why does the bristles of a brush spreads when the brush is in water and cling together when it is taken out from the water?
This phenomenon is due to surface tension. When the brush is in water, the water molecules at the surface of the water create a cohesive force that causes the water to pull together, creating surface tension. This tension pulls the bristles apart, allowing them to spread out. As the brush is taken out from the water, the surface tension decreases and the force pulling the bristles apart is reduced. As a result, the bristles cling together due to their natural cohesion.
When a brush with bristles is placed in water, several factors contribute to the spreading of the bristles. Here is a step-by-step explanation:
1. Capillary Action: Bristles are often made of materials such as nylon or natural hair, which have tiny capillary channels within them. When the brush is placed in water, these channels absorb water through capillary action. This causes the bristles to absorb water and swell up.
2. Hydrophobicity: The surfaces of the bristles interact differently with water and air. Bristles are generally hydrophilic, meaning they have an affinity for water. When water is absorbed by the bristles, their surface tension decreases, and they become more flexible.
3. Surface Tension: Water molecules have strong cohesive forces, causing them to stick together and form surface tension. The absorbed water in the bristles helps reduce the surface tension effect, allowing the bristles to spread apart.
When the brush is taken out of the water, the following steps occur:
1. Desorption: As the brush is removed from water, the absorbed water begins to evaporate. The bristles lose moisture and start to shrink back to their original size.
2. Hydrophobicity & Surface Tension: As the bristles lose moisture, their surface tension increases, causing them to return to their original position. The hydrophobic nature of the bristles allows them to repel water and minimize their interaction with it.
In summary, when a brush is placed in water, the bristles spread apart due to capillary action, the hydrophilic nature of the bristles, and the reduction of surface tension. When the brush is taken out of the water, the bristles contract back to their original position due to desorption, increased surface tension, and the hydrophobic nature of the bristles.