The study of droplet impact is crucial due to its myriad industrial applications like fuel injection, painting, windshield coatings, aircraft icing prevention, electronic-component welding, fuel combustion optimization, etc. (Worthington 1877; Bragg 1996; Martin et al. 2008; Bolleddula et al. 2010; Liu et al. 2013; Moghtadernejad et al. 2016). Some other applications include pesticide spraying on crops, insecticides spray on fly wings, tissue engineering, to name a few (Gart et al. 2015; Zhang et al. 2019). In most of these applications, the droplets impact on tissues, leaves, etc. which are soft surfaces. The fate of the droplets which impact on a surface depends upon several factors such as droplet impact velocity, droplet size, surface tension, viscosity, as well as the surface properties (Yarin 2006). The impact dynamics on hard surfaces have been rigorously studied due to its fundamental importance and its applications. The major forces governing the process of impact are inertial force, force of surface tension and viscous force. Accordingly, the strength of non-dimensional parameters like the Reynolds number \(\left(Re\right)\), Weber number \(\left(We\right)\), among others, determine the different impact outcomes such as deposition, rebound, prompt splash, corona splash, partial rebound, and receding breakup (Rioboo et al. 2001; Josserand and Thoroddsen 2016). For instance, droplets of low We undergo deformation, deposition and retraction (Rioboo et al. 2002; CLANET et al. 2004; Yarin 2006). On the other hand, high \(We\) impact is dominated by the phenomena of splashing wherein the inertial force dominates over stabilizing surface tension forces(Xu et al. 2005; Liu et al. 2010) .Surprisingly, a limited work has been done towards the understanding of the impact dynamics on soft/elastic surfaces (Rioboo et al. 2010; Chen et al. 2011; Alizadeh et al. 2013; Weisensee et al. 2016; Howland et al. 2016).
Unlike a solid surface, the dynamics of droplet impact on a soft surface is more complex due to deformation of the surface. The surface deformation alters the contact angle hysteresis depending upon the impact velocity (Rioboo et al. 2010). The energy lost due to deformation of soft substrates also reduces the rate of splashing of the droplets (Howland et al. 2016). Chen and Li demonstrated that the droplet rebounds more on soft surfaces due to the formation of an air film at the interface (Chen and Li 2010). Additionally, a droplet impact on a soft surface undergoes oscillations. Chen et al. gave insights into the oscillations of droplets on a PDMS coated surface by comparing it to a spring mass damper system and different outcome regimes were defined based on the Weber number (Chen et al. 2016). These results are related to Newtonian fluids. However, most of the biological and agricultural applications involve non-Newtonian fluids. The behaviour of these droplets is drastically different from the pure fluid droplets. For instance, protein-laden droplets rebounce less even on a hard surface due to high rate of viscous dissipation (Smith and Bertola 2010). The droplets alter the wetting properties of the substrate which could also lead to a reduction of rebounce of the droplets (Izbassarov and Muradoglu 2016). Such polymer added droplets promote deposition and are highly preferable in applications like spray of pesticide over leaves (Wirth et al. 1991; Bergeron et al. 2000). The enhanced deposition is attributed to the rheological properties of the fluids which induces resistance to the motion of droplets (Bartolo et al. 2007). Vega and Pita showed that addition of small amount of polymer completely suppresses splashing mainly due to elastic energy of the polymer chains (Vega and Castrejón-Pita 2017). However, the behaviour of such polymer droplets has not been explored on a soft substrate. Therefore, a systematic approach is required to understand the dynamics of such impinging droplets.
In this work, we study the impact of polymer droplets upon soft surfaces. We deployed water based PEO droplets on PDMS surfaces of different softness. We delineate three different stages during the entire droplet lifetime and analysed these stages using major governing forces. The effect of the surface softness and the droplet impact velocities has been elucidated.