Description
We report a contactless, gas-actuated interfacial instability that turns a quiescent, surfactant-laden water surface into a robust, low-mode recirculating flow above a sharp gas-flow threshold. The platform is a simple circular well (Petri-dish geometry) capped by a plate with a central orifice of radius rc: an impinging gas jet spreads radially and applies a tangential shear stress that decays approximately as one over r along the gas–water interface. Below a critical gas flow rate Qc, the interface remains essentially motionless. Above Qc, we observe a reproducible azimuthal symmetry breaking dominated by a dipolar mode, producing a pair of counter-rotating surface vortices and strong interfacial transport, quantified by particle tracking velocimetry.
To rationalize the onset and mode selection with a minimal model suitable for microfluidic design, we derive a ring-wise linear stability theory for the surfactant concentration. The applied gas traction is mapped to an interfacial shear response, so that in the axisymmetric base surfactant profile shear stress competes directly with Marangoni stress due to surfactant concentration gradients. Regularity at the center and rotational symmetry imply decoupled azimuthal Fourier modes; a Frobenius/Taylor regular-branch of sin(m*theta) form yields a compact inner-edge closure for the radial log-slope at the ring, approximately m/rc, enabling an explicit dispersion relation. The resulting growth rate has a simple “traction versus diffusion” form, predicting a threshold Qc that scales with interfacial elasticity, orifice radius rc, and surfactant diffusivity. To capture the observed recirculating topology away from the ring, we complement the traction-driven (potential) surface response with a screened-Stokes/Brinkman streamfunction response driven by the azimuthal Marangoni traction at r equals rc, which naturally selects an eddy length scale and reproduces the dipolar pattern.
Beyond fundamental interest, the effect provides a practical actuator for open-surface microfluidics: gas flow rates above Qc yield on-demand micromixing in shallow wells, with the option to produce chaotic advection by utilizing two independent orifices. Conversely, because Qc depends sensitively on interfacial properties, the onset of flow itself can serve as a rapid readout for surface contaminants, turning a single gas flow sweep into a quantitative assay.