This paper analyzes experimentally and numerically the steady bubble rising in water with a surfactant dissolved at very low concentrations. We explain how traces of surfactant can significantly change the bubble dynamics. The tiny surface tension variation produced by the surfactant monolayer has a negligible effect on the capillary pressure. However, this variation occurs within an extremely thin diffusive boundary layer, which produces a Marangoni stress three orders of magnitude larger than the tangential viscous stress in a surfactant-free bubble. Although the Marangoni stress is confined within the surface boundary layer, it manages to immobilize most of the bubble's south hemisphere. The increase in skin friction and the reduction of the terminal velocity cannot be attributed to the viscous stress exerted on the immobilized interface but to the stress in the diffusive surface boundary layer. The stagnant-cap approximation applies despite the small surfactant concentration considered.

We study the bursting of bubbles in weakly viscoelastic liquids. The dissolved macromolecules form a monolayer at the water–air interface, influencing the bubble dynamics during the cavity collapse. For an optimum polymer concentration, the interfacial effects dampen short-wavelength waves, which intensifies the focusing of energy powering the jet ejection. This results in a significant increase (decrease) in the first-emitted droplet velocity (radius). The jet formation produces strain rates leading to a significant increase in the extensional viscosity. This extensional thickening reduces (increases) the first-emitted droplet velocity (radius). Bulk viscoelasticity produces a large difference between the velocity of the jet front at the tank surface level and the velocity of the first-emitted droplet. This droplet coalescence with others that are subsequently emitted, even for small polymer concentrations. Overall, viscoelasticity considerably hinders the ejection of small droplets, even for quasi-Newtonian liquids. The droplet emission is suppressed for smaller polymer concentrations when the bubble radius is decreased.

We study experimentally the stability of micrometer weakly viscoelastic jets produced with transonic flow focusing. Highly stable jets are formed when a low molecular weight polymer is added to water at a given low concentration, and the injected flow rate is reduced to its minimum value. In this case, the capillary instability is delayed, and the jet breakup occurs at distances from the ejector of the order of tens of thousands the jet diameter. The results indicate that the intense converging extensional flow in the ejection point builds up viscoelastic stress that does not relax in the jet even for times much longer than the polymer relaxation time. We hypothesize that the drag (shear) force exerted by the outer gas stream prevents the stress relaxation. It is also possible that partial polymer entanglement at the jet emission point contributes to this effect. We measure the jet length and the diameter at the ejector orifice and breakup point. The diameter takes values just above 2 μm at the breakup point regardless of the liquid flow rate and gas pressure.

We find and study the viscoelastic transition in transonic flow focusing when polymers of low molecular weights are dissolved in water at the appropriate concentration. This phenomenon is explained in terms of the coil-stretch transition of the polymers crossing the tapering meniscus, where the airstream produces very large strain rates. The resulting elastic stress stabilizes the flow, reducing the minimum liquid flow rate leading to jetting. As a consequence, we produce jets much thinner and longer than their Newtonian counterparts, which has important consequences at the technological level. We present a scaling analysis to show the role played by the polymer relaxation time and extensional viscosity in the viscoelastic transition.

Microfluidic devices for drop and emulsion production are often built using fire-shaped (or fire-polished) glass nozzles. These are usually fabricated manually with inexpensive equipment. The shape limitations and poor reproducibility are pointed as the main drawbacks. Here, we evaluate the capabilities of a new fire-shaping approach which fabricates the nozzle by heating a vertical rotating capillary at the Bottom of a Lateral Flame (BLF). We analyze the effect of the heating conditions, and the capillary size and tolerances. The shape reproducibility is excellent for nozzles of the same size produced with the same conditions. However, the size reproducibility is limited and does not seem to be significantly affected by the heating conditions. Specifically, the minimum neck diameter standard deviation is 3%. Different shapes can be obtained by changing the heating position or the capillary dimensions, though, for a given diameter reduction, there is a minimum nozzle length due to the overturning of the surface. The use of thinner (wall or inner diameter) capillaries allows producing much shorter nozzles but hinders the size reproducibility. Finally, we showed an example of how the performance of a microfluidic device is affected by the nozzle shape: a Gas Dynamic Virtual Nozzle (GDVN) built with a higher convergent rate nozzle works over a wider parametric range without whipping.