An image processing method to measure droplet impact and evaporation on a solid surface
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Sådhanå (2020) 45:287 https://doi.org/10.1007/s12046-020-01520-0
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An image processing method to measure droplet impact and evaporation on a solid surface AMITOSH DASH1, PRATHAMESH G BANGE1, NAGESH D PATIL2 and RAJNEESH BHARDWAJ1,* 1
Department of Mechanical Engineering, Indian Institute of Technology Bombay, Mumbai 400076, India Department of Mechanical Engineering, Indian Institute of Technology Bhilai, Raipur 492015, India e-mail: [email protected]
2
MS received 19 April 2020; revised 14 September 2020; accepted 6 October 2020 Abstract. We employ high-speed visualization to quantify the impact and evaporation of a droplet on a solid surface. The time-varying droplet shapes are recorded using a high-speed camera and an image processing method is developed for measurement for droplet dimensions, dynamic contact angle, and volume. The method is based on the liquid-gas and liquid-solid interface detection using Canny’s method and Otsu’s method for threshold digitalization. To obtain the dynamic contact angle, a higher-order polynomial is fitted along the pixels near the contact line. The droplet volume is measured assuming an axisymmetric geometry of the droplet and numerically integrating the identified pixels in the droplet. We demonstrate the detection of the complex topology of the deforming liquid-gas interface of an impacting and evaporating droplet on a solid surface. We plot the time-variation of droplet dimensions, namely, wetted diameter, maximum diameter, height; dynamic contact angle, and droplet volume. The contact angle and volume measurements are verified with known analytical values. Three case studies are considered to demonstrate the capability of the proposed method, namely, sessile evaporating droplet, bouncing droplet, and spreading viscous droplet on a solid surface. Keywords.
Droplet impact; droplet evaporation; image processing method; contact angle.
1. Introduction Droplet impact and evaporation are useful for several technical applications such as ink-jet printing, spray coating, and surface cooling via spray evaporative cooling [1] and pool boiling [2]. Recently, the mechanisms of the formation of dried deposits of colloidal particles by an evaporating sessile droplet has been a much-studied problem (for example, see review of such studies in [3, 4]). The impact and evaporation process is highly transient [5, 6] and high-speed visualization is one of the key techniques to record time-varying droplet shapes (see a review of visualization techniques in [7]). Experimental protocols for recording the sample preparations and recording images of advancing and receding contact angles of a water droplet on a solid surface were recently reported by Huhtama¨ki et al [8]. The accurate determination of the droplet dimensions, volume, and dynamic contact angle from the visualization images are important to reduce measurement errors. Previous studies on the contact angle measurement mostly focused on static droplets with a non-moving
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