On the growth of liquid bridges

What is it about?

While we may not always think about them, we experience the effect of liquid bridges everywhere in our daily lives. For instance, it takes considerably more force to lift a wet cooking pan from the countertop compared to a dry one because the liquid film holds the pan to the surface. When you stir a cup of coffee and lift the spoon, a small thread of liquid often stretches between the spoon and the surface for a moment before breaking. Another familiar example is a sandcastle, which holds its shape because the grains are held together by tiny water bridges. While the final shape of these bridges is well understood, the process of how they grow to reach that shape has remained less clear. This study explores how these bridges grow over time by observing what happens when a solid sphere is lowered onto a thin layer of oil. As the sphere makes contact, a liquid bridge begins to form, drawing oil from the surrounding film. The key finding is the formation of a temporary ‘dimple’, a very thin, bowl-shaped region, in the oil film right next to the growing bridge. This dimple acts like a bottleneck, creating significant resistance to the flow of oil toward the bridge. We found that if the initial oil film is below a certain thickness, this dimple becomes very deep, dramatically slowing down the bridge’s growth. This can cause the bridge to take orders of magnitude longer to stabilize than previous studies predicted, a finding confirmed both experimentally and numerically.

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Photo by Aaron Burden on Unsplash

Photo by Aaron Burden on Unsplash

Why is it important?

The forces exerted by these bridges are critical for understanding how surfaces stick together and how lubricants behave in confined spaces, such as inside a ball bearing. Oil transport to the ball bearing's contact point is facilitated by the temporary negative pressure created in the liquid bridge, which helps draw oil toward the ball. However, in the extremely thin liquid films next to the bridge, the dimple creates a huge resistance to this flow. This resistance can eventually cause the bearing to ‘run dry’ over time, leading to mechanical failure. This research is therefore crucial for fields like tribology and mechanical engineering, as it provides a new physical mechanism to consider when designing and maintaining lubricated systems.