Shedding light on how spray formula surfactants affect droplet size and distribution

Beyond just common household products, liquid sprays figure in countless industrial, agricultural, and medicinal applications, many of which require precise droplet size. Droplet width is a significant parameter in pharmaceutical sprays, for instance. Similarly, agricultural pesticides require small enough droplets to adequately cover crops, yet not so small as to pollute via airborne drifts.

“Understanding and ultimately controlling droplet size from sprays is of paramount importance, and recent progress allows this for simple liquids by quantifying the dynamics of a liquid sheet that breaks up into drops,” said author Rick Sijs. “But the effect of surfactants [compounds included in many spray formulas to reduce surface tension at the air-liquid interface] on droplet size has been relatively unknown.”

Sijs et al. examine spray dynamics with two types of agricultural spray nozzles to investigate how average drop size and distribution change for sprays with water and those with different surfactants. They find reduced surface tension shrinks drops, though not as much as anticipated.

“During spraying, new water-air surface is created so rapidly that the surfactants don’t have sufficient time to completely diffuse,” said Sijs. “So, the effective surface tension during spraying is between the value for the clean water surface and that of a surfactant-covered surface.”

Quantitatively, these results are understood by measuring dynamic surface tension, which, in turn, provides for accurate prediction of droplet size for surfactant solutions.

“Now that we know how surfactant sprays break up and how to predict droplet size and distribution, it becomes possible to use this in practice to fine-tune droplet size, which has profound implications for many applications,” said Sijs.

Source: “How surfactants influence the drop size in sprays from flat fan and hollow cone nozzles,” by R. Sijs, S. Kooij, and D. Bonn, Physics of Fluids (2021). The article can be accessed at https://doi.org/10.1063/5.0066775 .