Drying of skin and personal care products, to create thin films, is a process employed to deliver product-specific functions to targeted areas. Uniform, defect-free films are essential, as defects and instabilities can adversely affect product performance. To optimise product functionality, understanding the drying process and resulting final film morphology is essential. This project investigated film formation by drying droplets and thin films of a polymer solution, specifically polyvinylpyrrolidone (PVP) and ethanol, a formulation often utilised in skin and personal care products.

An imaging rig was designed and optimised to study the drying and spreading dynamics, as well as dried morphology of PVP droplets at ambient conditions. Drying PVP–ethanol droplets on glass substrates resulted in two distinct edge morphologies: a ringlike pattern, similar to the well known "coffee-ring", and a scalloped pattern. This scalloped pattern formed due to a surface tension-driven flow, initiated by the presence of water within PVP. This instability was suppressed by the removal of water from PVP. These findings were supported by a lubrication approximation analysis, which predicts the instability wavelength and exhibits good agreement with experimental observations.

Octamethyltrisiloxane was added to PVP–ethanol mixtures to reverse the Marangoni flow and facilitate uniform deposition. However, evaporation from this mixture resulted in liquid–liquid phase separation. This was found to be caused by the transfer of water vapour into the droplet, altering the mixture’s miscibility. The surrounding relative humidity significantly influenced droplet evaporation dynamics, with lower humidity levels preventing phase separation.

A confocal Raman microscope was used to characterise PVP distribution during drying. In 1D PVP–ethanol film, the initial film thickness was found to determine the drying uniformity, PVP distribution, and solvent retention. Films of 30–40 μm thickness exhibited a uniform PVP distribution and minimal solvent retention, whereas films of 500 μm developed a concentrated polymer skin layer that hindered drying. In typical thickness ranges found in the application of personal care products (30–40 μm), PVP–ethanol displays limitations in solvent retention and slow release.