute for Dynamics and Self-Organization. His research interests include hydrodynamics, non-linear physics and geophysics, and pattern formation. From 2008 Dr. Weiss published more than 20 journal papers with 5 in physical review letters and 9 in journal of fluid mechanics.