Experimental Study On The Dynamic Contact Angle Affected By Micron-sized Factors

With the development of global economy and the progress of science and technology, human beings require higher and higher for materials` performances, particularly for surface wetting properties. Inspired by the functions of surface self-cleaning of natural animals and plants, people are researching, inventing and fabricating self-cleaning surfaces. It is essential for self-cleaning surface with low friction between solid and liquid, so the liquid can move easily and take away the contaminants on the surface. There are two manners for surfaces self-cleaning, the contaminants are flushed by droplet(even water film) on hydrophilic surfaces and directly took away by droplet on super-hydrophobic surfaces, capturing insects of Nepenthes pitcher plants and the lotus effect can be classed to the former and the later, respectively. The difference between advancing and receding contact angle(so-called dynamic contact angles) is one important parameter to characterize the self-cleaning property, and affected by many factors, such as solid surface roughness, surface chemical heterogeneity, surface molecular weight and the chain length, liquid molecular polarity, velocity of the droplet, temperature and humidity and so on. However, for the influence of impurities and microstructure scale on dynamic wetting, there is less research.For this kind of dynamic contact angle changes, the problem of contact angle deviation from equilibrium is essentially caused by defects. The changes of contact angle reflect the properties of solid-liquid-vapor three phase contact line, in fact, whether it is solid, liquid or gas cannot reach the ideal state. Especially, the effect of liquid and solid properties on contact line is more important. To investigate the effect of impurities regarded as the defects in liquid, we add some micrometer PMMA particles into liquid. Besides, for the effect of solid, the main defect is the roughness. To explore the influence of solid surface topological changes, we use cylindrical pillar microstructure arrays to carry out experiments. In a word, we pay our attentions to the effect of the impurity concentration and surface microstructure scale on dynamic contact angle changes.When droplet evaporating, the contact angle is decreasing with the decreasing of droplet volume, however, the mechanism is not clear up to now. We experimentally observed that droplet three phase contact line shrinks inward, the contact line length decreases, the density of impurities on the contact line and the friction resistance on the contact line also increases, resulting in contact angle reducing. Based on the contact line sweeping, we propose a particles sweeping model which considering the effect of impurities concentration, friction and initial volume of droplet. The model gives the explanation that the higher concentration induces higher impurities friction; the larger initial volume causes faster decreasing of contact angle and leaves a bigger deposition ring. The model also quantitatively describe the correlation of droplet contact diameter and contact angle based on the three aforementioned factors, and predict the final deposition pattern size, and agree with the experimental results.Finally, we research the influence of surface microstructure scale on dynamic wetting properties. Based on the controlling volume method, the experimental results show that as the droplet volume increasing, the advancing contact angle decreasing; as the droplet volume decreasing, the receding contact angle decreasing, that means the contact angle hysteresis has 5° decrease. We can give the conclusion that increasing the droplet volume is benefit for surface self-cleaning, because of the droplet hysteresis and the motion resistance decreasing. On different microstructure surfaces, the contact angle hysteresis is larger than the values on smooth surfaces(2-3 times), as the increasing of droplet volume, the hysteresis decreasing reaches up to 26°; the smaller of microstructure scale, the larger hysteresis decrease due to contact line divided into more short line and promote the contact line moving. So, rationally controlling the droplet volume and selecting the microstructure scale, we can determine the droplet dynamic wetting properties.