3D Lattice Boltzmann simulations for water droplet's impact and transition from central-pointy icing pattern to central-concave icing pattern on supercooled surfaces. Part II: Rough surfaces

Abstract

In this paper (Part II of a paper series), a 3D pseudo-potential lattice Boltzmann method in combination with a solid-liquid phase change model with volume expansion taken into consideration (the same model used in Part I) is applied to study a water droplet's impact and its subsequent freezing patterns on rough supercooled substrates, consisting of an array of micro-pillars in a saturated vapor environment. Freezing patterns of the droplet in three different modes on rough surfaces depending on the contact angle are investigated. In the Hemi-wicking Cassie-Baxter mode where the water droplet fills into grooves between micro-pillars completely, heat conduction rate increases because of the increasing contact area of the water droplet and the micro-pillars, and the contact angle becomes smaller which results in the central-concave icing pattern. When the droplets are in the Wenzel mode and the Cassie-Baxter mode on the rough surface, saturated vapor exists between the droplet and the micro-pillars, which brings additional thermal resistance between the supercooled substrate to droplets. This results in lower heat conduction rate which delays ice nucleation time and allows the droplet to complete its recoiling motion that leads to the formation of the central pointy icing pattern. Because static contact angles of rough substrates in these two modes are higher than those of the corresponding smooth substrates, droplets’ recoiling motions on rough substrates are stronger which lead to the formation of central-pointy icing patterns. Regime maps, showing effects of contact angle and Stefan number on formation of central-pointy icing or central-concave icing patterns of a water droplet (with D0 = 100 and Pr = 13.5) after its impact on rough substrates at supercooled temperatures at We= 320 and Re = 164.9 are presented.