Contact Angle Hysteresis in Droplet Wetting

A liquid drop wetting a solid surface is everywhere in our daily life as well as in engineering and science. In this thesis, there are four major parts to investigate the effect of surface roughness on droplet wetting. (1) Contact angle hysteresis (CAH) on slippery liquid-infused porous surfaces (SLIPS) is essentially absent due to the smooth and defectless liquid-liquid interface. Our SLIPS is facilely fabricated by a stretched polytetrafluoroethylene (PTFE) tape infused with fluorinated lubricant. Because the PTFE film is thin, SLIPS can be transparent and flexible. The absence of hysteresis loops of contact angle (CA) or base diameter for both water and hexadecane shows neglected CAH of SLIPS. The excellent anti-smudge performance of our SLIPS is exhibited by two experiments: the removal of stains by sliding drops at an inclined plane and the wetting competition of a hexadecane drop between SLIPS and a lipophobic surface. (2) The encounter of a nanodrop with a trench on a CAH-free surface is explored by many-body dissipative particle dynamics (MDPD). A free nanodrop exhibits Brownian motion and the diffusivity decays exponentially with the liquid-solid contact area. In contrast, a nanodrop sitting on a trench shows the restricted random motion. Work must be done to overcome the energy barriers for the transition between free and trapped states. According to the force-displacement plot, the potential energy landscape is constructed. It is shown that the trench acts as a hydrophobic blemish for capture but like a hydrophilic blemish for escape. The macroscopic experiments are performed by SLIPS. The experimental observations agrees qualitatively with simulation outcomes. (3) Nanodrops on patterned rough surfaces are explored by MDPD. On a rough surface, the contact line is pinned and the most stable CA different from the intrinsic CA is acquired due to surface roughness. The extent of CAH is determined by two approaches which resemble the inflation/deflation method and inclined plane method for experiments. The hysteresis loop is acquired and both approaches yield consistent results. The influences of wettability and surface roughness on the most stable CA and extent of CAH are examined. The deviation of the most stable CA from that estimated by the Wenzel or Cassie-Baxter models is explained by the extent of impregnation which varies with the groove position and wettability. Moreover, the extent of CAH depends more on the groove width than the depth. (4) The stability of air pockets formed in grooves on a surface is relevant to CAH and it is investigated by imbibition experiments and Surface Evolver (SE) simulations. The air pocket formed by placing a liquid drop atop a conical hole on a polymethyl methacrylate (PMMA) substrate and its stability depends on surface wettability. Four kinds of imbibition behaviors are observed. The imbibition pathway is clearly observed by using the olive oil drop. The energy-barrier profile associated with the imbibition pathway acquired by SE can interpret the outcome of imbibition. CAH of various liquids on a PMMA substrate with drilled holes is also determined. Their wetting behaviors are categorized into three types and can explain the experimental results reasonably.