Asymmetric liquid wetting on controllable magnetic micro-pillar surfaces
Date of Issue2015
School of Mechanical and Aerospace Engineering
Wetting is the ability of a liquid to maintain contact with a solid surface, and spreading is the displacement process from a surface of air or one fluid by another. The wetting and spreading is a ubiquitous phenomenon in nature and daily life and has attracted numerous research interests in many applications, such as lubrication, coating, painting and drug delivery. Unidirectional liquid wetting and spreading on asymmetric nanostructured surfaces was also reported recently, such as water droplets roll off rice leaves due to their anisotropic micro-structured leaf surfaces. For developing new process and technologies in which wetting and spreading are involved, many research effects have been devoted in these areas. In this work, we studied the liquid wetting and spreading behaviors on different surfaces, and tried to understand the mechanism and utilize it in applications. First, we studied the wettability ageing behaviors of the silicon wafers which had been cleaned by piranha solution (a 3:1 mixture of sulfuric acid), SC1 (a mixture of NH4OH, H2O2 and H2O with a ratio of 1:1:5) and HF solution (a mixture of 40% NH4F and 49% HF with a ratio of 6:1), and treated by gaseous plasma. It is found that both piranha and SC1 solution cleaned silicon wafers were hydrophilic, and the water contact angles on the surfaces would increase along with ageing time, until they reached the saturated point ~ 70°. The contact angle increase rate of these wafers in vacuum was much faster than in open air, because of loss of water, which was physically adsorbed on wafer surfaces. The silicon wafer cleaned by HF solution was hydrophobic, and the contact angle decreased in atmosphere, while it increased in vacuum up to 95°. Gold thin films deposited on the hydrophilic wafers were smoother than the ones deposited on the hydrophobic wafers, because the numerous oxygen groups linked on hydrophilic surfaces would react with gold adatoms in the sputtering process to form continuous thin film at nucleation stage. The argon, nitrogen, oxygen gas plasma treatments could change the silicon wafer surfaces from hydrophobic to hydrophilic by creating a thin (around 2.5 nm) silicon dioxide film, which would be used to improve the gold thin film roughness and adhesion. Next, we invented a novel technique for fabricating high-aspect-ratio magnetic PDMS micro-pillars which are capable of vibrating with considerable amplitudes under a gradient magnetic field. We dispersed the special treated Fe3O4 super-paramagnetic nanoparticles (refer as Fe3O4 nanoparticles, similarly hereinafter) in acetone solution and sonicate for disaggregation, and pour the solution with Fe3O4 nanoparticles over the silicon mold, which has been pre-etched with deep micro-holes. Then, nanoparticles are attracted into micro-holes by a strong rotating permanent magnet at the backside of the silicon mold. Soft lithography is used to make the PDMS fluid flow into the micro-structures in a vacuum, and peel it off after sufficient baking. The PDMS magnetic pillars have been fabricated with diameters ranging from 1 μm to 10 μm, and with heights of 30 μm and 50 μm. The pillar tip displacement, for 1 μm diameter pillars with an aspect ratio of 50, could be as much as 12 μm under a gradient magnetic field. The magnetic pillar tip vibration in a liquid was also validated, which enables the application as actuators in micro-fluidics, miniaturized devices on a chip, and dynamic substrate for cell culture in biomimetic researches. The key to drive micro-pillar tip deflection is high-aspect-ratio. EDS data shows that the weight percentage of iron embedding in PDMS pillars is from 42% to 81.4% with a median value 59.6%, which is the highest value reported in the literature, to our best knowledge. Last, we studied the uni-directional liquid spreading phenomena on asymmetric silicone-fabricated nanostructured surfaces. The uniformly deflected Polydimethylsiloxane (PDMS) micro-pillars covered with silver films were fabricated. Asymmetric liquid wetting and spreading behaviors in a preferential direction were observed on the slanted micro-pillar surfaces and a micro-scale thin liquid film advancing ahead of the bulk liquid droplet was clearly observed by high-speed video imaging. The slanted micro-pillar array is able to promote or inhibit the propagation of this thin liquid film in different directions by the asymmetric capillary force. The spreading behavior of the bulk liquid was guided and finally controlled by this micro-scale liquid film. Different spreading regimes are defined by the relationship between the liquid intrinsic contact angle and the critical angles which were defined by the micro-pillar height, deflection angle and inter-pillar spacing. We incorporated Fe3O4 magnetic particles into the PDMS micro-pillars to make them be controllable to external magnetic field, which in turn was used to turn the liquid spreading direction. However, it was proven that the magnetic force is not stronger than capillary force for controlling the liquid flow direction. Once the drop is placed on the micro-pillar surfaces, the micro-pillars with partial wetting are deflected in different directions. Based on the magnetic pillars deflection angles measured, it was found the magnetic force calculated is around 5.9E-5 for 1 µm pillar in diameter and 5.9E-4 N/m for 10 µm in diameter. These forces are much smaller than the capillary force (0.072 N/m) exerted on micro pillar surface. Therefore, the conclusion is that magnetic force is not able to overcome the surface tension. Nevertheless, the capillary force could be tuned by appropriately selecting the wetting liquid or the liquid mixtures and the magnetic pillar surface coating materials and the features. It will be the next research focus. Moreover, the magnetic micro-pillars also could be used as actuators in micro-fluidic and other applications for controlling the liquid flow rate and direction.