Role of shear induced diffusion in acoustophoretic focusing of dense suspensions
TL;DR: In this paper, the authors investigate the interplay between acoustic and shear induced diffusion (SID) forces in acoustophoretic focusing of dense suspensions in a microchannel and present a theoretical model which accurately predicts the width of the focused band in terms of shear rate, acoustic energy density, and particle concentration.
Abstract: We investigate the interplay between acoustic and shear induced diffusion (SID) forces in acoustophoretic focusing of dense suspensions in a microchannel. A theoretical model is presented which accurately predicts the width of the focused band in terms of shear rate, acoustic energy density, and particle concentration. The role of SID is clearly demonstrated by switching off the acoustic field, which leads to the instantaneous spreading of the focused band. At a given acoustic energy density and particle concentration, there exists a critical shear rate Γcr above which the focusing of microparticles is prevented. For Γ<Γcr, an equilibrium focused band is formed whose width remains constant downstream.
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11 Dec 2019
TL;DR: An updated overview of the state of the art for passive label-free microparticle separation, with emphasis on performance and operational conditions is provided and the newly emerging approach based on shear-induced diffusion is highlighted.
Abstract: Massive growth of the microfluidics field has triggered numerous advances in focusing, separating, ordering, concentrating, and mixing of microparticles. Microfluidic systems capable of performing these functions are rapidly finding applications in industrial, environmental, and biomedical fields. Passive and label-free methods are one of the major categories of such systems that have received enormous attention owing to device operational simplicity and low costs. With new platforms continuously being proposed, our aim here is to provide an updated overview of the state of the art for passive label-free microparticle separation, with emphasis on performance and operational conditions. In addition to the now common separation approaches using Newtonian flows, such as deterministic lateral displacement, pinched flow fractionation, cross-flow filtration, hydrodynamic filtration, and inertial microfluidics, we also discuss separation approaches using non-Newtonian, viscoelastic flow. We then highlight the newly emerging approach based on shear-induced diffusion, which enables direct processing of complex samples such as untreated whole blood. Finally, we hope that an improved understanding of label-free passive sorting approaches can lead to sophisticated and useful platforms toward automation in industrial, environmental, and biomedical fields.
62 citations
TL;DR: In this paper, the physics behind the separation of plasma from whole blood using acoustophoresis (movement driven by sound waves) is not well understood, and experiments and simulations are used to provide an improved understanding.
Abstract: Separating blood plasma in microchannels is of great relevance to microfluidics-based biodetection However, the physics behind the separation of plasma from whole blood using acoustophoresis (movement driven by sound waves) is not well understood This study uses experiments and simulations to provide an improved understanding of plasma separation from whole blood using acoustophoresis It is seen that acoustophoretic focusing of cells depend on two time scales: that of shear-induced diffusion, and that of actual acoustophoresis These results will help in designing highly efficient blood-plasma separation devices for lab-on-a-chip applications
28 citations
TL;DR: In this article, the authors investigated the aggregation of a dense suspension of particles (volume fraction, = 0.1 ) in a PDMS microwell by employing surface acoustic wave (SAW) microcentrifugation.
Abstract: We investigate the aggregation of a dense suspension of particles (volume fraction, $$\varphi \sim 0.1$$
) in a PDMS microwell by employing surface acoustic wave (SAW) microcentrifugation. In spite of acoustic attenuation at the LiNbO3–PDMS interface, a significant portion of the energy (> 80%) is available for driving fluid actuation, and, in particular, microcentrifugation in the microwell via acoustic streaming. Rapid particle aggregation can then be affected in the microcentrifugation flow, arising as a consequence of the interplay between the hydrodynamic pressure gradient force $$F_{\text{p}}$$
responsible for the migration of particles to the center of the microwell and shear-induced diffusion force $$F_{\text{SID}}$$
that opposes their aggregation. Herein, we experimentally investigated the combined effect of the particle size $$a$$
and sample concentration $$c$$
on these microcentrifugation flows. The experimental results show that particles of smaller size and lower sample concentration (such that $$F_{\text{p}} > F_{\text{SID}}$$
) are concentrated efficiently into an equilibrium spot, whose diameter scales with the initial particle volume fraction as $$d_{\text{cs}} \sim \varphi^{0.3}$$
. In contrast, we found that as the local particle volume fraction at the center of the microwell approaches $$\varphi \sim 0.1$$
such that $$F_{\text{SID}} \ge F_{\text{p}}$$
, the particle aggregation fails. Additionally, we also investigate the effects of the well diameter, and the height, lateral positioning of microwell and the liquid volume on the microcentrifugation.
21 citations
TL;DR: In this article, the authors studied the mechanism of cross-stream migration and the coalescence of aqueous droplets flowing in an oil-based ferrofluid with a co-flowing aouous stream in the presence of a magnetic field and revealed that the migration phenomenon is governed by the advection and magnetophoretic (τm) time scales.
Abstract: Manipulation of aqueous droplets in microchannels has great significance in various emerging applications such as biological and chemical assays. Magnetic-field based droplet manipulation that offers unique advantages is consequently gaining attention. However, the physics of magnetic field-driven cross-stream migration and the coalescence of aqueous droplets with an aqueous stream are not well understood. Here, we unravel the mechanism of cross-stream migration and the coalescence of aqueous droplets flowing in an oil based ferrofluid with a coflowing aqueous stream in the presence of a magnetic field. Our study reveals that the migration phenomenon is governed by the advection (τa) and magnetophoretic (τm) time scales. Experimental data show that the dimensionless equilibrium cross-stream migration distance δ* and the length Lδ* required to attain equilibrium cross-stream migration depend on the Strouhal number, St = (τa/τm), as δ* = 1.1 St0.33 and Lδ*=5.3 St−0.50, respectively. We find that the droplet-stream coalescence phenomenon is underpinned by the ratio of the sum of magnetophoretic (τm) and film-drainage time scales (τfd) and the advection time scale (τa), expressed in terms of the Strouhal number (St) and the film-drainage Reynolds number (Refd) as ξ = (τm + τfd)/τa = (St−1 + Refd). Irrespective of the flow rates of the coflowing streams, droplet size, and magnetic field, our study shows that droplet-stream coalescence is achieved for ξ ≤ 50 and ferrofluid stream width ratio w* < 0.7. We utilize the phenomenon and demonstrated the extraction of microparticles and HeLa cells from aqueous droplets to an aqueous stream.
20 citations
TL;DR: In this article, the authors studied the physics of acoustics-driven coalescence of liquid droplets with a liquid stream and found that the phenomenon is also governed by the acoustic capillary number and relative widths of the coflowing oil and aqueous streams.
Abstract: The coalescence of liquid droplets with a liquid stream has profound importance in various emerging applications, such as biochemical assays. Acoustic force-based droplet manipulation, which offers unique advantages, is consequently gaining attention. However, the physics of acoustics-driven coalescence of liquid droplets with a liquid stream is not well understood. Here, we unravel the mechanism of coalescence of aqueous droplets flowing in an immiscible (oil) phase with a coflowing aqueous stream, when the system is exposed to acoustic radiation force due to bulk acoustic waves. Our study reveals that the acoustic coalescence phenomenon is governed by the interplay between two important timescales, acoustic migration timescale (τac) and advection timescale (τadv), that underpin the phenomenon. We find that the phenomenon is also governed by the acoustic capillary number (Cac) and relative widths of the coflowing oil and aqueous streams (i.e., Waq and Woil). Our results show that, if Cac Woil are satisfied to ensure the stability of the streams and positioning of the acoustic node in the aqueous phase, respectively, continuous coalescence is observed for (τadv/τac)≥0.85. We exploit the phenomenon for the extraction of droplet contents (beads and cells) into an aqueous stream. (Less)
13 citations
References
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TL;DR: The behavior of a vesicle suspension in a simple shear flow between plates (Couette flow) was investigated experimentally in parabolic flight and sounding rocket experiments by Digital Holographic Microscopy as mentioned in this paper.
Abstract: The behaviour of a vesicle suspension in a simple shear flow between plates (Couette flow) was investigated experimentally in parabolic flight and sounding rocket experiments by Digital Holographic Microscopy. The lift force which pushes deformable vesicles away from walls was quantitatively investigated and is found to be rather well described by a theoretical model by Olla (J Phys II (France) 7:1533, 1997). At longer shearing times, vesicles reach a steady distrib- ution about the center plane of the shear flow chamber, through a balance between the lift force and shear induced diffusion due to hydrodynamic interactions between vesicles. This steady distribution was inves- tigated in the BIOMICS experiment in the MASER 11 sounding rocket. The results allow an estimation of self-diffusion coefficients in vesicle suspensions and reveal possible segregation phenomena in polydisperse suspensions.
25 citations