title: Gravity-induced swirl of nanoparticles in microfluidics.
abstract: Parallel flows of two fluids in microfluidic devices are used for miniaturized chemistry, physics, biology and bioengineering studies, and the streams are often considered to remain parallel. However, as the two fluids do not always have the same density, interface reorientation induced by density stratification is unavoidable. In this paper, flow characteristics of an aqueous polystyrene nanofluid and a sucrose-densified aqueous solution flowing parallel in microchannels are examined. Nanoparticles 100 nm in diameter are used in the study. The motion of the nanoparticles is simulated using the Lagrangian description and directly observed by a confocal microscope. Matched results are obtained from computational and empirical analysis. Although solution density homogenizes rapidly resulting from a fast diffusion of sucrose in water, the nanofluid is observed to rotate for an extended period. Angular displacement of the nanofluid depends on the ratio of gravitational force to viscous force,Re/Fr2, whereReis the Reynolds number andFris the Froude number. In the developing region at the steady state, the angular displacement is related toy/Dh, the ratio between distance from the inlet and the hydraulic diameter of the microfluidic channel. The development of nanofluid flow feature also depends onh/w, the ratio of microfluidic channel's height to width. The quantitative description of the angular displacement of nanofluid will aid rational designs of microfluidic devices utilizing multistream, multiphase flows.
authors.affiliation: Department of Materials Science and Engineering and Bioengineering Program, Lehigh University, Bethlehem, PA, USA.
authors.forename: Chao
authors.initials: C
authors.lastname: Zhao
authors.affiliation: Department of Mechanical Engineering and Mechanics, Lehigh University, Bethlehem, PA, USA.
authors.forename: Alparslan
authors.initials: A
authors.lastname: Oztekin
authors.affiliation: Department of Materials Science and Engineering and Bioengineering Program, Lehigh University, Bethlehem, PA, USA.
authors.forename: Xuanhong
authors.initials: X
authors.lastname: Cheng
keywords: Angular displacement; Density stratification; Microfluidics; Miscible fluids; Nanoparticles