Evaporation of dimethylformamide sessile drops on stationary and vibrating substrates

Abstract

Contact and interaction between droplet and solid surface is a fundamental transport phenomena problem, with ubiquitous presence in various applications. In this paper, we study the effect of imposing vertical and horizontal ultrasonic vibration on dynamics and evaporation of sessile droplets of dimethylformamide (DMF), a pure volatile model solvent. Droplet contact angle and contact radius are the two main parameters that may change during evaporation. Hence, droplet evaporation may be categorized into different modes: constant angle (CA), constant radius (CR), and a complex combination of CA and CR modes. Imposing substrate vibration affects the evaporation rate and mode by changing the thermodynamics and hydrodynamics of the sessile droplet on the substrate. The former happens by changing the heat transfer coefficient and the latter by pinning or unpinning the droplet from the substrate. Experimental analysis using an optical tensiometer has been conducted for a small DMF sessile drop on a Teflon substrate. Among our results, it is observed that the DMF droplet on a Teflon substrate evaporates in the CR mode until it reaches its receding contact angle. Then, its contact radius recedes to the next equilibrium position. Imposing vertical ultrasonic vibration pins the droplet to the substrate and reduces the receding contact angle, while horizontal ultrasonic vibration unpins it. Furthermore, imposing vibration accelerates the evaporation rate more than 5 times higher than that of the natural convection. The increase is more significant for the horizontal vibration.