Fabrication and Rheological Analysis of Microfluidic Systems with Nanostructured Surfaces

Abstract

We explored a liquid slip, referred to as the Navier slip, at liquid-solid interface. While the liquid slip has been observed experimentally at micro and nanoscale, its fundamental mechanism is still controversial and has yet to be fully understood. In general, it is known that liquid molecules are capable of slipping at liquid-solid interface when the liquid possesses a low wettability due to superhydrophobicity, thus resulting in drag reduction. This is the so called Navier slip for a Newtonian liquid. Such a slip is provoked by the physicochemical features of the liquid-solid system. The goal of this study was to investigate the effect of a nanoengineered surface structure on liquid slip by fabricating the self-assembly structure of nano Zinc Oxide (n-ZnO). We have also examined how the liquid-solid surface interaction controlled by hydrophobic chemical treatment affects the liquid slip. The findings showed that liquid slip increases with decreasing the characteristic length scales (e.g., channel height and depth), resulting in drag reduction. It was also found that dewetted (Cassie) state due to the generation of air gap developed by n-ZnO was more critical for the liquid slip than the minimization of interface interaction. The linear and non-linear Navier slip models showed that liquid slip behavior is more obvious when increasing the non-linearity. This study will contribute to understanding of the underlying physics behind fluid slip phenomena, such as the Navier slip for Newtonian liquids and Maxwells slip for Newtonian gases. In chapter 2, superhydrophobicity of multi-scale hierarchical structures and bouncing phenomenon of a water droplet on the superhydrophobic surface were studied. Superhydrophobicity is a unique characteristic of surface structure provoking extreme water repellency and a contact angle (CA) over 150°. For these reasons many researchers have produced biomimetic superhydrophobic surfaces with extremely high water repellency for industrial applications such as self-cleaning windows, windshield, exterior paints, antifouling, roof tiles, and textiles. Theoretically, the surface roughness morphology of multi-scale nanostructures is desired not only to maximize CA but also to minimize the free energy barrier. Additionally, it guarantees the superhydrophobic state more stable and thus incurs larger self-cleaning effect, higher dynamic bouncing behavior, lower CAH, and smaller sliding angle. The dynamic bouncing effect is determined by the droplet velocity, surface roughness, and liquid properties. Compared with references related to the CA, a limited number of studies on the bouncing effect were reported so far. Therefore, the present study is meaningful to investigate physical insight for the bouncing phenomenon. To do this, the multi-scale hierarchical structures of carbon nanotube/ZnO and ZnO/carbon nanofiber were produced by the hydrothermal method. The multi-scale hierarchical structure showed superhydrophobicity with a static contact angle (CA) larger than 160° due to increased air pockets in the Cassie-Baxter state. Water bouncing effect observed on the multi-scale hierarchical nanostructure was explained by the free energy barrier (FEB) analysis and finite element simulation. The multi-scale hierarchical nanostructure showed low FEBs which provoke high CA and bouncing phenomenon due to small energy dissipation towards receding and advancing directions. In chapter 3, we investigated a novel concept for the separation of particles using the incorporation of elasto-inertial effect and dean effect termed as Dean-coupled Elasto-inertial effect. Under laminar flow regime, when viscoelastic fluid is used as a medium particles experience the Elasto-inertial force and are focused at the center. In this study an additional secondary force caused by a curved geometry, the dean drag force, is exerted on the particles and the concept of Dean-coupled Elasto-inertial force is proposed. Theoretical analysis indicates that a particle focusing band with respect to channel aspect ratios exists due to the two main forces. It is revealed that the competition of the dean drag force and the elastic force affects the lateral displacement of suspended particles depending on the particle sizes. The elastic force is induced by the nature of a viscoelastic medium and the dean drag force is contributed by the curved channel geometry. The cooperation of the two forces produces an extraordinary particle focusing region. Numerical analysis of viscoelastic fluid was employed in order to explain theoretical background and a mechanism for the lateral migration which follows the relationship between the elastic and dean drag forces with respect to particle size. It was demonstrated that this platform is very promising to achieve the multiplex particle focusing at the equilibrium position of the particle which is determined by the incorporation of the rheological properties of the fluid and the channel geometry.