Marangoni destabilization of bidimensional-confined gas–liquid co-flowing streams in rectangular microfluidic channels

Abstract

In microchannels, the stability of a fluid jet injected into another immiscible fluid strongly depends on its degree of geometric confinement. When the width of the jet, w, is larger than the channel height, H, the surface tension driven Rayleigh-Plateau instability is suppressed so that the 2D (bidimensional)-confined jet is absolutely stable and never collapses into bubbles (or drops) in contrast to what occurs when w{less than or equal to}H. We here demonstrate both experimentally and theoretically, that this picture is indeed no longer valid when Marangoni effects are considered. We experimentally show that the addition of small length alcohol molecules into the liquid phase destabilizes a 2D-confined gas-water microfluidic stream (w>H) leading to the generation of steady non-linear waves and further to the production of bubbles. Using a simple hydrodynamic model, we show through a linear analysis that the destabilization of the gas stream may result from a Marangoni instability due the fast adsorption of the alcohol molecules which occurs on a time scale comparable to that of the microfluidic flow.

Foam Generation Through a Single Pore with Rectangular Cross-Section: Hysteretic Behavior and Geometric Limitation of the Volume Fraction of Bubbles

“…2 (the radius of curvature of the gas/liquid interface along its normal remains constant as it equals to H/2). 33,34,36 For this reason, as previously noted by Dollet et al, 33 we experimentally witness the formation of bubbles only when d c becomes equal to H, that is, when the cylindrical jet adopts a circular cross-section, a geometry that is unstable with respect to the surface tensiondriven Rayleigh-Plateau instability. 37 3.3.…”

“…Finally, our numerical results show that the dimensionless steady-state width of the gas jet depends on the ratio q = Q normalg Q normalw and henceforth on the gas volume fraction f g = q 1 + q . In the absence of Marangoni effects, a gas jet is unstable when it becomes 3D-unconfined, that is to say, when d normalc H = 1 . ,, Subsequently, for a given FFG, there exists a critical gas volume fraction, f g c = q normalc q c + 1 , below which only a foam can be witnessed. A quick glance at Figure reveals that for a fixed value of H , f g c is a decreasing function of W c whose maximum value, reached for a square cross-section of 0.998, is very close to 1.…”

“…Recall that the thickness of the lubrication film scales as HCa 2/3 , where is the capillary number at play (in this expression, V represents the mean fluid velocity) . For such a 2D-confined configuration, a perturbation of the width of the gas jet, δ d c ( x , t ) along x , the flow direction, leads to a Laplace pressure perturbation term, δ P ( x , t ), that, in the absence of Marangoni effects, is always stabilizing as it writes (the radius of curvature of the gas/liquid interface along its normal remains constant as it equals to H /2). ,, For this reason, as previously noted by Dollet et al, we experimentally witness the formation of bubbles only when d c becomes equal to H , that is, when the cylindrical jet adopts a circular cross-section, a geometry that is unstable with respect to the surface tension-driven Rayleigh-Plateau instability…”

“…We have identified the existence of two steady-state regimes: a monodisperse bubble regime and another one in which a stable gas jet flows. As theoretically expected, ,, we observe that the formation of bubbles always occurs at the pore neck (constriction of the FFG) when the width of the gas jet becomes comparable to the channel height. By performing systematic experiments, tuning the value of Q w while maintaining the P g constant has shown that the transition between these two regimes exhibits a bistability region.…”

“…For rectangular cross-section channels, in the absence of Marangoni effects, the stability of the gas inner thread depends strongly on its degree of geometric confinement. When the width of the jet is greater than the height of the channel, H , the Rayleigh–Plateau instability due to surface tension is suppressed so that the gas flow is absolutely stable and never collapses into bubbles, unlike what happens when its width becomes less than H . ,, Note that if the gas is replaced by a liquid, the same phenomenon is witnessed . For experiments performed at constant gas pressure, as the width decreases as the liquid flow rate increases, the inner gas jet becomes unstable above a critical value of the flow rate, generating a foam made of monodisperse bubbles.…”