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. © 2016 Author(s).

Ashis Kumar Sen

Department of Mechanical Engineering

S. Karthick

Acoustic fields

Shear deformation

Acoustic energy

CRITICAL SHEAR RATES

DENSE SUSPENSION

Micro-particles

Particle concentrations

SHEAR-INDUCED DIFFUSION

Theoretical modeling

Focusing

Fulltext

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IIT Madras

Role of shear induced diffusion in acoustophoretic focusing of dense suspensions

Applied Physics Letters

We investigate the aggregation of a dense suspension of particles (volume fraction, φ∼ 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 (&gt; 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&nbsp;force Fp responsible for the migration of particles to the center of the microwell and shear-induced diffusion force FSID 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 Fp&gt; FSID) are concentrated efficiently into an equilibrium spot, whose diameter scales with the initial particle volume fraction as dcs∼ φ0.3. In contrast, we found that as the local particle volume fraction at the center of the microwell approaches φ∼ 0.1 such that FSID≥ Fp, 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. © 2019, Springer-Verlag GmbH Germany, part of Springer Nature.

Microfluidics and Nanofluidics

Aggregation of a dense suspension of particles in a microwell using surface acoustic wave microcentrifugation

We report the behavior of a dense suspension (blood sample, φ&gt;0.1) coflowing with an aqueous buffer in a microchannel exposed to an acoustic standing wave studied by experiments and a theoretical model. The coupled effect of acoustophoretic particle migration and bulk relocation due to an inhomogeneous-fluid configuration leads to interesting phenomena. Our study reveals that depending on the acoustic impedance of the suspension (Zs) and buffer (Zb) streams, different flow regimes (complete relocation, acoustic migration, nonrelocation and nonmigration, and partial relocation) can be observed. The occurrence of different regimes is explained by the impedance contrast (i.e., ΔZsb and ΔZpb) and is characterized in terms of the particle residence (τres), acoustic relocation (τrel), and acoustic migration (τmig) timescales. We discover the partial-relocation regime, wherein acoustic migration of particles results in a band of a highly concentrated suspension at the suspension-buffer interface that alone relocates to the pressure node. The partial-relocation regime thus offers a technique for acoustic-driven splitting of a suspension stream into denser and diluted suspension streams. © 2019 American Physical Society.

Physical Review Applied

Acoustic Behavior of a Dense Suspension in an Inhomogeneous Flow in a Microchannel

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&lt;0.9 and Waq&gt;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. © 2019 American Physical Society.

Continuous Droplet Coalescence in a Microchannel Coflow Using Bulk Acoustic Waves

We report the dynamics of coflowing immiscible liquid streams exposed to an acoustic standing wave in a microchannel. Relocation of the liquid streams is experimentally demonstrated and a theoretical model that explains the underlying phenomena is presented. Our experiments and theoretical model suggest that the relocation phenomena are governed by the interplay between the primary acoustic radiation force F ac and the interfacial tension force F int-which is represented in terms of a new dimensionless number called "acoustocapillary number", . Using various combinations of immiscible liquids, we show that relocation of the higher acoustic impedance liquid stream to the pressure node occurs above a critical acoustocapillary number . The understanding of the above phenomena provides a new paradigm related to the manipulation of immiscible liquids under acoustic field. © 2019 EPLA.

Relocation of coflowing immiscible liquids under acoustic field in a microchannel

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 ζ = (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 ζ ≤ 50 and ferrofluid stream width ratio w* &lt; 0.7. We utilize the phenomenon and demonstrated the extraction of microparticles and HeLa cells from aqueous droplets to an aqueous stream. © 2019 Author(s).

Journal	Data powered by TypesetApplied Physics Letters
Publisher	Data powered by TypesetAmerican Institute of Physics Inc.
ISSN	00036951
Open Access	No