Capillary pressure-based measurement of dynamic interfacial tension in a spontaneous microfluidic sensor

The size of droplets and bubbles, and the properties of emulsions and foams strongly depend on dynamic interfacial tension (gamma(d)) - a parameter that is often inaccessible due to the very short time scales for droplet and bubble formation, and the inaccessibility of (e.g., food) production lines. To solve this challenge, we developed a microfluidic tensiometer that can measure gamma(d) by monitoring the formation time of both droplets and bubbles. Our tensiometer is a pressure-driven microfluidic device that operates based on the principle of a pressure balance: the formation of a droplet (or a bubble) is initialized when the Laplace pressure of the interface is decreased below the externally applied pressure, and this decrease is caused by a reduction in gamma(d) that can be calculated from the applied pressure and the Young-Laplace equation. The decay of gamma(d) due to surfactant adsorption can be followed at the characteristic time scale, which is dependent on surfactant type and concentration. For 0.05-1% wt sodium dodecyl sulfate (SDS), we were able to measure gamma(d) at time scales down to 1 ms and 0.1 ms for droplet and bubble interfaces, respectively, at increasing applied pressures and SDS concentrations. Our tensiometer proves to be a simple, robust method that inherently allows access to nearly the full range of dynamic interfacial tension at relevant time scales.

Automated summary

The size and properties of droplets, bubbles, emulsions, and foams are greatly influenced by dynamic interfacial tension (gamma(d)). However, this parameter is often difficult to measure due to the short time scales involved in droplet and bubble formation. To overcome this challenge, researchers have developed a microfluidic tensiometer that can measure gamma(d) by monitoring the formation time of droplets and bubbles. This tensiometer operates based on the principle of pressure balance and provides a simple and robust method to measure dynamic interfacial tension at relevant time scales.