As the plate speed is increased, the foam is no longer spread evenly, but drifts along on a sheet of liquid. At even faster speeds, a thin trail of foam is left behind.
Newswise — Tokyo, Japan – Tokyo Metropolitan University researchers have successfully unraveled the underlying physics governing the spreading of foams on surfaces. Their study involved placing foam balls on a flat substrate and then using a plate to scrape them across the surface while closely observing the process. The researchers discovered distinct patterns during the spreading, with the scraping speed playing a crucial role, as it involves a competition of various physical phenomena. Their findings have broad applicability to soft materials that require even distribution on surfaces, such as mayonnaise on bread or insulation on walls.
The spreading of soft materials on flat surfaces, whether it's shaving foam, wall insulation, or margarine on toast, holds immense importance in everyday practicality and industrial process optimization. Surprisingly, limited knowledge existed about the specific behavior of foams during spreading, especially concerning the interaction with a flat blade or plate, which efficiently transforms them into uniform layers.
Professor Rei Kurita and his team of researchers from Tokyo Metropolitan University were intrigued by the behavior of foams on surfaces and decided to conduct a comprehensive investigation. To delve deeper into the matter, they devised an experiment involving small detergent foam domes placed on a flat surface. Employing an acrylic plate, they carefully scraped the foam domes while maintaining a consistent distance between the plate and the surface. The entire process was meticulously observed using a video camera.
Remarkably, their observations unveiled a fascinating phenomenon: the spreading dynamics of the foam underwent a complete transformation based on two crucial factors - the velocity of the plate and the liquid's affinity to the surface (hydrophilic or hydrophobic).
On a hydrophobic surface, when the plate operated at low scraping speeds, the foam spread evenly, forming an elongated section with the same width as the original dome. However, as the plate's speed increased, the foam ceased to spread conventionally and instead glided along the surface atop a thin layer of fluid. This resulted in the plate leaving behind minimal foam. Surprisingly, at the highest speeds tested, the spreading regime reappeared, but now the width of the foam's tail was thinner than the original dome.
Conversely, when the experiment was conducted on a hydrophilic surface, the first regime with even spreading was absent entirely. The absence of this regime indicates a distinct behavior of the foam in response to the surface's hydrophilic nature.
The team's attention was drawn to the disparity between the two surfaces, prompting them to explore the influence of "wetting," which refers to the liquid's tendency to cover the surface. Concentrating on the behavior at low scraping speeds, they observed that on hydrophobic surfaces, the detergent films constituting the foam tended to anchor onto the surface due to "dewetting" of the liquid. This revealed a process where the plate progressively spread the foam away from the dome as it moved. However, when the foam was forced across the surface rapidly enough to wet it, a lubricating layer of liquid formed at the base. As a consequence, the walls in the foam (Plateau borders) lost their ability to adhere to the substrate firmly, and the foam slid across the surface, leaving only a thin section where the dome was initially situated and a trail of liquid behind.
The investigation not only explored the impact of plate speed but also delved into the significance of gap width and plate thickness. These findings shed light on the less understood aspects of a common occurrence, with potential implications extending beyond foams to various soft materials, including paint, protective coatings, and even mayonnaise. The research has the potential to enhance our understanding of these materials and their spreading behavior on surfaces, thus opening new avenues for practical applications in various industries.
This work was supported by JSPS KAKENHI Grant Numbers 20K14431, 17H02945 and 20H01874.
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