Browsing by Author "Harikrishnan, A. R."

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Post-impact lamella evolution of drop impact on superhydrophobic cylindrical surfaces at high Weber number
(AIP, 2024-02) Harikrishnan, A. R.
Drop surface interaction is omnipresent in nature and vital to many engineering applications. Most previous studies on drop impact dynamics on superhydrophobic cylindrical surfaces have focused on low-impact Weber (We) numbers, wherein the asymmetric bouncing behavior is the prominent outcome. However, it is observed that an impacting drop at higher impact We numbers (>100) exhibits more complex dynamics. The asymmetric post-impact lamella evolution in axial, and azimuthal directions are analyzed in detail. At higher impact velocities, the lamella expands in an azimuthal direction over the solid cylindrical surface, sweeping a certain angle followed by further expansion in air and over the solid surface until attaining the critical detachment angle or swiping angle, which is found to be a function of surface curvature and impact velocity. Thereafter, the expansion proceeds only in the air until the lamella completely shatters away, indicating the absence of a retraction phase in the azimuthal direction contrary to that during low-velocity impact. Lamella nucleation and film rupturing together, along with the ejection of satellite droplets, further add complexity. The present experimental study comprehensively evaluates the effect of higher Weber numbers (We up to 660) and surface curvature. Universal scaling relations are proposed for the lamella evolution in the axial and azimuthal directions based on the impact parameters to rationalize the same. A minimization of the surface energy approach has been hypothesized to predict the detachment angle utilizing the proposed scaling relations and is found to predict well with the experimental data
Role of retraction dynamics in bouncing to pinning transition during drop impact on cold superhydrophobic surfaces
(AIP, 2024-09) Harikrishnan, A. R.
A deeper understanding of the post-impact phenomenology of droplets on cold surfaces is crucial for comprehending and developing anti-icing surfaces for various applications. In the present study, a systematic experimental investigation has been done in a controlled environment with a wide range of subcooled surface temperatures (⁠ ⁠), slightly over the freezing point of water. The inertia force dominates during the spreading phase, and the time for maximal spreading is independent of the surface temperature. However, surface temperature has a major impact on the recoiling phase and governs the post-impact outcome. During the receding phase, the dynamic receding angle varies drastically and is also found to be strongly dependent on surface temperature. It is proposed that the micro-cavity condensation induced water bridge formation and viscous dissipation critically influences the receding dynamics. The retraction becomes partial retraction and finally pins at low temperatures with an enhanced retraction time, thus aiding the proposed mechanism. An empirical relationship is found for the average receding contact angle as a function of surface temperature. A scaling relation for retraction time is proposed that takes into account both the transient and surface temperature dependent variation of receding contact angle variation and the changes in thermophysical properties of the fluid. A theoretical framework has been proposed to predict the pinning to bouncing regimes for drop impact over subcooled superhydrophobic surfaces. The postulated scaling relation and prediction models are in good agreement with the experimental results.
Solutal Marangoni augmented levitation time of an aqueous surfactant drop over an immiscible oil pool
(Elsevier, 2024-07) Harikrishnan, A. R.
Droplets are found to exhibit a finite residence time over the volatile liquid pool due to the confined evaporation mediated sustained thin film draining. The levitated drops are quite critical in biomedical applications such as for the biophysical micro reactors. It is hypothesized that the presence of surfactants can induce solutal Marangoni and can further enhance the levitation time. Moreover, the surfactant drops can closely mimic the bio drop assay. Droplets of surfactant solutions are allowed to impact over a series of volatile hydrocarbon oils to understand the effect of surfactant concentration, impact height and the properties of the oil pool using high speed imaging technique. Flow visualization is employed in the pendant droplet mode to understand the Marangoni induced internal circulation. The droplets of surfactant solutions are found to exhibit enhanced residence time compared to the case of non-surfactant drops. The levitation time is a function of the surfactant concentration and the volatility of the oil pool. A mathematical model is developed incorporating the effect of solutal Marangoni to test the hypothesis and is found to predict the levitation time with reasonable accuracy. A universal floating–sinking regime map was developed incorporating the influence of governing parameters.