What property causes a drop of liquid to ball up rather than spread as a film?

The phenomenon where a drop of liquid forms a bead-like shape rather than spreading out into a film is attributed to surface tension. Surface tension is caused by the cohesive forces between liquid molecules at the surface. Molecules within the liquid experience equal attractive forces from all directions, while those on the surface are pulled inward by the molecules beneath them. This imbalance results in the surface layer contracting and minimizing its surface area, leading to the formation of droplets.

In contrast, viscosity refers to a liquid's resistance to flow, which affects how quickly a liquid moves but does not directly dictate its shape in the form of droplets. Density relates to the mass of the liquid relative to its volume, influencing whether a liquid sinks or floats rather than how it spreads. Adhesion refers to the attraction between molecules of different substances, which can affect how a liquid interacts with a surface but does not directly cause the droplet's shape.