High-Speed Imagery Analysis of Droplet Impact on van der Waals and Non-van der Waals Soft Oil-Infused Surfaces

Droplet impact on Slippery Liquid-Infused Porous Surface (SLIPS) has been widely explored due to its significant scientific implications and industrial relevance. This study investigates how the surface functionalization, oil coating, and oil absorption on smooth polydimethylsiloxane (PDMS) substrates influence the droplet impact behavior. Smooth PDMS samples were functionalized with octadecyltrichlorosilane (OTS) to reduce the surface energy and enhance water repellency. Contact angle(CA) measurements indicate that OTS treatment significantly decreased water adhesion. Following functionalization, these surfaces were either coated or absorbed with two different lubricants, i.e., 5cSt-silicone oil (SO-5cSt) and hexadecane. SO-5cSt exhibits strong interaction with PDMS and PDMS-OTS, as indicated by ultrasmall contact angle, i.e., eq CA∼1°. Meanwhile, hexadecane has a finite CA, indicating weak affinity for PDMS and PDMS-OTS. High-speed imaging was used to capture water droplet impact on this surface across a broad range of Weber numbers, enabling detailed analysis of rebound phenomena. PDMS with OTS functionalization results in partial droplet rebound upon impact. Surface wettability and droplet impact outcomes of PDMS_{(SO-5cSt-coated)}, PDMS_{(Hexa-coated)}, and bare PDMS surfaces were largely similar. High-speed imaging confirmed no significant changes in spreading or rebound dynamics across these surfaces, suggesting that the thin oil layer likely lacked sufficient coverage. In contrast, when PDMS and PDMS-OTS surfaces absorbed SO-5cSt, they exhibited full droplet rebound behavior even at low Weber numbers. This rebound is attributed to the presence of a thin layer of SO-5cSt on the absorbed surface, leading to low CA hysteresis and reduced energy dissipation. Conversely, PDMS and PDMS-OTS samples absorbed with hexadecane showed no or partial rebound, respectively, due to hexadecane's inability to form a stable thin film on the surface. These findings enhance understanding of how surface functionalization and lubrication strategies influence droplet impact behavior on the smooth porous surfaces and are relevant for applications in self-cleaning, microfluidics, and controlled droplet manipulation.