Srinivas Vanapalli

A volatile sessile liquid droplet or a sublimating solid manifests levitation on its own vapor when placed on a sufficiently heated surface, illustrating the Leidenfrost phenomenon. In this study, we introduce a non-invasive capacitance method for investigating this phenomenon, offering a potentially simpler alternative to existing optical techniques. The designed sensor features in-plane miniaturized electrodes forming a double-comb structure, also known as an interdigitated capacitor. Initially, the sensor’s capacitance is characterized for various distances between the sensor and a dielectric material. The influence of the sensor substrate material and the spacing between the electrodes on the sensor’s capacitance is also investigated. To demonstrate the feasibility of the method, a sublimating dry ice pellet is placed on the capacitive sensor, and its performance is evaluated. We present results for the dimensionless vapor layer thickness and the pellet’s lifetime at different substrate temperatures, derived from the capacitance output. The results are compared with Optical Coherence Tomography (OCT) data, serving as a benchmark. While the temporal evolution of the sensor’s output, variation in the dimensionless vapor layer thickness, and the lifetime of the dry ice pellet align with expected results from OCT, notable quantitative deviations are observed. These deviations are attributed to practical experimental limitations rather than shortcoming in the sensor’s working principle. Although this necessitates further investigation, the methodology presented in this paper can potentially serve as an alternative for the detection and measurement of Leidenfrost vapor layers.

Cryogenic liquids such as liquid nitrogen are of relevance for numerous processes in engineering, and the food and pharmaceutical industries. However, as a result of its strong evaporation at ambient conditions, its handling for laboratory purposes and experimentation is so far cumbersome. In the present work an original design approach for a supply device for liquid nitrogen is developed and characterized in detail. With the device pure liquid nitrogen is supplied from a pressurized dewar flask to a hypodermic needle without contamination of the liquid with its own vapor or frost, finally enabling to generate a free liquid jet or single droplets in a way comparable to the handling of non-cryogenic liquids using a syringe and a hypodermic needle. Compared to previous approaches for the generation of liquid nitrogen droplets in scientific studies which mostly rely on a reservoir for liquid nitrogen from which droplets form at a bottom outlet due to gravity, the present design allows generation of droplets and free liquid jets in a significantly better controlled and more flexible way. The device is experimentally characterized for varying operational conditions during generation of a free liquid jet, and its versatility for laboratory research purposes is further briefly demonstrated.