A group at Tokyo Metropolitan University has clarified the mechanism by which foam sprayed on the wall loses liquid, and succeeded in finding conditions to prevent liquid outflow and keep the foam for a long time.
Foam refers to a state in which a large number of bubbles are densely packed in a liquid, and is used in various daily situations such as hand soap, face wash foam, and dishwashing detergent.The foam sprayed on the wall can stay on the wall for a certain period of time, so that the effect of the active ingredient can be exerted for a longer period of time than when a liquid is simply applied.However, under gravity, the liquid flows from the upper part to the lower part of the foam, and the phenomenon that the liquid collected at the lower end of the foam is torn off from the foam (pinch off) is also confirmed.
In this study, we succeeded in elucidating the physical conditions and mechanism by which this pinch-off occurs.A simple model experiment and theory for observing the behavior of foam droplets sandwiched between parallel plates under gravity led to a formula explaining the conditions under which the liquid pinches off from the foam droplet end.
From this theoretical formula, it was found that in order to prevent the liquid from flowing out due to pinch-off in the foam, it is necessary to prevent the mass of the liquid accumulated at the foam dropping end from exceeding a certain threshold value.For this, it is important to reduce the absolute amount of liquid contained in the foam, and in order to prolong the time to pinch off, the liquid is drained (transported downward) by making the foam bubbles finer. ) Was also found to be effective.
The results of this research provide hints for controlling foam in gravity-affected environments, such as applying shaving foam to the face and spraying cleaning foam on the walls.Preventing pinch-off, keeping foam in the same place for a long time, and efficiently exerting the function of the active ingredient will greatly contribute to the achievement of SDGs.
Paper information:[Soft Matter] Pinch-off from a Foam droplet in a Hele-Shaw cell