Dynamic wetting behavior of droplets on the surface of porous materials with micro-airflow

Abstract

<p indent=0mm>The dynamic wetting behavior characteristic of droplets is of significant importance in various industrial applications and high technologies, such as surface self-cleaning, surface drag reduction, anti-frosting, and droplet directional transportation. The modifications of coating surface energy and micro/nano structures on substrate surfaces are the conventional and indirect ways to manipulate the droplet dynamic wetting behavior. At present, many direct and external incentive ways, like substrate vibration, electrowetting and additional magnetic field, also have been proposed to control the droplet dynamic wetting behavior. In the present study, inspired from the phenomenon of droplet floating in the air without gravity, an innovative manipulation method had been proposed, which utilized the additional micro-airflow field as a momentum field to control the force distribution and wetting characteristic of droplets. The core idea of this innovative manipulation method was similar as the Leidenfrost phenomenon, which used the gas buoyancy to counter the droplet gravity effect. A porous substrate sintered by copper particles was used as the experimental sample, a micro-airflow field was located at the down side of porous substrate, and a droplet was wetted on the up side of porous substrate. The magnitude of upright momentum force could be changed by adjusting the pressure of micro-airflow field. Under different pressure of micro-airflow field, the dynamic wetting behaviors of droplets on porous substrate were visualized, and the change regulations of dynamic wetting parameters as a function of wetting time were analyzed. According to the results, three different dynamic wetting modes of droplets on the surface of porous materials with micro-airflow were identified, namely, intact infiltration mode, broken infiltration mode and non-infiltration mode. In the intact infiltration mode and broken infiltration mode, the droplets were finally in the state of complete infiltration, while in the non-infiltration mode, the droplets always maintained the suspension state similar as the super-hydrophobic wetting state. When the droplet was in the intact infiltration mode on the surface of the porous sample, the corresponding dynamic wetting process was simultaneously affected by gravity, surface tension, lifting force, adhesion force and capillary force. In this mode, the apparent contact angle and volume of droplet decreased exponentially at the initial stage during the dynamic wetting process. At the following stage, the decreasing amplitude of two dynamic wetting parameters became slower and presented a linear declining trend. The wetting area at the bottom of droplet rapidly increased to the maximum value at the initial stage, and then decreased slowly. However, when the ratio of droplet height to bottom diameter was less than 0.03, the decreasing amplitude of bottom wetting area increased rapidly, and the droplet would quickly approach to the state of complete infiltration. Besides, the maximum wetting area at the bottom of droplet increased with the increase of pressure difference driven by micro-airflow. The larger the pressure difference was, the smaller the changing range of apparent contact angle and volume of droplet in the same wetting time was, but the longer the whole dynamic wetting period was.