Study On Stick-slip And Orientated Slipping Behaviors Of Solid/liquid Interface Under Electric Field

Microfluidic technology is used to manipulate microfluidics,which is widely used in engineering,physics,chemistry,micromachining,biological engineering and other multidisciplinary research fields.As one of the most potential microfluidic technology,electrowetting on dielectric(EWOD)was used to actuate micro-droplets because of the advantages of non-polluting,real-time actuation,fast response,long-term stability and reliability.Previous studies mainly focused on the electric properties of dielectric materials to decrease the applied voltage.However,the influence of solid/liquid interface characteristics and slew rate voltage on EWOD wettability and frictional behaviors were not studied.Consequently,the effects of surface roughness,surface microstructure and the slew rate voltage on the dynamic characteristics of electric wettability were studied by contact angle meter and applied voltage,aiming to clarify of the stick-slip and oriented slipping of micro-droplet at solid-liquid interface under applied voltage.Furthermore,a circuit protection device was designed to protect electric equipment.Based on the above studies,the main conclusions of this study are provided as follows:(1)The effect of surface roughness on EWOD response were studied.Results showed that threshold voltage exponentially increased with increasing surface roughness.The actuation voltage for micro-droplet on solid surface was closely related with solid-liquid interfacial pinning and adhesion force.The pinning force varied with surface roughness and adhesion forces increased with applied voltage.The increasing of adhesion and pinning force resulted from the realignment of the interfacial water molecules in the diffuse layer under applied voltage.Furthermore,the quantity of the interfacial water molecules depended on the actual contact area.Therefore,the combined actions of adhesion and pinning force contribute to the interfacial friction force.(2)Longer sliding distance of droplet was achieved using direct current voltage by increasing the slew rate voltage.The motion of the droplet is a stick-slip process under applied voltage.The triple contact lines slipped at least seven times before saturation at 1 V/s,however,it slipped three times and twice at 10 V/s and 50 V/s respectively.Moreover,the average slip distance per sliding increased more than four times when the incremental velocity of voltage increased from 1 V/s to 50 V/s.Dynamics analysis indicated that the farther slip distance should be attributed to the higher solid-liquid interface molecular-kinetic frequency and inertial effect at higher incremental velocity of voltage.(3)Surface microstructures were used to control the solid/liquid interface oriented slipping behaviors.The irregular distribution of microstructures on solid surfaces caused the difference between right and left contact angle.The slip distances and directions mainly depended on distribution of microstructures on the solid surface,which were caused by the inhomogeneous force at the triple contact line.Moreover,there were no significant differences of the slip behaviors among three different salt solutions.Based on the experiments,the microstructures with different height were made on the solid surface to control slip behaviors of micro-droplet.When the ridges of microstructures far higher than surface roughness of the hydrophobic surface,the slip distance increased and slip direction deviated from the direction of the microstructures.