On a dirty car, the surface is rough and uneven due to the presence of dirt particles, dust, and other contaminants. These irregularities create microscopic pockets and crevices on the surface, which trap air and prevent the water droplets from spreading evenly. As a result, the water droplets appear to spread out and form a continuous film on the dirty surface.
In contrast, a polished car has a smooth and even surface, with fewer irregularities and contaminants. The absence of these imperfections allows the water droplets to form spherical beads, as the surface tension of the water acts to minimize its surface area. The cohesive forces between the water molecules are stronger than the adhesive forces between the water and the polished surface, causing the droplets to coalesce and collect into beads.
Additionally, the surface tension of water is influenced by the presence of substances known as surfactants, which are molecules that can reduce the surface tension of a liquid. Dirt particles and contaminants on a dirty car can act as surfactants, reducing the surface tension of the water and contributing to the spreading out of the droplets. On a polished surface, the absence of these contaminants allows the water to exhibit its natural surface tension, resulting in the formation of beads.
In summary, the spreading of water on a dirty car and the collection of water in beads on a polished car can be explained by the interplay between surface tension, surface roughness, and the presence of surfactants. The smooth, even surface of a polished car allows water droplets to form beads due to the strong cohesive forces between the water molecules, while the irregularities and contaminants on a dirty surface disrupt the formation of beads, causing the water to spread out.
