16 research outputs found
Numerical study of thermoviscous effects in ultrasound-induced acoustic streaming in microchannels
We present a numerical study of thermoviscous effects on the acoustic
streaming flow generated by an ultrasound standing-wave resonance in a long
straight microfluidic channel containing a Newtonian fluid. These effects enter
primarily through the temperature and density dependence of the fluid
viscosity. The resulting magnitude of the streaming flow is calculated and
characterized numerically, and we find that even for thin acoustic boundary
layers, the channel height affects the magnitude of the streaming flow. For the
special case of a sufficiently large channel height we have successfully
validated our numerics with analytical results from 2011 by Rednikov and Sadhal
for a single planar wall. We analyze the time-averaged energy transport in the
system and the time-averaged second-order temperature perturbation of the
fluid. Finally, we have made three main changes in our previously published
numerical scheme to improve the numerical performance: (i) The time-averaged
products of first-order variables in the time-averaged second-order equations
have been recast as flux densities instead of as body forces. (ii) The order of
the finite element basis functions has been increased in an optimal manner.
(iii) Based on the International Association for the Properties of Water and
Steam (IAPWS 1995, 2008, and 2011), we provide accurate polynomial fits in
temperature for all relevant thermodynamic and transport parameters of water in
the temperature range from 10 C to 50 C.Comment: 13 pages, 8 eps figures, Revtex 4.
Theoretical study of time-dependent, ultrasound-induced acoustic streaming in microchannels
Based on first- and second-order perturbation theory, we present a numerical
study of the temporal build-up and decay of unsteady acoustic fields and
acoustic streaming flows actuated by vibrating walls in the transverse
cross-sectional plane of a long straight microchannel under adiabatic
conditions and assuming temperature-independent material parameters. The
unsteady streaming flow is obtained by averaging the time-dependent velocity
field over one oscillation period, and as time increases, it is shown to
converge towards the well-known steady time-averaged solution calculated in the
frequency domain. Scaling analysis reveals that the acoustic resonance builds
up much faster than the acoustic streaming, implying that the radiation force
may dominate over the drag force from streaming even for small particles.
However, our numerical time-dependent analysis indicates that pulsed actuation
does not reduce streaming significantly due to its slow decay. Our analysis
also shows that for an acoustic resonance with a quality factor Q, the
amplitude of the oscillating second-order velocity component is Q times larger
than the usual second-order steady time-averaged velocity component.
Consequently, the well-known criterion v << c for the validity of the
perturbation expansion is replaced by the more restrictive criterion v << c/Q.
Our numerical model is available in the supplemental material in the form of
Comsol model files and Matlab scripts.Comment: 14 pages, Revtex, 8 eps figure
Focusing of sub-micrometer particles and bacteria enabled by two-dimensional acoustophoresis.
Handling of sub-micrometer bioparticles such as bacteria are becoming increasingly important in the biomedical field and in environmental and food analysis. As a result, there is an increased need for less labor-intensive and time-consuming handling methods. Here, an acoustophoresis-based microfluidic chip that uses ultrasound to focus sub-micrometer particles and bacteria, is presented. The ability to focus sub-micrometer bioparticles in a standing one-dimensional acoustic wave is generally limited by the acoustic-streaming-induced drag force, which becomes increasingly significant the smaller the particles are. By using two-dimensional acoustic focusing, i.e. focusing of the sub-micrometer particles both horizontally and vertically in the cross section of a microchannel, the acoustic streaming velocity field can be altered to allow focusing. Here, the focusability of E. coli and polystyrene particles as small as 0.5 ÎĽm in diameter in microchannels of square or rectangular cross sections, is demonstrated. Numerical analysis was used to determine generic transverse particle trajectories in the channels, which revealed spiral-shaped trajectories of the sub-micrometer particles towards the center of the microchannel; this was also confirmed by experimental observations. The ability to focus and enrich bacteria and other sub-micrometer bioparticles using acoustophoresis opens the research field to new microbiological applications
Theoretical study of time-dependent, ultrasound-induced acoustic streaming in microchannels
Based on first- and second-order perturbation theory, we present a numerical
study of the temporal build-up and decay of unsteady acoustic fields and
acoustic streaming flows actuated by vibrating walls in the transverse
cross-sectional plane of a long straight microchannel under adiabatic
conditions and assuming temperature-independent material parameters. The
unsteady streaming flow is obtained by averaging the time-dependent velocity
field over one oscillation period, and as time increases, it is shown to
converge towards the well-known steady time-averaged solution calculated in the
frequency domain. Scaling analysis reveals that the acoustic resonance builds
up much faster than the acoustic streaming, implying that the radiation force
may dominate over the drag force from streaming even for small particles.
However, our numerical time-dependent analysis indicates that pulsed actuation
does not reduce streaming significantly due to its slow decay. Our analysis
also shows that for an acoustic resonance with a quality factor Q, the
amplitude of the oscillating second-order velocity component is Q times larger
than the usual second-order steady time-averaged velocity component.
Consequently, the well-known criterion v << c for the validity of the
perturbation expansion is replaced by the more restrictive criterion v << c/Q.
Our numerical model is available in the supplemental material in the form of
Comsol model files and Matlab scripts.Comment: 14 pages, Revtex, 8 eps figure
Numerical study of thermoviscous effects in ultrasound-induced acoustic streaming in microchannels
We present a numerical study of thermoviscous effects on the acoustic streaming flow generated by an ultrasound standing-wave resonance in a long straight microfluidic channel containing a Newtonian fluid. These effects enter primarily through the temperature and density dependence of the fluid viscosity. The resulting magnitude of the streaming flow is calculated and characterized numerically, and we find that even for thin acoustic boundary layers, the channel height affects the magnitude of the streaming flow. For the special case of a sufficiently large channel height, we have successfully validated our numerics with analytical results from 2011 by Rednikov and Sadhal for a single planar wall. We analyzed the time-averaged energy transport in the system and the time-averaged second-order temperature perturbation of the fluid. Finally, we have made three main changes in our previously published numerical scheme to improve the numerical performance: (i) The time-averaged products of first-order variables in the time-averaged second-order equations have been recast as flux densities instead of as body forces. (ii) The order of the finite-element basis functions has been increased in an optimal manner. (iii) Based on the International Association for the Properties of Water and Steam (IAPWS 1995(IAPWS , 2008, and 2011), we provide accurate polynomial fits in temperature for all relevant thermodynamic and transport parameters of water in the temperature range from 10 to 5