The role of viscosity ratio in Janus drop impact on macro-ridge structure

Abstract

An interaction of liquid and solid surfaces upon impact has made great progress in understanding the principle behind impinging compound drops, such as single-interface Janus and core–shell configurations, for controlling drop mobility on the surfaces. Despite advancement of recent technologies, fundamentals of how viscosity ratios of Janus drops affect post-impact dynamics on anisotropic surfaces are still unknown. Here, we numerically investigate the asymmetric impact dynamics of Janus drops on a non-wettable ridged surface to demonstrate the feasibility of the separation of the low-viscosity part from the high-viscosity part by reducing the residence time. The separation is investigated for various viscosity ratios, Weber numbers ( We), and initial angle, which are discussed in terms of the temporal evolution of the mass and momentum distributions. A regime map for the separation reveals that the low-viscosity parts are more likely to be separated from high-viscosity parts as the viscosity ratio increases. The phenomenon can be related to a retraction time, which is explained by a hydrodynamic model for the low-viscosity part. This study suggests that We thresholds for the separation can be significantly reduced with the help of center-assisted retraction along the ridge. The asymmetric bouncing of Janus drops on a ridged surface can open up possibilities for the efficient control of liquid separation.

Characterizing the Bounce and Separation Dynamics of Janus Drop on Macrotextured Surface

“…Approximately 80% of the W-part or more (Ω ≥ 0.8) can be separated from the G-part at the We w thresholds. It is found that the We w threshold for separation at D 0 = 3.0 mm is almost similar to that observed at D 0 = 2.0 mm (We c,w ~40) [28]. It is found that Ω decreases slightly with increasing We w and α, which can be seen by the triangle symbol's size described in Figure 4e.…”

“…In other words, the higher the viscosity ratio, the easier it is for the W-part to separate from the G-part. It is found that the Wew threshold for separation at D0 = 1.33 mm is slightly higher than that observed at D0 = 2.0 mm (Wec,w ~ 40) [28]. In addition, the volume ratio of the separated W-part decreases slightly with increasing Wew and α, which can be seen by the triangle symbol's size described in Figure 3e.…”

“…The inset represents the normalized residence time as a function of Oh g /Oh w , obtained from numerical data (symbols) and predictions (lines) under the high-We w regime. To explain the trend of the decreasing t c,w /τ o,w , we employed a theoretical model related to a receding velocity of the film and conservation of mass from the previous study [16,28]. The residence time can be determined by durations for spreading and retraction as t c = t s + t r , where the dimensionless time t is denoted as t = t/τ o,w .…”

“…Many researchers have focused on an exploration of the basic physical principle and practical meaning as related to liquid-solid interaction, including drop impact on micro-and nano-structured surfaces [16][17][18][19][20][21][22][23][24][25][26][27][28]. Bird et al [16] studied that superhydrophobic surfaces with macrotextures could decrease the contact time of millimeter-sized water drops by means of center-assisted retraction.…”

“…The role of viscosity of impinging drops in bouncing dynamics was studied numerically by investigating the residence time on single-ridge surfaces [27]. Most recently, our group demonstrated the possibility of the separation of the low-viscosity component from the high-viscosity component on macroridge structures by reducing the residence time [28]. The previous study predicted the bounce and separation behavior for various Weber numbers, viscosity ratios, and initial inclined angles with respect to the ridgeline.…”