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arxiv: 2604.05419 · v1 · submitted 2026-04-07 · ⚛️ physics.flu-dyn

Ultrasound-controlled stream splitting in a microfluidic coflow

D. Ghosh , S. Z. Hoque , T. Sujith , N. S. Satpathi , L. Malik , A. K. Sen This is my paper

Pith reviewed 2026-05-10 19:33 UTC · model grok-4.3

classification ⚛️ physics.flu-dyn
keywords microfluidicsacoustic controlstream splittingdroplet generationcoflowcapillary numberliquid-liquid interfaceultrasound
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The pith

An external acoustic field induces a partial splitting regime in a microfluidic coflow where droplets form at tunable locations with a persistent residual stream.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

This paper examines how a standing acoustic field applied to a liquid-liquid coflow inside a rectangular microchannel destabilizes an otherwise stable interface. The excitation produces a series of reversible regimes that include waviness, splitting, relocation, and breakup. A key regime allows the stream to split partially into droplets at positions that can be adjusted by the acoustic input while a thin residual stream continues unbroken. Droplet dimensions are set by the flow conditions and fluid properties, but the acoustic field decides when and where the splitting occurs. This separation of controls works across a wide range of capillary numbers and avoids the full flow interruption that occurs in ordinary droplet breakup.

Core claim

Application of a standing acoustic field to a liquid-liquid coflow in a rectangular microchannel destabilizes the interface to produce a sequence of regimes. A distinct splitting regime appears in which the stream partially divides into droplets at locations controlled by the acoustic field, while a thin residual stream persists. Droplet size and residual thickness are set by hydrodynamic conditions, and the acoustic power governs the onset and position of splitting. This holds over a wide range of capillary numbers and enables droplet generation without complete flow disruption.

What carries the argument

The standing acoustic wave that destabilizes the liquid-liquid interface, interacting with the coflow hydrodynamics to set the conditions for regime transitions and the characteristics of the partial split.

If this is right

  • Acoustic power and frequency can be used to select the position along the channel where droplets begin to form.
  • The size of generated droplets and the thickness of the residual stream scale with the flow rates and fluid properties rather than acoustic strength.
  • Partial splitting allows droplet generation to continue at capillary numbers higher than those permitting standard complete breakup.
  • Removal of the acoustic field returns the interface to a stable coflow, making the process reversible.
  • This provides a route to on-demand, spatially controlled droplet production in continuous microfluidic streams.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The residual stream could carry reagents or particles past the splitting zone for separate downstream handling without mixing with the droplet phase.
  • Similar acoustic-hydrodynamic balance might produce analogous regimes in channels of different aspect ratios if the interface tension and viscosity ratios remain comparable.

Load-bearing premise

The interfacial regimes and their transitions arise from a general interplay of acoustic destabilization and hydrodynamics that applies outside the specific rectangular geometry, fluid combinations, and frequencies used in the experiments.

What would settle it

If the partial splitting regime fails to appear or its transition thresholds change dramatically when the channel width-to-height ratio is altered while keeping all other parameters fixed, the claim of broad applicability would be weakened.

Figures

Figures reproduced from arXiv: 2604.05419 by A. K. Sen, D. Ghosh, L. Malik, N. S. Satpathi, S. Z. Hoque, T. Sujith.

Figure 1
Figure 1. Figure 1: (a) Schematic of the experimental setup (top left). The system consists of a silicon– [PITH_FULL_IMAGE:figures/full_fig_p004_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: (a) Experimental images illustrating the different flow regimes: stable coflow, [PITH_FULL_IMAGE:figures/full_fig_p012_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: (a) Schematic of a liquid protrusion, resulting from interfacial deformation, [PITH_FULL_IMAGE:figures/full_fig_p015_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: (a) Experimental images illustrating the influence of acoustic excitation, expressed [PITH_FULL_IMAGE:figures/full_fig_p020_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: (a) Temporal evolution of the interfacial oscillation amplitude [PITH_FULL_IMAGE:figures/full_fig_p022_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Variation of normalized interfacial wavelength ( [PITH_FULL_IMAGE:figures/full_fig_p023_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: Characterization of droplet size and thin residual stream (TRS) thickness in [PITH_FULL_IMAGE:figures/full_fig_p025_7.png] view at source ↗
read the original abstract

Precise control of multiphase microfluidic flows underpins applications ranging from chemical processing to biomedical diagnostics. We investigate the response of a liquid--liquid coflow in a rectangular microchannel to an externally applied standing acoustic field. Acoustic excitation destabilizes an otherwise stable interface, giving rise to a sequence of reversible interfacial regimes: waviness, splitting, relocation, and stream-droplet breakup. Remarkably, a distinct splitting regime emerges, where a continuous stream partially splits into droplets at tunable locations while retaining a thin residual stream. Unlike conventional droplet breakup, this regime avoids complete disruption of the main flow, enables droplet generation at high capillary numbers, and allows spatial control over droplet formation. Extending across a broad range of capillary numbers, we examine how variations in flow conditions and applied acoustic power influence these regimes. Combining experiments, numerical simulations, and theoretical scaling, we elucidate the mechanisms governing this droplet generation mode and the associated regime transitions. Systematic measurements show that droplet size and residual stream thickness are governed primarily by hydrodynamic parameters, whereas the acoustic field controls the onset and spatial location of the breakup. These results establish a simple avenue for stream splitting and drop generation on-demand in a microfluidic coflow, opening new possibilities for spatially programmable manipulation of multiphase flows.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript investigates the response of a liquid-liquid coflow in a rectangular microchannel to an externally applied standing acoustic field. It identifies a sequence of reversible interfacial regimes (waviness, splitting, relocation, stream-droplet breakup) and focuses on a distinct partial-splitting regime in which a continuous stream breaks into droplets at acoustically tunable locations while retaining a thin residual stream. The authors combine experiments across a range of capillary numbers, numerical simulations, and theoretical scaling to argue that hydrodynamic parameters primarily control droplet size and residual-stream thickness, while the acoustic field governs onset and spatial location. This is presented as enabling droplet generation at high Ca without complete flow disruption and opening avenues for spatially programmable multiphase manipulation.

Significance. If the quantitative regime boundaries and scaling hold, the work offers a practical route to on-demand, spatially controlled droplet generation in microfluidics that operates without fully interrupting the main flow and extends to higher capillary numbers than conventional breakup. The integration of experiments, simulations, and scaling provides mechanistic insight and is a methodological strength. Broader significance for applications in chemical processing or diagnostics would be enhanced by explicit checks on geometric and fluid variations.

major comments (2)
  1. [Scaling analysis] Scaling analysis (theoretical section following the regime description): The hydrodynamic scaling for residual-stream thickness and droplet size is derived under the constraints of one rectangular channel geometry and one immiscible fluid pair. No parametric variation of aspect ratio or viscosity contrast is reported, yet these parameters directly influence capillary-wave dispersion and acoustic streaming that set breakup location. This makes the load-bearing claim that the regime 'enables droplet generation at high capillary numbers' and 'opens new possibilities for spatially programmable manipulation' an extrapolation beyond the demonstrated domain.
  2. [Results section] Results on regime transitions and systematic measurements: The manuscript states that droplet size and residual thickness are governed primarily by hydrodynamics while acoustics control location, but does not report quantitative metrics (e.g., R² values, error bars on regime boundaries, or goodness-of-fit for the scaling relations) across the tested Ca range. Without these, the distinction from post-hoc regime labeling and the support for reversible transitions remain difficult to verify.
minor comments (2)
  1. [Abstract] Abstract: Inclusion of at least one key quantitative result (e.g., the Ca range explored or a representative residual-stream thickness) would strengthen the summary without lengthening it.
  2. [Figures] Figure captions: Ensure all panels explicitly list the acoustic power levels, flow rates, and fluid properties corresponding to each regime image or data point.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their thoughtful and constructive comments on our manuscript. We address each major point below and have revised the manuscript to strengthen the presentation of our results and clarify the scope of our claims.

read point-by-point responses

  1. Referee: [Scaling analysis] Scaling analysis (theoretical section following the regime description): The hydrodynamic scaling for residual-stream thickness and droplet size is derived under the constraints of one rectangular channel geometry and one immiscible fluid pair. No parametric variation of aspect ratio or viscosity contrast is reported, yet these parameters directly influence capillary-wave dispersion and acoustic streaming that set breakup location. This makes the load-bearing claim that the regime 'enables droplet generation at high capillary numbers' and 'opens new possibilities for spatially programmable manipulation' an extrapolation beyond the demonstrated domain.

    Authors: We acknowledge that the experimental and numerical results are confined to a single channel aspect ratio and fluid pair, and that systematic variation of these parameters would provide additional support for broader applicability. The scaling relations are obtained from a force balance incorporating channel height, width, and fluid properties as explicit parameters, and the full Navier-Stokes simulations with acoustic body force are geometry-agnostic in principle. Nevertheless, we agree that the manuscript overstates the generality of the claims. In the revised version we will (i) add a paragraph discussing the expected influence of aspect ratio and viscosity ratio on capillary-wave dispersion and acoustic streaming, drawing on existing literature, and (ii) qualify the concluding statements to reflect the demonstrated parameter space while retaining the mechanistic insight that hydrodynamics set droplet size and residual thickness within the explored regime. revision: yes

  2. Referee: [Results section] Results on regime transitions and systematic measurements: The manuscript states that droplet size and residual thickness are governed primarily by hydrodynamics while acoustics control location, but does not report quantitative metrics (e.g., R² values, error bars on regime boundaries, or goodness-of-fit for the scaling relations) across the tested Ca range. Without these, the distinction from post-hoc regime labeling and the support for reversible transitions remain difficult to verify.

    Authors: We appreciate the referee’s call for greater quantitative rigor. In the revised manuscript we will add error bars (standard deviation from repeated measurements) to all plotted data, report R² values for the hydrodynamic scaling fits, and supply explicit, reproducible criteria for the regime boundaries (e.g., critical interface displacement amplitudes). These additions will be placed in the Results section and the supplementary material, thereby strengthening the evidence that hydrodynamics primarily govern droplet size and residual-stream thickness while the acoustic field sets onset and location. revision: yes

Circularity Check

0 steps flagged

No significant circularity detected

full rationale

The paper's central claims rest on experimental observations of regime transitions, numerical simulations of the flow, and theoretical scaling arguments that separate hydrodynamic control of droplet size/residual thickness from acoustic control of location. No load-bearing derivation reduces by construction to a fitted parameter, self-definition, or self-citation chain; the scaling is presented as derived from first-principles hydrodynamics and capillary-wave considerations applied to the observed data rather than tautologically reproducing the inputs. The analysis remains self-contained against external benchmarks of microfluidics and acoustics.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

The abstract does not introduce or specify any free parameters, axioms, or invented entities; the central claim rests on experimental observation of regimes, numerical simulation of the acoustic-flow interaction, and theoretical scaling arguments whose explicit forms are not given.

pith-pipeline@v0.9.0 · 5539 in / 1188 out tokens · 53697 ms · 2026-05-10T19:33:52.363198+00:00 · methodology

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Reference graph

Works this paper leans on

5 extracted references · 5 canonical work pages

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    flow focusing

    Al-Housseiny, Talal T., Tsai, Peichun A. & Stone, Howard A.2012 Control of interfacial instabilities using flow geometry.Nature Physics8(10), 747–750. Andrieux, S ´ebastien, Muller, Pierre, Kaushal, Manish, Macias Vera, Nadia Sof ´ıa, Bollache, Robin, Honorez, Cl ´ement, Cagna, Alain & Drenckhan, Wiebke2021 Microfluidic thin film pressure balance for the ...

    work page 2012

  2. [2]

    Guillot, Pierre, Colin, Annie, Utada, Andrew S

    Guillot, Pierre, Colin, Annie & Ajdari, Armand2008 Stability of a jet in confined pressure-driven biphasic flows at low reynolds number in various geometries.Physical Review E78(1). Guillot, Pierre, Colin, Annie, Utada, Andrew S. & Ajdari, Armand2007 Stability of a jet in confined pressure-driven biphasic flows at low reynolds numbers.Physical Review Lett...

    work page 2024

  3. [3]

    Robert de Saint Vincent, Matthieu & Delville, Jean-Pierre2016 Fragmentation mechanisms of confined co-flowing capillary threads revealed by active flow focusing.Physical Review Fluids1(4). Santra, Somnath, Mandal, Shubhadeep & Chakraborty, Suman2020 Phase-field modeling of multicomponent and multiphase flows in microfluidic systems: a review.International...

    work page 2010

  4. [4]

    between acoustic radiation forces, interfacial tension, and viscous drag asymmetry, consistent with the mechanism described in the main text

    resulting in the waviness regime. between acoustic radiation forces, interfacial tension, and viscous drag asymmetry, consistent with the mechanism described in the main text. The reduced-order model captures the essential spatial structure of the acoustic field, while the full-device simulations provide improved quantitative agreement with experimental o...

    work page 1967

  5. [5]

    The Reynolds numbers are defined as𝑅1 =𝑈 0𝜌1𝑊2/𝜇1 and𝑅 2 =𝑈 0𝜌2𝑊2/𝜇2, where𝑈 0 is the average velocity in the channel

    2(𝜇 𝑟 +𝑛) , where𝜇 𝑟 =𝜇 1/𝜇2 is the viscosity ratio,𝑄 𝑟 =𝑄 1/𝑄2 is the flow rate ratio, and𝑛=𝑊 1/𝑊2 is the width ratio of the two streams. The Reynolds numbers are defined as𝑅1 =𝑈 0𝜌1𝑊2/𝜇1 and𝑅 2 =𝑈 0𝜌2𝑊2/𝜇2, where𝑈 0 is the average velocity in the channel. The pressure gradient parameter𝐾is given by Yih (1967); Hazraet al.(2024) 𝐾= (1+𝑛) (𝑄 2/𝑄𝑡 ) 𝑅2 h −...

    work page 1967