InterPore2025

Deposition of droplets on inclined porous surfaces

21 May 2025, 12:05

15m

Oral Presentation (MS10) Advances in imaging porous media: techniques, software and case studies MS10

Speaker

Maude Dias

Description

Water infiltration in buildings is an increasing issue that has intensified with climate change : weather conditions (floodings, storms) are progressively becoming more intense and may cause a faster deterioration of buildings. In order to analyze the consequences of wind driven rain on built materials, the impact of rainfall on an inclined porous surface is observed at droplet scale. The droplet spreading dynamics are recorded on high speed cameras from two points of view.
The shadowgraphy technique is used to observe the droplet fall and spreading through the high speed camera with great contrast and lightning. A direct light is placed facing the camera in order to observe only the shadow of the drop, which allows to accurately measure its diameter while it is falling and spreading. The cameras are set at a precise distance to capture the drop impact and maintain sufficient resolution. An optical zoom lens is set up to seize the complete droplet spreading in any regime: deposition, bouncing or splashing. The camera setup, spatial and time resolution, allows to record the spreading and beginning of absorption process of the impinging drop.

Some characteristics of both the liquid and the porous surface are determined, in order to draw a phase diagram that weighs the importance of the different parameters. Some of them are the droplet diameter, the viscosity and surface tension of the liquid, the porosity of the surface. The high speed cameras allow to record the spreading diameter of the droplet and its surface to obtain the dynamic surface energy. The velocity of the droplet and the angle of impact are the two varying parameters. With an energy equilibrium model, the dynamic parameters are linked to the set characteristics, in order to predict the spreading ratio. A few hypotheses take into account the spreading shape of the droplet, to simplify the surface energy, that is compared to the experimental data. The experimental data agrees with the correlation obtained theoretically. Understanding porous surface wetting by a single droplet is essential to upscale the analysis. Analyzing the droplet behavior throughout the spreading and absorption process is necessary to quantify the moisture content of the substrate and what surface is affected. This has to be considered in order to estimate the area potentially contaminated by the droplet and to better understand the drying mechanism of building materials. The effects of a droplet spreading can help to predict the wetting, drying and runoff of rain water in the built environment at urban scale.