Complex formulations drying on complex substrates

Abstract

Prediction and control over particle deposit patterns obtained from sessile droplet evaporation are essential for many industrial practices such as printing, forensics and crop-care. For the latter, and particularly in the case of agrochemicals drying on plant leaves, the spread of the deposit can impact the uptake of the active ingredient by the leaf. Hence, deposit pattern control is a crucial step for achieving high performance pesticides. Numerous formulations have been studied in order to understand how the different components affect the final dried deposit; yet the testing substrates are generally far from those used in real applications.

Therefore, a key challenge resides in understanding how different surfaces affect the drying of a particle-laden droplet and the final distribution of the particles after evaporation. To address this challenge, a staggered approach is followed in this work. First, the sole influence of the substrate’s wettability is studied, via hydrophobisation of flat glass slides. Second, surface roughness is introduced, and the results compared to those from a flat substrate with the same surface chemistry. Then, the effect of certain additives, in this case sodium chloride salt, is compared to that of the addition of surface roughness. Finally, the knowledge gained is implemented into a real-life leaf model with the aim of creating a crop-care tailored prototype. The main results found are first that the coffee-ring effect decreases with decreasing wettability on flat surfaces. This is explained to be as a consequence of a change in direction of the particle flow inside the droplet. It is speculated that this finding could be derived from geometrical constraints that could lower the rate of evaporation at the triple phase contact line compared to the apex of the droplet. Second, for the rough substrates this correlation is maintained when compared to a flat substrate. An increase in the roughness causes a decrease in the wettability and a subsequent decrease in the coffee-ring effect.

Third, an increase of the ionic strength of the solution, which is known to induce a uniform deposit was also found to increase the wettability. However, it was found that certain salt concentrations had no added effect on rough substrates, thus this effect is hindered by the roughness. Finally, no significant differences were observed between the behaviour of real and polydimethylsiloxane copies of leaves, meaning that freshly cut leaves could potentially be substituted by more durable and easier to make copies.

A large array of applications are based on the evaporation of sessile droplets. As such, and on the basis of the results obtained, it is concluded that the substrate plays an essential role and should not be overlooked during formulation design.