The onset of droplet motion along a horizontal anodized aluminum surface arising from aerodynamic loading imposed by an accelerating wall-bounded shear flow was investigated for water droplet volumes ranging from 75 to $120\mu \mathrm{L}$. Two accelerating shear flows were considered, a flat plate boundary layer and an impinging jet with orientation angles spanning ${30}^{\circ }$ to ${90}^{\circ }$. The flows were linearly accelerated at three different rates, 1.2, 2.2, and $4.4\mathrm{m}/{\mathrm{s}}^2$, to a maximum flow speed of $20\mathrm{m}/\mathrm{s}$. Droplets in the flat plate boundary layer were observed to have a constant depinning threshold Weber number of ${\mathrm{We}}_{h,\mathrm{crit}}=7.5\pm 0.5$. By contrast, droplets in impinging jets exhibited lower thresholds in the range of $2⪅{\mathrm{We}}_{h,\mathrm{crit}}⪅4$. Droplet volume and flow acceleration had marginal influence on depinning threshold for the considered parameter range. Rigorous dimensional analysis, supplemented by statistical examination of the present data and results from previous studies, is employed to identify dominant dimensional groups governing ${\mathrm{We}}_{h,\mathrm{crit}}$. A new dimensionless group that encapsulates interrelated parameters connected to droplet shape and substrate wettability is proposed, producing good collapse of available data and allowing for a simplified empirical estimation of critical depinning velocity.

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• Contact line dynamics
• Shear flows
• Wetting