Instability of a Thermal Stokes Layer

We examine experimentally the stability of a Stokes layer in a fluid near a boundary whose temperature is modulated as T₀cosωt. We define an appropriate Rayleigh number for the problem and determine its critical value. Increased stabilization is observed to accompany a reduction in the Prandtl number. We observe hysteresis effects near the critical Rayleigh number, including a double hysteresis loop, which appear qualitatively similar to recent predictions of Roppo, Davis, and Rosenblat.