Controlling Impact Contact Times (JHT)

P. B. Weisensee, J. Ma, N. Miljkovic, W. P. King, “Controlling the Contact Times of Bouncing Droplets: Droplet Impact on Vibrating Surfaces“, J. Heat Transfer 140 (2018) 030901

Droplet impact on vibrating solids is ubiquitous in nature and industrial applications, including impact on turbine blades, insect wings, or during spray cooling of electronic systems and steel manufacturing processes. Using high speed imaging, we demonstrate that through substrate vibration (60 – 320 Hz), droplet contact times tc, which are independent of impact speed on rigid stationary substrates, can be actively manipulated and controlled. We show that droplet dynamics and contact times are most sensitive to impact phase, followed by vibration frequency, with vibration amplitude having negligible effects (Figure 1, Figure 2b). We determine a critical impact phase φc at which contact times transition rapidly from a minimum (tc ≈ 0.5tc,th) to a maximum (tc ≈ 1.6tc,th), where tc,th is the theoretical contact time on a stationary rigid substrate (insert Figure 2a). Averaging contact times over all impact phases, we show that for low frequencies (< 80 Hz) average contact times increase relative to impact on stationary substrates, while contact times decrease for impact at higher vibration frequencies (> 100 Hz) (Figure 2a). The present findings provide guidelines for the rational design of applications where the contact time influences heat transfer. During spray cooling, for example, the per droplet heat transfer rates increase (decrease) for longer (shorter) contact times. Thus, by tailoring the vibration frequency of the substrate, the average contact time, and consequently the average heat transfer, can be actively controlled.