Structure of the surface microrelief of a droplet evaporating from a rough substrate as a possible cause of contact angl
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Structure of the Surface Microrelief of a Droplet Evaporating from a Rough Substrate as a Possible Cause of Contact Angle Hysteresis I. N. Pavlov, I. L. Raskovskaya*, and A. V. Tolkachev National Research University “MPEI,” Krasnokazarmennaya ul. 14, Moscow, 111250 Russia *e-mail: [email protected] Received June 16, 2016
Abstract—Based on the refraction images of a droplet evaporating on a rough substrate, we simultaneously observed the dynamics of its surface microrelief, contact angle, and contact line deformations along the entire perimeter of the contact line. This has led us conclude that the microrelief structure is directly related to the phenomenon of contact angle hysteresis and the jump-like pattern of contact line deformation. We suggest a possible mechanism for the occurrence of contact angle hysteresis during droplet evaporation and derive the relations that specify the range of possible contact angles at known microrelief parameters. DOI: 10.1134/S1063776117030141
Information about the properties and state of the surfaces at interfaces underlies a number of modern technologies, and, therefore, surface phenomena are currently among the most topical subjects of research [1–5]. Obviously, surface microinhomogeneities at the place of their localization will affect the contact angles of wetting; therefore, the local contact angles and droplet surface microrelief parameters in the contact line region can serve as informative parameters in determining the quantitative surface characteristics of a solid substrate. In addition, the phenomena of contact line delay and displacement [6], which take place during droplet evaporation and are accompanied by a change in the local contact angle and microrelief both with time and along the contact line, are of interest in their own right. The existence of contact angle hysteresis during droplet evaporation on a rough substrate is pointed out in a large number of papers (see, e.g., [7– 12]), but the mechanism of its occurrence is not considered in detail. In this paper we make an attempt to explain the existence of some range of possible contact angles and the jump-like pattern of three-phase contact line (TPCL) deformation during droplet evaporation from the viewpoint of the droplet surface microrelief structure in the TPCL region. For a quantitative analysis of the phenomena observed during droplet evaporation we used an experimental technique that allows one to reconstruct the characteristic parameters of the global droplet relief and the microrelief in the contact line region, to measure the contact angle, and to observe the TPCL deformation dynamics by analyzing the refraction images.
In our experiment we used a scheme for the vertical probing [13] of a droplet on a transparent rough substrate by a wide collimated laser beam with a diameter of 8 mm (Fig. 1). The setup consisted of a JDSU helium–neon laser with a power of 10 mW, an optical system for the formation of a wide collimated beam with a given diameter, a rotating prism with which the beam w
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