Enhanced mixing in laminar flows using ultrahydrophobic surfaces

Abstract

Under laminar, microscale flow conditions, rapid mixing can be difficult to achieve. In these low Reynolds number flows, mixing rates are governed by molecular diffusion, and in the absence of enhanced mixing techniques, mixing lengths and residence times can be much longer than most applications will allow. A number of active mixing techniques have been developed to improve mixing; however, they can be complex to implement and expensive to fabricate. In this paper, we describe a passive mixing method that utilizes a series of ultrahydrophobic surfaces. Our previous experiments have demonstrated that a shear-free air-water interface supported between hydrophobic microridges results in large slip velocities along these ultrahydrophobic surfaces, and significant drag reduction. 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-shaped 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 degrees , beyond which the mixing length was found to increase with further increases to the angle of the microridge. The mixing enhancement was found to be much less sensitive to changes in microridge width or separation.

Hydrodynamic interaction with super-hydrophobic surfaces

“…Such surfaces have been already used for reduction in pressure-driven flows 15 and enhancement of mixing. 36 The problem of flow past stripes has also been examined theoretically. Effective slip lengths b ,⊥…”

A novel process for continuous thermal embossing of large‐area nanopatterns onto polymer films

“…A reliable, fast, and low-cost process which has the potential to produce robust, homogeneous patterns on areas 0.1m 2 and larger is critical to success in these industries. [1][2][3][4][5][6][7][8][9][10][11][12] Many existing methods currently employed in the production of micropatterns are unsuitable for incorporation into a large area process. Research in areas such as thermal and UV light curing imprint lithography have yielded robust, defect free structures with a resolution as low as 10nm; however, these methods are limited to areas of tens of cm 2 or less.…”

Regimes of wetting transitions on superhydrophobic textures conditioned by energy of receding contact lines

“…14 For instance, dilute SH stripes show the remarkable drag-reducing ability. 15,16 Dense SH grooves generate transverse hydrodynamic phenomena 17 and can be successfully used to separate tiny particles 18,19 or enhance mixing rate 20,21 in microfluidic devices. These striped textures amplify electrokinetic pumping 22,23 and are employed for sorting droplets.…”