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| Shape of Meniscus |
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| The free surface of a liquid called meniscus, assumes a flat, convex or concave shape, depending on the solid and liquid surface. Consider the following diagram |
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| The forces acting on the liquid molecule O, at the free surface are |
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 the weight mg of the molecule, acting vertically downward |
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 the force of adhesion acting along OA and |
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 the force of cohesion acting along OC, as shown in diagram (a). |
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| On resolving FC into its components we have, FC cos 450, acting vertically downwards and FC sin 450, acting horizontally away from OA. |
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There, the forces acting on the molecule O horizontally, are
 depending
on the magnitudes of Fa & Fc
and  acting vertically downward. |
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1. If  acts on 'O' and
every point on the free liquid surface remains perpendicular and meniscus is flat as shown in diagram b. |
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2. If  the net horizontal
force acts along OA outward and the resultant of horizontal and vertical forces, acts along OR outward but perpendicular to the liquid surface at O. Therefore, resultant force on all liquid molecules on the free surface, acts perpendicular to the liquid surface as shown below in diagram (e), giving rise to a concave meniscus. |
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Similarly, if  as in diagram (d), the resultant
of horizontal and vertical forces, acts along OR pointing inwards, resulting in a convex surface. |
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| When the capillary tube is dipped in a beaker containing water, water rises through the capillary tube. The force of surface tension acts tangentially inwards, all along the sides, tending to minimise its surface area. This force creates a force of reaction 'R', acting along OT but outwards. This force of reaction tries to pull the liquid upward, (this capillary tube is clamped) due to which, the liquid rises till the upward reaction is balanced by the weight of liquid column, acting vertically downward. |
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| On resolving R, we have Rcosq acting all along the sides of the glass and R sin q, acting horizontally outward. |
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| The total upward force is Rcosq.2pr (where r is radius of the capillary tube) and the total weight of liquid column is mg. |
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| where h is height of liquid rise, r the density of liquid and g is the acceleration due to gravity. |
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 In equilibrium, |
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| Since R = T |
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| For pure water and glass surface, q = 0 |
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 Surface tension of oil is lesser than that of water. Therefore, oil spreads on water. |
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 Cleansing action of detergents is based on the fact that when soap is added to water, the force of adhesion between soap water and the fabric increases and this reduces the surface tension of water. This results in spreading of soap water. |
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 Lubricating oil and paints have low surface tension so that they can spread properly. |
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 Surface tension prevents water from passing through the pores of an umbrella or tent. |
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