Wetting Characteristics Of Droplets On Microstructured Surfaces With Gradient Wettability

With the rapid advancement of material science,a large number of functional surfaces are manufactured,including wettability gradient surfaces concerned by many researchers due to the potential applications in heat and mass transfer,antifreezing,deicing,petrochemical industry,aerospace and other fields.There are two main types of wettability gradient surfaces: microstructured surfaces and chemical surfaces.In this thesis,we designed copper and PDMS surfaces structured by gradient micro-squared pillars and holes.The wetting characteristics of droplets on microstructured surfaces with gradient wettability was studied from three parts: static,dynamic and adhesive characteristics.Firstly,the static characteristics of droplet on different surfaces are studied.It is shown that on the surface of the copper substrate and PDMS substrate,the static contact angle will increase with area fraction and roughness.Therefore,the Wenzel equation will cause an error in estimating the apparent contact angle when the microstructure scale is micro.In Wenzel state,the spacing of the microstructures will affect the aspect ratio of the droplet,and the droplet was stretched longer in the direction with a smaller spacing.The surface with low surface free energy and hole structures can keep the droplet in the Cassie state for a long time.In Cassie state,the droplet is not subject to the capillary force of the structure and remains approximately circular.Then,tilting method is used to measure the critical sliding angle and contact angle hysteresis of the droplet,and we find that the larger the droplet volume or the smaller the surface roughness,the smaller the sliding angle.Additionally,under the action of Laplace force caused by liquid-gas interface,the sliding angle of the Cassie droplet is smaller than that of the Wenzel droplet.Secondly,the dynamic characteristics of droplet are discussed by applying microvibration.We find that the droplets at each measurement position have several dynamic modes in different vibration conditions.Specifically,when the vibration energy is small,the droplet will stick to the surface or move unilaterally;the droplet will break while the vibration energy is too large.When the vibration energy is between the previous two,the droplet will move,and the motion can be divided into forward mode and reverse mode.The vibration energy that determines the droplet to start moving relate with vibration frequency.When the vibration frequency approaches the natural frequency of the droplet,the energy barrier required for moving will decrease.Further research find that the movement of the droplet is also related to the expansion of the three-phase contact line.The larger the roughness,the smaller the expansion of the three-phase contact line.Because of the asymmetry,the droplet will gradually creep.In addition,the larger the roughness,the more difficult it is for the droplet to transmit from the Cassie state to the Wenzel state during the vibration process.Therefore,in places with larger roughness,the droplet will be additionally affected by Laplace force.A model for the critical droplet motion is established to analyze the influence of Laplace force and surface static friction force on the moving direction.Finally,the adhesion characteristics of droplets is investigated by establishing a new model suitable for microstructured surfaces with gradient wettability.It is shown that the adhesion energy per unit area is no longer constant on the structured gradient surface,but decreases with the increase of the relative size between the droplet and the structure,and tends to the flat.Also,the adhesion energy per unit area on both sides of the droplet is different due to the asymmetric structures.Under the same conditions,the sliding angle of the Cassie state is smaller than that of the Wenzel droplet.By substituting the adhesion energy value into the motion model,the static friction force of the droplet in the critical state is calculated,which determine the dynamic state of the droplet together with the Laplace force(Cassie state).We used the modified model to calculate the adhesion energy per unit area of the droplets on the microholed,microgrooved and chemical gradient surfaces,and all of them showed similar trend,indicating that this model has a good applicability.It also proves that the movement of droplet is related to the adhesion energy per unit area.