The impact dynamics of picolitre aerosol droplets depositing on surfaces: Effect of wettability, inertia and viscosity

The rich literature on the impact of droplets on surfaces is dominated by studies of relatively accessible, microlitre droplets, predominantly motivated by the relevance to industrial processes such as inkjet printing. However, detailed analysis of the impact dynamics of picolitre aerosol droplets remains an under-researched field, mainly due to experimental challenges, despite their wide-ranging relevance. Here we provide a comprehensive experimental analysis of the impact dynamics of picolitre droplets at relatively low inertia (<i>We</i> ∼1.89–19.6), while varying viscosity (giving <i>Oh</i> ∼0.0168–0.0408) and surface wettability. A 30% increase in maximum contact diameter is seen across the <i>We</i> range, as well as a 10% increase as the impact surface changes from hydrophobic to hydrophilic. The surface wettability has an even more pronounced impact on the receding velocity and receding contact angle, whilst a non-linear relationship is observed between the receding velocity and <i>We</i>. Increasing <i>Oh</i> results in a 50% reduction in the receding velocity, along with 30% increase and decrease in the advancing and receding contact angles, respectively, with negligible impact on the maximum contact diameter. The measurements are compared with existing models (e.g., for larger droplets) and a more accurate relationship between the maximum spreading factor and <i>We</i> for picolitre aerosol droplets, which also incorporates the dependence on surface wettability, is derived.