In this presentation, we explore the surface tensions of a range of different surfactant, organic and inorganic salt microscopic droplets. These will be detailed over various concentrations and droplet sizes alongside statistical modelling data.<br/>We use the droplet oscillation method to quantify the surface tension of aqueous surfactant solutions (Stückrad 1993). In the work presented here, droplets of picolitre volume and 30-80 μm in diameter are reproducibly dispensed from a microdroplet dispenser with a surface age &lt;1 ms. The resulting oscillations in droplet shape are characterised with 1 μs time resolution. The surface tension is inferred from the frequency of oscillating liquid droplet, time-resolved by stroboscopic imaging (500 ns), (Miles 2019). The temporal development of the dynamic surface tension will be compared to bulk solution measurements to determine the effects of confinement on surface-bulk partitioning timescales (Bzdek 2020). Preliminary work has investigated organic acids such as oxalic acid, malonic acid and pyruvic acid coupled with ammonium sulfate in order characterise the surface tension suppression of inorganic salts by the addition of organic material. Additional work on surfactants has shown a surface tension reduction in 30 μm diameter droplets containing sodium dodecyl sulfate (SDS) (Wang 2014). As the surfactant concentration increases from 3 mM to 100 mM, the measured droplet surface tension decreases, with increasing differences between droplet and solution measurements owing to the timescales of surface-bulk partitioning (Zhang 2014). Diffusion coefficients, calculated from this data, decrease as the concentration of surfactant is increased (data), whereas the adsorption coefficients show an increase as the surfactant concentration is increased (data).

Characterising the dynamic surface tension of droplets containing organic acids, inorganic salts and surfactants by the oscillating droplet method.

ACS Spring 2022

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