Mechanisms of rectangular groove-induced multiple-microdroplet coalescences

The mechanism of microdroplet coalescence is a fundamental issue for droplet-based microfluidics. We developed an asymmetric expansion (a rectangular groove) along one side of a microchannel to achieve multiple-microdroplet trapping, collision, and coalescence. Compared with reported symmetric expansions, this asymmetric groove could easily trap microdroplets and control two or three microdroplet coalescences precisely without a requirement for temporal and spatial synchronization. To reveal the mechanisms of multiple-droplet coalescences in a groove, we observed five different coalescence patterns under different flow conditions. Moreover, we characterized the flow behavior quantitatively by simulating the velocity vector fields in both the microdroplets and continuous phase, finding good agreement with experiments. Finally, a map of coalescence forms with different capillary numbers () and flow ratios () was obtained. The results could provide a useful guidance for the design and application of droplet-based microfluidic devices.

Electrokinetic mixing of two fluids with equivalent conductivity

Electrokinetic(EK)micromixers have been widely studied in the past decade for biochemical applications,biological and chemical analysis,etc.Unfortunately,almost all EK mixers require different electrical conductivity between the two fluids to be mixed,which has greatly limited their wide applications,in cases where the two streams to be mixed have equivalent electrical conductivity.Here we show that mixing enhancement between two fluids with identical conductivity can be achieved in an EK micromixer with conductive sidewalls,where the electric field is in transverse direction of the flow.The results revealed that the mixing became stronger with increased conductivity value.This mixing method provides a novel and convenient strategy for mixing two liquids with the same or similar electrical conductivity in microfluidic systems,and could potentially serves as a powerful tool for sample preparation in applications such as liquid biopsy,and environmental monitoring,etc.

Simulation Studying Effects of Multiple Primary Aberrations on Donut-shaped Gaussian Beam

In this paper, we demonstrate the variation of donut-shaped depletion pattern which influenced by multiple primary aberrations. The simulation is base on a common stimulation emission of depletion (STED) system composed by Gaussian laser and vortex phase plate. The simulation results are helpful guidelines for analyzing the aberration of depletion patterns in real situations.

A tentative study of the transport of energy and other scalar quantities in forced turbulence driven by∇^(n)A-type volume forces

The transport of the energy and other scalar quantities in turbulence can be controlled by scalar-based volume forces,e.g.,the buoyancy in the turbulent thermal convection or the electric body force in the electrokinetic(EK)turbulence,and expressed as∇^(n)A,with A being a control scalar.This paper presents a unified theoretical framework for the transport of the energy and other scalar quantities in the turbulence driven by scalar-based volume forces.Several isolated results that have previously been reported in relation to the turbulent thermal convection(related to n=0)and the EK turbulence(related to n=1)are unified in this theoretical framework.With the theory,the following interesting results are predicted:(1)When n<2/3,the transport of the kinetic energy is dominated by the Kolmogorov scaleηand another small scale l_(A).When n>2/3,the transport of the kinetic energy is controlled by three characteristic small scales:l_(K),l_(ηK) and l_(A).(2)The scaling law range can be divided into an inertial subrange and a volume-force-dominated subrange.(3)In the latter case,the exponents of the power spectra of the velocity and the relevant scalar quantity areη_(u)=(4n-11)/5,η_(A)=-(2n+7)/5,respectively.(4)In the equilibrium state,n cannot exceed 4.(5)The positive exponent of l_(A)∼0.024Ra^(0.107±0.016)_(A.l_(0)) is confirmed in the turbulent thermal convection.