Experimental Investigation of the Motion and Deformation of Droplets in Curved Microchannel

The trajectory and topology of an immiscible droplet moving in a microchannel can be influenced by the flow structure or a vortex within the flow. Channel geometry is a common effective parameter of the flow structure. Investigating the effect of a curved channel on the droplet trajectory and topology helps one to understand the effect of such geometries on the content of the droplets in various applications. The effect of Reynolds number, 3.5 ≤ Re ≤ 7, surface tension, and droplet size has been experimentally studied on the droplet shape and trajectory. Droplets were generated from a 2-propanol–water mixture with a broad range of equivalent diameters of 95 μm ≤ deq ≤ 610 μm in two microchannels with a 180° and 270° curvature angle. It was found that lateral migration and deformation of the droplet are insensitive to variations in channel Reynolds number. Surface tension, however, has a direct impact on the deformation of the droplets. It can also affect the trajectory of the droplets and direction of lateral migration. Furthermore, droplet size was shown to significantly affect the deformability of the droplet

Effect of the Thermocouple on Measuring the Temperature Discontinuity at a Liquid–Vapor Interface

The coupled heat and mass transfer that occurs in evaporation is of interest in a large number of fields such as evaporative cooling, distillation, drying, coating, printing, crystallization, welding, atmospheric processes, and pool fires. The temperature jump that occurs at an evaporating interface is of central importance to understanding this complex process. Over the past three decades, thermocouples have been widely used to measure the interfacial temperature jumps at a liquid–vapor interface during evaporation. However, the reliability of these measurements has not been investigated so far. In this study, a numerical simulation of a thermocouple when it measures the interfacial temperatures at a liquid–vapor interface is conducted to understand the possible effects of the thermocouple on the measured temperature and features in the temperature profile. The differential equations of heat transfer in the solid and fluids as well as the momentum transfer in the fluids are coupled together and solved numerically subject to appropriate boundary conditions between the solid and fluids. The results of the numerical simulation showed that while thermocouples can measure the interfacial temperatures in the liquid correctly, they fail to read the actual interfacial temperatures in the vapor. As the results of our numerical study suggest, the temperature jumps at a liquid–vapor interface measured experimentally by using a thermocouple are larger than what really exists at the interface. For a typical experimental study of evaporation of water at low pressure, it was found that the temperature jumps measured by a thermocouple are overestimated by almost 50%. However, the revised temperature jumps are still in agreement with the statistical rate theory of interfacial transport. As well as addressing the specific application of the liquid–vapor temperature jump, this paper provides significant insight into the role that heat transfer plays in the operation of thermocouples in general