Splash: Understanding the Dynamics of High-Speed Drop Impact

The dynamics of capillary-driven free surface flows, together with turbulent behaviour are, arguably, some of the most challenging topics in fluid dynamics. They involve complex flows evolving across several length and time scales. The dynamic fluid processes involved during drop formation and disintegration are fascinating but extremely complicated, with time-dependent fluid interface disruptions. A drop impacting on to a surface (solid, liquid, or granular) can lead to simple spreading, bouncing, or splashing. The resulting dynamics depends not only on the liquid properties and speed of the drop, but on a variety of other parameters, such as the surface's roughness, stiffness, chemistry, and temperature, and surrounding conditions. While in some industrial applications splashing is desired (e.g. cooling and combustion), it is to be avoided at all cost in others (e.g. inkjet printing or in the prevention of the spreading of infectious diseases as the Covid-19 pandemic has shown).

For my research, I am currently using advanced mathematical techniques combined with state-of-the-art computational power, and technological advances in experimental imaging techniques, which allow me to observe and model the dynamics in unprecedented detail and at unprecedented speeds.

Splashing is one of the most fascinating, albeit challenging, topics in the field of drops. However, the exact mechanisms triggering a splash have remained elusive. With the help of cutting-edge ultrahigh speed photography, modern ultrahigh resolution numerical simulations and asymptotic theory the main objective of the proposed research is to reveal the dynamics underlying and triggering a splash. In particular, we aim at:

1- Identifying the parameters leading to a splash in when a droplet impacts a pool of miscible and immiscible fluids. This includes developing models capable of resolving three phase flows under these violent conditions.
2- Reveal the (vertical) speed of the contact line (drop/pool) upon impact. Preliminary results demonstrate that a complex relationship between the ratio of densities and viscosities between the liquids of the drop and the pool play an important role here.
3- Understand the contributions due to pure viscous and viscoelastic effects of both the drop and target.
4- Explore the influence that the curvature of the target has on the resulting dynamics.
5- Based on the above, explore techniques to suppress splashing.

The first part (1 and 2) of this this research has already been performed carrying out a series of systematic experiments of the impact of drops onto (immiscible) substrates of varying viscosity. Volume of Fluid simulations are currently being carried out which are providing detail not available from experiments (e.g. internal velocity fields) to enable greater understanding of the underlying dynamics and how this can lead to splashing. Whilst the aforementioned area of research is largely concerned with the motion of the droplet in the case of liquid on liquid impact the motion of the pool itself is also one of great interest but is often overlooked, especially in the early times upon impact. Whilst some models exist for the pool motion, these are largely limited to when the droplet and pool fluid are the same fluid and are often used when they are often not appropriate. For this reason, another area to be researched is the motion of the pool both before and after impact and how this pool motion is affected by the relevant fluid properties and how this pool motion can affect splashing.

These objectives are major undertakings, and a close collaboration with Dr. R. Cimpeanu at the University of Warwick.

The proposed research falls within the EPSRC areas of Fluid Dynamics and Aerodynamics, Complex Fluids and Rheology, as well as Manufacturing the Future for the importance identified above in industrial applications such as inkjet printing.