Investigation On Contact Angle Hysteresis Of Droplets On Hydrophilic And Hydrophobic Surfaces

Hysteresis phenomenon of droplet moving on surfaces widely exsits in nature, industrial manufacture and daily life. However, the main reason of the hysteresis phenomenon has not yet been fully solved in theory. As to the droplet in critical condition, analysis and calculation of hysteretic force and driving force of a droplet is one of the key points to quantitatively describe the hysteresis phenomenon.Through the state observation of beginning movement of droplets on hydrophilic and hydrophobic surfaces by means of high speed camera, the hysteretic forces of droplets on hydrophilic and hydrophobic surfaces were studied here. The results show that the positions of initial movement of droplets on hydrophilic and hydrophobic surfaces are different. Through theoretical analysis and verification with experimental data, we find that the hysteretic forces of droplet moving on hydrophilic and hydrophobic surface are corresponded to spreading work and adhesion work respectively. And the experimental results coincide with theoretical analysis. The hysteretic force reflects the essence of hysteresis phenomenon and provides theory basis for furthering research on contact angle hysteresis.When a droplet is on the critical state of moving, it should get driving force in order to be in balance. According to the theoretical derivation and Wilhelmy experiment data, the driving force of a droplet on the state of advancing contact angle (ACA) and receding contact angle (RCA) were analyzed. And based on the force balance of the droplet in critical condition, the theoretical formulas of ACA and RCA were derived for hydrophilic and hydrophobic surfaces. Meanwhile, through two steps chemical etcing method, we got aluminum and copper hydrophobic surfaces with different contact angles. ACA was measured by slowly adding a small volume of the probe liquid to droplet on the surface, and RCA was measured by slowly removing the probe liquid from the droplet. Combining with experimental data from literatures, the theoretical formulas were verified. It is indicated that contact angle hysteresis (CAH) depends on both surface topography and interface chemical property, CAH does not exist only when equilibrium contact angle is0°or180°. The theoretical formulas can be used to predict both ACA and RCA and have instructive effect on designing hydrophilic and hydrophobic surfaces with low CAH.