Droplet impingement on nonuniform heated substrates

Abstract

Droplet impingement on heated substrates is a complex phenomenon that has major significance in a wide range of applications including but not limited to, inkjet printing, surface coating, and heat transfer in spray cooling. This project investigates the effect of a nonuniform substrate on droplet impingement interactions. Two stainless steel specimens were fabricated with a horizontal and vertical lay orientation relative to a dividing line that separates areas of different surface roughness. The experiment parameters consist of deionised water with Weber numbers of 18.5, 46.3 and 74.1 along with five initial surface temperature ranges (25°C, 175°C, 200°C, 250°C and 300°C). When the initial surface temperature is 25°C, droplets impinging on both vertical and horizontal lay surfaces recede to an equilibrium position with greater contact in the rough regions for We=46.3 and We=74.1. For We=18.5, the wetted length in both regions are equal after the receding phase in both lay orientations. During the receding phase, the contact line in the rough region appears to be pinned and did not retract as much compared to the contact line in the smooth region for all Weber numbers and lay orientations. Upon impingement on a heated nonuniform vertical lay surface, droplets exhibit a tendency to move towards the smooth region across all Weber numbers which suggests an inclination towards a region with lower contact angles. However, this inclination is only observed at an initial surface temperature range of 200°C to 250°C on the horizontal lay surface. Droplets impinging on the horizontal lay surface tend to stay on the dividing line with contact on both regions at a surface temperature of 300°C. The maximum spread factors for droplets on the surface with a horizontal lay orientation are generally higher across all Weber numbers and temperature ranges. After the receding phase, the wetted area for droplets impinging on the vertical lay surface are smaller than that for droplets impinging on the horizontal lay surface particularly in the higher temperature ranges. This phenomenon is attributed to the increased energy of the droplet following the receding phase, which results in earlier rebound or the formation of a vertical liquid column in cases of no rebound. At a surface temperature of 250°C, the contact line of droplets on the rough and smooth regions of the vertical lay surface fluctuates and decrease rapidly as compared to droplets on the horizontal lay surface. The maximum temperature decrease recorded for all Weber numbers and temperature ranges are greater in the horizontal lay surface particularly at the rough side. The behaviour of droplets under heated surface conditions can be influenced by the nonuniform surface roughness on different lay orientations.