## Weber number $\mathrm{We}=\frac{\text{Drag Force}}{\text{Cohesion Force}}=\left( \frac{8}{C_D} \right) \frac{\left( \frac{\rho v^2}{2} C_D \pi \frac{l^2}{4} \right)}{(\pi l \sigma)} = \frac{\rho v^2 l}{\sigma}$ $C_{D}$ is the drag coefficient of the body cross-section; $l$ is the characteristic length, typically the droplet diameter; $\sigma$ is the surface tension. >[!Question] >In animated works, one could often see the scene that a droplet (tears, for example) breakup into smaller parts right in the air without contracting other obstacles. However, this is less likely to happen in real world (at least for someone running). What are the conditions for such droplet breakup? >An intuitive idea is to consider surface tension and external dragging, so Weber number is important. And for high viscous liquid, [[Reynolds number]] could also be critical. If combined, we may reach the [[Ohnesorge number]]. >At least I do not think without vehicle, just by running, one could break 5mm diameter droplet into even smaller parts. >[!Info] >See more on wiki: https://en.wikipedia.org/wiki/Weber_number.