04 Feb To Seal or Not to Seal
Splashes are ubiquitous in nature and engineering and govern processes from diving, to ocean oxygenation. Yet to date, we do not understand when or why sometimes the splash seals, and other times does not. This work presents the first detailed analysis and physical understanding of why a splash does not always seal, supported by a developed theory and experimental observations. Contrary to current understanding, projectile impact velocity is not the governing parameter determining the occurrence of the surface seal, and this phenomenon is also dependent on the projectile size and density. We discovered the existence of a critical dimensionless number that predicts the occurrence of the surface seal. We developed an analytical model to describe the trajectory and dynamics of the splash curtain in the water entry of hydrophobic spheres and validated it with a series of experiments. We revealed that non-dimensionalized velocity of the airflow rushing into the cavity behind the sphere is the one scaling relationship for the dimensionless surface seal time which is able to determine a critical dimensionless airflow velocity beyond which the surface seal occurs.
When an object impacts the free surface of a liquid, it ejects a splash curtain upwards and creates an air cavity below the free surface. As the object descends into the liquid, the air cavity eventually closes under the action of hydrostatic pressure (deep seal). In contrast, the surface curtain may splash outwards or dome over and close, creating a surface seal. In this paper, we experimentally investigate how the splash curtain dynamics are governed by cavity pressure difference, gravity, and surface tension, and how their interplay controls the occurrence, or not, of the surface seal. Based on the experimental observations and measurements, we develop an analytical model to describe the trajectory and dynamics of the splash curtain. The model enables us to reveal the scaling relationship for the dimensionless surface seal time and discover the existence of a critical dimensionless number that predicts the occurrence of the surface seal. This scaling indicates that the most significant parameter governing the occurrence of a surface seal is the velocity of the airflow rushing into the cavity, which is in contrast to the current understanding that considers the impact velocity as the determinant parameter.