Effect of micro-structure array on the liquid flow behaviors of near-surface layer

Study of the liquid flowing behavior through the micro-structure array has aroused the significant interest due to its key roles in the fields of microfluidics, micro-mixers, micro-heat exchangers, tribology, etc. Micro-structure array can significantly affect the liquid flowing characteristics of the near-surface layer and the solid-liquid interfacial properties, like adhesion, surface wetting, shear viscous resistance, interfacial slip, etc. The researches indicate that the stripe- and square-patterned electrodes can improve the storage properties of the lithium-ion battery due to its ability to promote the diffusion of the liquid electrolyte. Micro-structure array patterned micro-channel can reduce the friction drag of liquid flowing through it. And the surface fabricated with lotus-leaf-like dual-scale structure array can achieve the super-hydrophobicity.For a micro-structure array, its influences on the liquid flowing behaviors greatly depend on the shape and size of the micro-structure, and the porosity, arrangement and size of the array. Here, we mainly focus on the influences of the micro-structure shape and surface topography on the liquid flowing behaviors, by adopting the same array porosity, arrangement and size, and the same feature size of the micro-structure. In the present paper, we prepare three different surfaces, which are the micro-pillar array surfaces, micro-hole array surface, and dual-scale micro-pillar array surface (i.e., micro-pillar with rough top surface), respectively. Their influences on the liquid flowing characteristics of the near-surface layer are investigated by quartz crystal microbalance (QCM). The QCM is a powerful and promising technique in studying the solid/liquid interfacial behaviors. Its main output parameters are frequency shift and half-bandwidth variation, which are closely related to the rheological properties and flow characteristics of the near-surface liquid layer. When the QCM chip is patterned with micro-structure array, it will inevitably influence the liquid motion and makes it more complicated, like the generation of non-laminar motion, the trapping of liquid in the gap, and the conversion of the in-plane surface motion into the surface-normal liquid motion. The experimental results show that for the same tested liquid, the frequency shift caused by the micro-hole array is higher than that by the micro-pillar array with the same feature size. And the dual-scale micro-pillar array surface results in a higher half-bandwidth variation than the micro-pillar array surface with the same feature size. It demonstrates that micro-hole tends to confine the liquid motion and make the trapped liquid oscillate with the substrate like a rigid film, thus resulting in a higher frequency shift. The dual-scale micro-structure will render the flow behavior of the near-surface layer more chaotic, thus showing a larger half-bandwidth variation. This study provides an experimental basis for selecting the type of micro-structure used in the microfluidic chip to better control the liquid flowing and mixing.