Laminar flow control with ultrahydrophobic surfaces
With the miniaturized of mechanical technology and the increasingly wide use of microfluidic devices, the development of the drag reducing and mixing enhancing technique could have a significant economic impact. In this study, a series of experiments will be presented which demonstrate significant drag reduction and mixing enhancement for the laminar flow through microchannels using hydrophobic surfaces with well-defined micron-sized surface roughness. A shear-free air-water interface can be formed between the hydrophobic micro surface structures and the slip velocity at the interface can be engineered for various applications. These ultrahydrophobic surfaces were fabricated from silicon wafers using photolithography and designed to incorporate patterns of microridges of various sizes, spacings and angles. The ridges are made hydrophobic through a chemical reaction with an organosilane. In some cases, the micro structures fabricated on silicon wafer were used as a master to transfer the patterns to PDMS via soft-lithography. In the drag reduction experiments, an experimental flow cell is used to measure the velocity profile and the pressure drop as a function of the flow rate for a series of rectangular cross-section microchannel geometries and ultrahydrophobic surface designs. The velocity profile across the microchannel is determined through micro particle image velocimetry (μ-PIV) measurements capable of resolving the flow down to lengthscales well below the size of the surface features. Through these detailed velocity measurements, it is demonstrated that slip along the shear-free air-water interface supported between the hydrophobic micron-sized ridges is the primary mechanism responsible for the drag reduction observed for flows over ultrahydrophobic surfaces. A maximum slip velocity of more than 60% of the average velocity in the microchannel is found at the center of the shear-free air-water interface while the no-slip boundary condition is found to hold along the surface of the hydrophobic ridges. The experimental velocity and pressure drop measurements are compared to the predictions of numerical simulations and an analytical theory based on a simple model of an ultrahydrophobic surface composed of alternating shear-free and no-slip bands with good agreement. In the mixing enhancement experiments, a passive mixing method was developed that using ultrahydrophobic surfaces with oblique microridges. By aligning the microridges and therefore the air-water interface at an oblique angle to the flow direction, a secondary flow is generated which is shown to efficiently stretch and fold the fluid elements and reduce the mixing length by more than an order of magnitude compared to that of a smooth microchannel. The designs of the ultrahydrophobic surfaces were optimized through experiments and numerical simulations. A Y-channel was used to bring two streams of water together, one tagged with a fluorescent dye. A confocal microscope was used to measure fluorescence intensity and dye concentration. Quantitative agreement between the experiments and the numerical simulations was achieved for both the flow patterns and degree of mixing. Increasing the angle of the microridges was found to reduce the mixing length up to a critical angle of about 60° beyond which the mixing length is found to increase with further increases to the angle of the microridge. The mixing enhancement was found to be a much less sensitive to changes in microridge width or separation. To study the detail of the helical flow inside the channel, a three-dimensional, three-component micro particle image velocimetry (μ-PIV) measurement technique based on conservation of mass principles is presented using standard two-dimensional μ-PIV experimental equipment and modest additional computational effort and programming. The proposed method starts by using a commercial PIV code to correlate the two-dimensional motion of fluorescent seed particles using volume-illuminated images obtained with an epi-fluorescent microscope. To reduce the depth of field and therefore the influence of out of focus particles, a high numerical aperture oil-immersion objective is used resulting in a series of 187μm x 187μm x 0.82μm images of particle motion. A three-dimensional, two-component vector field is then built by systematically moving the focal plane of the microscope through the microchannel in increments of 1.7μm. To obtain the third component of the velocity vector field normal to the interrogation plane conservation of mass is applied to control volumes whose vertices are defined by the location of the 3D, 2C velocity vectors. As an example, the technique is implemented in microchannel designed for enhanced mixing using ultrahydrophobic surfaces consisting of 30mm wide microridges spaced 30mm apart and aligned at a 45° to the flow direction. The microridges are observed to produce a strong secondary helical flow with a strong z-component. The results of the 3D, 3C measurements are found to qualitatively agree with the predictions of numerical simulations.