Abstract
To clarify a driving mechanism for the self movement of a droplet across hydrophobic textured surfaces in series and to develop applications for a microfluidic device, we report a theoretical model, a microfabrication technique and experimental measurements. The contact angle of a droplet on a composite surface, the stable surface energy level and the energy barrier caused by hysteresis were investigated. With increasing pattern density of the microstructure, the contact angle and stable surface energy decreased gradually, but the energy barrier increased. Both the analytical results and the experimental measurements show that the surface energy for a suspended status is greater than the one for a collapsed status, which produces an increased energy to generate the movement of a droplet. An analysis of interactions between actuation force, resistive force and viscous force during the motion of a droplet is based on the equilibrium between forces. From a perspective of energy conversion, the difference of surface energy between a higher state and a lower state would drive a single droplet and make it move spontaneously if it could overcome the static friction force resulting from hysteresis and the kinetic friction force under the droplet movement. The mean velocity in the present device, measured as 62.5 mm/s, agrees satisfactorily with the theoretical prediction. The model developed for the energy levels enables us to assess the contact mode of a droplet placed on the patterned surface. For a prediction of the transport capability of the designed devices, a theoretical interpretation of the conversion between surface energy and kinetic energy of the droplet establishes a criterion that the pattern density of a textured surface should be less than 0.76. The effective rate of energy conversion is estimated to be 20.6 %. Hybrid-structured surfaces consisting of microgrooves and nanocrystals have been modified with a self-assembled monolayer (CF3(CF)7CH2CH2SiCl3) via low-cost, mass-production, and highly integrated nano/microfabrication. The microgrooves decorated with nanocrystals were patterned and fabricated on a silicon substrate to yield an ultrahydrophobic surface with an anti-sticking property. The nanocrystals were etched by means of oxidation of the silicon surface. Contours of nanostructured surfaces were inspected with a SEM and an AFM; the surface roughness and level of hydrophobicity depended on the duration of etching. Comparison of contact angles for microdroplets on those designed surfaces showed that the hydrophobicity of the solid surfaces became amplified with nanocrystals and accurately modulated with a pattern density (f1), ranging from 112o to 173.1o, to generate a much increased gradient of Gibbs surface energy that served to transport the microdroplet. To characterize the anti-sticking capability of those hybrid-structured surfaces in quantity, we measured the heights and frequencies of rebounding microdroplets on those test surfaces with varied roughness. Similar to the interfacial characteristics of a lotus leaf, our designed surfaces feature superior aqueous repellence, little hysteresis, and slight adhesion, such that microdroplets hence roll off effortlessly and bounce off repeatedly. We also utilized microfabrication processes to develop a digital-microfluidic devices and further studied the related interfacial collision and chaotic mixing mechanisms between two droplets. To make it sure that the microdroplets could be transported on those designed transport paths, the stature of microgroove more than 50 □m and the volume of droplet less than 15 □L would be suggested on basis of theoretic predictions. Sequences of transient images about two droplets transporting and colliding with each other have been captured from high-speed camera, which show that the average velocities of two droplets were 58.14 mm/s and 54.35 mm/s, respectively. The transient mixing index (TMI) inside the merged droplet has been obtained from the variation in gray level of each image and been calculated with Matlab program, which reaches 0.8 within the mixing time 600 s in the case of collision among three droplets. Microdroplets spontaneously transported by wettability gradient have successfully been simulated with the commercial software CFD-ACE+. The strength of vorticity and the characteristic streamlines in topology have further been calculated and visualized according to the flow field inside a merged droplet at each moment during the mixing process. With regard to droplet collision and mixing, when two droplets collide, the momentum of the moving droplets, an important parameter, is variable and controllable through the varied types of collision to influence directly the mixing quality. The design parameters of the transport device were optimized with the simulation results to abbreviate the duration of mixing, so to promote effectively the quality of mixing. The simulation results demonstrate that the chaotic mixing efficient inside the merged droplet could be enhanced by the three mechanisms as follows: deflective collision, increasing interfacial area in collision, and the constrain effect from the sidewalls. Finally, it would be expected that those related results of the present study could be integrated and applied to those microfluidic devices and biochips.