Materials used to make rain proof coats and tents are water proof due to:

Question

When two soap bubbles of radii \( a \) and \( b \) (\( b>a \)) coalesce, the radius of curvature of the common surface is:

Answer 1

Compare a rain proof coat with an ordinary absorbent towel to see the difference clearly.

A plain cotton towel is meant to soak up water, so water wets its fibres easily and gets drawn deep into the narrow gaps between threads by capillary action.
A rain proof fabric is coated so that the angle at which a water drop touches the fibre becomes large, instead of small.
When this contact angle is large, the surface tension of water pulls each drop into a tight bead rather than letting it spread and soak into the cloth.
The bead then simply rests on top of the fabric and rolls off, keeping the material dry underneath.

This water repelling behaviour comes directly from surface tension acting on the treated surface, so option A is correct.

Answer 2

Compare a rain proof coat with an ordinary absorbent towel to see the difference clearly.

A plain cotton towel is meant to soak up water, so water wets its fibres easily and gets drawn deep into the narrow gaps between threads by capillary action.
A rain proof fabric is coated so that the angle at which a water drop touches the fibre becomes large, instead of small.
When this contact angle is large, the surface tension of water pulls each drop into a tight bead rather than letting it spread and soak into the cloth.
The bead then simply rests on top of the fabric and rolls off, keeping the material dry underneath.

This water repelling behaviour comes directly from surface tension acting on the treated surface, so option A is correct.

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