Surface Tensions of Picoliter Droplets with Sub-Millisecond Surface Age

Abstract

Aerosols are key components of the atmosphere and play important roles in many industrial processes. Because aerosol particles have high surface-to-volume ratios, their surface properties are especially important. However, direct measurement of the surface properties of aerosol particles is challenging. In this work, we describe an approach to measure the surface tension of picoliter volume droplets with surface age <1 ms by resolving their dynamic oscillations in shape immediately after ejection from a microdroplet dispenser. Droplet shape oscillations are monitored by highly time resolved (500 ns) stroboscopic imaging, and droplet surface tension is accurately retrieved across a wide range of droplet sizes (10-25 μm radius) and surface ages (down to ~100 μs). The approach is validated for droplets containing sodium chloride, glutaric acid, and water, which all show no variation in surface tension with surface age. Experimental results from the microdroplet dispenser approach are compared to complementary surface tension measurements of 5-10 μm radius droplets with aged surfaces using a holographic optical tweezers approach and 2 predictions of surface tension using a statistical thermodynamic model. These approaches combined will allow investigation of droplet surface tension across a wide range of droplet sizes, compositions, and surface ages.

The surface tension of surfactant-containing, finite volume droplets

“…In fact, the surface tension values of droplets produced by nebulization of the same solution were uniformly consistent (<±3 mN•m −1 ) over several weeks even using different nebulizers, with clear and consistent differences in measured droplet surface tensions apparent only when the primary solute:surfactant ratio was changed. Third, in previous work with mixtures of glutaric acid and NaCl, the retrieved droplet RI and corresponding surface tension gave excellent agreement with model predictions and macroscopic solution measurements, indicating the relative ratio of glutaric acid and NaCl in solution was conserved upon nebulization (26,52).…”

“…Such partitioning may fully or partially counteract the surface tension lowering effect of surfactants and must be considered when predicting particle activation (18)(19)(20)(21)(22). Accounting for this partitioning is challenging because few approaches directly measure aerosol particle surface tension (23)(24)(25)(26)(27) and so far none have investigated surfactant partitioning in detail. Most approaches infer surface tension from hygroscopic growth or critical supersaturation measurements (28).…”

Chemical equilibria of aqueous ammonium–carboxylate systems in aqueous bulk, close to and at the water–air interface

“…A recent surface tension study of surfaces of small millisecond old droplets, show a significant decrease in surface tension of droplets containing glutaric acid with increasing concentration in comparison with pure water. 45 Aqueous solutions of sodium chloride show on the other hand increasing surface tension with increasing concentration. Furthermore, it requires a 16-fold excess of sodium chloride to counter-balance the surface tension decrease of glutaric acid.…”

“…27,28,32,33 It has been shown in a recent study that surface tension can be studied in very small aqueous droplets. 45 It is shown for very small water droplets that organic compounds accumulated in the surface region decrease the surface tension significantly, while the presence of inorganic salts as sodium chloride increases it. 45 Most chemical species require water molecules in all three dimensions for efficient hydration, such as the carboxylate, phosphate and sulfate ions, 46,47 and they will be increasingly depleted from the water-air interface with increasing hydration strength as approaching this region decreases the possibility for complete hydration, 26,48 mainly due to entropy effects, and thereby chemical stability.…”

High time resolution measurements of droplet evaporation kinetics and particle crystallisation

“…Note that droplet shape oscillations are also observed early in the droplet lifetime as marked within the first 100 µs. 38,39 The inset shows the rapid change in velocity as the droplet decelerates to the terminal settling velocity. The immediate deceleration following droplet generation is discussed in more detail below.…”