Drop spreading and penetration into pre-wetted powders

We present results from an experimental study of the impact of liquid drops onto powder beds which are pre-wetted with the impacting liquid. Using high-speed video imaging, we study both the dynamics of the initial spreading regime and drainage times once the drop has reached its maximum spread on the surface. During the initial spreading stage, we compare our experimental data to a previously developed model which incorporates imbibition into the spreading dynamics and observe reasonable agreement. We find that the maximum spread is a strong function of the moisture content in the powder bed and that the total time from impact to complete drainage is always shorter than that for dry powder. Our results indicate that there is an optimum moisture content (or saturation) which leads to the fastest penetration. We use simple scaling arguments which also identify an optimum moisture content for fastest penetration, which agrees very well with the experimental result.

Graphical abstractWe present results from an experimental study of the impact of liquid drops onto powder beds which are pre-wetted with the impacting liquid. Using high-speed video imaging, we find that the maximum spread is a strong function of the moisture content and that there is an optimum saturation which leads to the fastest penetration.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► We study experimentally the impact of water drops with pre-wetted powders. ► The penetration time for pre-wetted powders is always shorter than for dry powder. ► The maximum spread of the drop is a function of the moisture content in the target. ► We find an optimum saturation of 17 %, leading to the fastest penetration.