Capillary length sets the basic budget for how much force the interface can hold before breaking. For freshwater at 20 C, the capillary length is roughly 2.7 mm, which means the weight must be distributed over an area that prevents the meniscus from dipping past that scale. Water-walking spiders exploit this by spreading each tarsus into a shallow footprint shaped by hydrophobic setae.
When the leg presses down, the water surface bends with a slope that ties back to the Young-Laplace equation. The contact angle remains above 90 degrees thanks to the waxy coating, which flips the pressure sign and resists penetration. A quick calculation: a 10 mg load shared by eight legs yields about 1.25 mg per leg. Spread over a 3 mm diameter footprint, the pressure stays well below the capillary limit, so the interface remains intact.
Slower movements allow the meniscus to relax, but rapid impulses add dynamic pressure. That is why spiders time their strides: the leg never lingers long enough for the surface to sag. The practical takeaway is that any biomimetic device should keep instantaneous forces below the capillary threshold and preserve a contact angle that favors non-wetting.