The results of the collaborative work of scientists have been published in the journal International Communications in Heat and Mass Transfer (Q1, IF: 6.4).
The interaction of liquid droplets with surfaces is a crucial issue in experimental, numerical, and theoretical research, as it helps to explain numerous phenomena in nature and technical applications. Currently, the most promising methods for substantiating the processes occurring during a droplet's impact on a surface are numerical modeling and video and photo documentation with subsequent post-processing. However, often only by combining these approaches can a clearer understanding of the physics of the process be achieved, and sometimes additional research methods are required.
Scientists from the Heat and Mass Transfer Laboratory at Tomsk Polytechnic University, in collaboration with colleagues from the Institute of Thermophysics of the Siberian Branch of the Russian Academy of Sciences, have developed a numerical model illustrating how "fingers" form and how the dynamics of droplet spreading occur upon impact with a solid surface. To verify the accuracy of the developed model, the researchers compared it with experimental data.
“In experiments, a droplet and its individual components, such as the rim of the spreading droplet, deform differently depending on the wall characteristics and the initial parameters of the droplet (its speed and diameter). Understanding the droplet deformation process, such as the appearance of "fingers" during spreading along a surface, will enhance our understanding of the physics of the interaction between liquids and surfaces. In this study, we only worked with water, but the developed method can be adapted for other liquids and surfaces,” notes one of the study's authors, Associate Professor at the TPU Scientific and Educational Center, I.N. Butakov, Maxim Piskunov.
The developed numerical approach accounts for the contact angle of wetting, which depends on the speed of the contact line of the spreading water droplet and the hysteresis (instantaneous response to influence) of the contact angle in the discontinuous Hoffman function at the moment of maximum spreading of the water droplet along the surface. This modification of the Hoffman function significantly increased the accuracy of predicting the maximum droplet spreading, as the numerical model began to more adequately predict the dynamic deformation of the droplet rim up to the moment of its maximum spreading.
Moreover, the scientists jointly established certain patterns in the calculations of droplet deformation. For instance, research showed that the shape of the droplet and the formation of "fingers" on it depend on the impact speed: at low impact speeds (less than 0.5 meters per second), the droplet retains a circular shape; at moderate speeds (from 1.2 to 2 meters per second), the droplet forms a polygon; and at high speeds (more than 3 meters per second), "fingers" appear on the droplet.
The study results indicated that the presence and quantity of "fingers" depend on the contact angle. For example, if the droplet has a convex rim shape at low impact speeds, "fingers" form from it.
“The results of our numerical study allowed us to estimate the number of "fingers" on droplets with up to seven percent greater accuracy. The developed method sets a very high standard regarding the precision of liquid droplet deposition on specific surfaces, closely approximating the process of single droplet deformation and its application to the surface to real working processes. Additionally, the research demonstrated the potential to scale the results to coatings and layers, adjusting detail and quality integrally when transferring to actual production processes,” notes one of the study's authors, Senior Researcher at the Physical Hydrodynamics Laboratory of the Institute of Thermophysics of the Siberian Branch of the Russian Academy of Sciences, Ivan Vozhakov.
In the future, the researchers plan to expand their study by investigating the impact of wall roughness on the formation of critical wavelength and the number of "fingers" during the deformation of a liquid droplet and its rim, as well as conducting a more detailed examination of the processes near the contact line.