Shape of Drops
Category : JEE Main & Advanced
Whether the liquid will be in equilibrium in the form of a drop or it will spread out; depends on the relative strength of the force due to surface tension at the three interfaces.
\[{{T}_{LA}}=\] surface tension at liquid-air interface, \[{{T}_{SA}}=\] surface tension at solid-air interface.
\[{{T}_{SL}}=\] surface tension at solid-liquid interface, \[\theta =\] angle of contact between the liquid and solid.
For the equilibrium of molecule
\[{{T}_{SL}}+{{T}_{LA}}\cos \theta ={{T}_{SA}}\] or \[\cos \theta =\frac{{{T}_{SA}}-{{T}_{SL}}}{{{T}_{LA}}}\]
Special Cases
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\[{{T}_{SA}}>{{T}_{SL}},\,\,\cos \theta \] is positive i.e. \[{{0}^{o}}<\theta <{{90}^{o}}\] This condition is fulfilled when the molecules of liquid are strongly attracted to that of solid. Example : (i) Water on glass. (ii) Kerosene oil on any surface. |
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\[{{T}_{SA}}<{{T}_{SL}},\,\,\cos \theta \] is negative i.e. \[{{90}^{o}}<\theta <{{180}^{o}}\]. This condition is fulfilled when the molecules of the liquid are strongly attracted to themselves and weakly w.r.t. that of solid. Example : (i) Mercury on glass surface. (ii) Water on lotus leaf (or a waxy or oily surface) |
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\[({{T}_{SL}}+{{T}_{LA}}\cos \theta )\text{ }>{{T}_{SA}}\] In this condition, the molecule of liquid will not be in equilibrium and experience a net force at the interface. As a result, the liquid spreads. Example : (i) Water on a clean glass plate. |
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