Ph.D. (Engg): Splashing of impacting drops on rigid and flexible superhydrophobic surfaces

Understanding droplet impact dynamics on superhydrophobic (SHP) surfaces is crucial for exploiting their exceptional liquid-repellent properties in various industrial applications. Studies on high-velocity droplet impacts on SHP surfaces, particularly under splashing conditions, are of significant importance for applications such as anti-icing and erosion protection of turbine blades. This thesis presents an experimental investigation of splashing of impacting drops on flexible cantilever and rigid superhydrophobic (SHP) surfaces. The SHP surfaces are prepared by spray-coating plastic polymer films (overhead projector sheets) with commercially available NeverWet hydrophobic solution. For the flexible cantilever SHP surfaces, the coated film is cut into rectangular beams, with one end of the beam is firmly fixed on an aluminium support. For the rigid SHP surface, the entire coated film is firmly fixed to the aluminium support. In this study, four cantilever beams of different lengths are considered to study the effect of beam stiffness on the drop impact dynamics. Three different droplet liquids – water (W), 20% glycerine and 80% water (G20), and 50% glycerine and 50% water (G50) – varying mainly in their dynamic viscosity are considered.

Water droplet impact on the rigid superhydrophobic (R-SHP) surface exhibits five regimes with increasing Weber number, We, from complete rebound to prompt splashing, and is mapped on a We–Re chart alongside reported SHP surfaces. The various parameters of droplet impact such as maximum spread factor (β_max), spreading time, contact time, maximum retraction rate, and coefficient of restitution (COR) are characterized as functions of We. The splashing of impacting water drops on the R-SHP surface is studied and an additional regime (sixth regime) of prompt splashing with hole nucleation is documented. Quantitative measurements of the temporal variation of lamella diameter, number of holes formed, time at which the first hole nucleates, critical impact velocity at which the hole-nucleation begins, and contact time are extracted. These measurements clearly show that the contact time reduction of splashing droplets decreases with increase in droplet viscosity. Moreover, the number of holes formed in the lamella film scales with the number of micro-bumps underneath the droplet at its maximum spreading which, in turn, decreases with increase in droplet viscosity. The time instant at which the first hole nucleates on the lamella film is seen to be independent of the droplet viscosity. A modified model is proposed to describe the effects of droplet viscosity and surface micro-characteristics (height and pitch of surface micro-bumps) on the critical velocity for hole nucleation, U_c,h. The predictions from this modified model seem to explain the experimental observations on U_c,h in the current study as well as in the literature.

Droplet impacts on the flexible superhydrophobic (F-SHP) surface follow the same sequence of regimes as the R-SHP surface, but the regime boundaries are shifted to higher We due to partial energy transfer from the droplet into beam bending. For a given We, the maximum spread factor, receding velocity, and maximum rebound height are all reduced relative to that of the R-SHP surface. A modified Weber number, We^#, which accounts for the elastic bending energy stored in the beam during the drop impact dynamics is proposed, and the variation of F-SHP data with We^# collapses with the corresponding data recorded on the R-SHP surface for both spreading and receding dynamics. The beam oscillation response, including deflection, oscillation amplitude, and the effect of droplet re-impact on the oscillation cycle, is also characterized. Splashing dynamics of impacting drops on the F-SHP surface shows regime transitions comparable to those on the R-SHP surface but shifted to higher We and Re. Hole nucleation on the F-SHP surface is delayed relative to the R-SHP surface. The recorded value of U_c,h increases from 1.75 to 2.00 m/s (14.3%) for W and from 2.35 to 2.53 m/s (7.7%) for G20 drops, and the number of holes is lower for both liquids at a given We, attributed to part of the impact kinetic energy being stored as strain energy in the beam. For G50 drops, no holes are formed on both surfaces within the tested We range. Before the hole nucleation, the contact time is unaffected by the surface flexibility, and while beyond U_c,h, the recorded contact time is longer on the F-SHP surface by about 10% for W and 8% for G20 drops. The surface flexibility therefore affects the contact time only indirectly, by delaying and suppressing hole nucleation.

The effect of beam stiffness on droplet impact dynamics is studied by carrying out water drop impacts on four F-SHP surfaces. The maximum spread factor follows β_max = α(We)^0.25, with the empirical constant α decreasing systematically from 1.012 for the stiffest beam to 0.899 for the most flexible beam, reflecting greater energy loss to beam bending as stiffness decreases. The variation of β_max with We^# collapses the data across all the F-SHP and R-SHP surfaces into a single trend, established in the present study as β_max = α^#(We^#)^0.25 with α^# estimated as 1.00 ± 0.07 for both the R-SHP and F-SHP surfaces. Similarly, the receding velocity decreases with decreasing beam stiffness but collapses across all surfaces when plotted against We^#. The contact time is found to be largely unaffected by the beam flexibility. The maximum rebound height of the bouncing drop on the F-SHP surfaces decreases with decreasing beam stiffness. Since the pinched-off droplet formed in the rebounding process carries 6 to 14% of the impacting droplet mass, the COR is redefined in the present study using an effective rebound velocity that accounts for both droplets, and the modified COR decreases with increasing beam length at a given We. The maximum beam tip deflection of the F-SHP surfaces increases with both We and beam length, L, and is well predicted by spring-mass model incorporating the impact force and beam natural frequency, f_b. Finally, a theoretical model is proposed to predict COR on the F-SHP surfaces. The proposed model predicts the experimental COR across all F-SHP and the R-SHP surfaces with errors of 5.0 – 15.2% and also captures the dependence of COR on the beam length.