Onset of Marangoni convection in low viscosity silicon oil inside a heated capillary tube

In the present experimental investigation the onset of Marangoni convection inside a heated capillary tube filled with low viscosity silicon oil is presented and discussed. The 1 cSt viscosity silicon oil used evaporates spontaneously at ambient temperature. The evaporation of silicon oil inside the 1 mm internal diameter tube is not uniform, being larger near the meniscus triple line region than in the centre; this creates gradients of temperatures (which have been measured by InfraRed thermography) and therefore of surface tension. For the unheated tubes this gradient of surface tension is found to be not big enough to set a convection motion (Marangoni convection) which was reported by one of the present authors in previous studies using alcohols. With increasing power supplied to the tube by an electric heater, the Marangoni convection sets in and strengthens at increasing powers till the pinned meniscus detaches from the tube mouth and recedes inside the capillary.