page_title: Surface tension
section_title: Puddles on a surface
hierarchical_section_title: Surface tension / Effects / Puddles on a surface
context_page_description: Surface tension is the tendency of liquid surfaces to shrink into the minimum surface area possible. Surface tension allows insects, usually denser than water, to float and slide on a water surface.
  At liquid–air interfaces, surface tension results from the greater attraction of liquid molecules to each other than to the molecules in the air.
  There are two primary mechanisms in play. One is an inward force on the surface molecules causing the liquid to contract. Second is a tangential force parallel to the surface of the liquid. The net effect is the liquid behaves as if its surface were covered with a stretched elastic membrane.
  Because of the relatively high attraction of water molecules to each other through a web of hydrogen bonds, water has a higher surface tension than most other liquids. Surface tension is an important factor in the phenomenon of capillarity.
  Surface tension has the dimension of force per unit length, or of energy per unit area. The two are equivalent, but when referring to energy per unit of area, it is common to use the term surface energy, which is a more general term in the sense that it applies also to solids.
context_section_description: Pouring mercury onto a horizontal flat sheet of glass results in a puddle that has a perceptible thickness. The puddle will spread out only to the point where it is a little under half a centimetre thick, and no thinner. Again this is due to the action of mercury's strong surface tension. The liquid mass flattens out because that brings as much of the mercury to as low a level as possible, but the surface tension, at the same time, is acting to reduce the total surface area. The result of the compromise is a puddle of a nearly fixed thickness.
  The same surface tension demonstration can be done with water, lime water or even saline, but only on a surface made of a substance to which water does not adhere. Wax is such a substance. Water poured onto a smooth, flat, horizontal wax surface, say a waxed sheet of glass, will behave similarly to the mercury poured onto glass.
  The thickness of a puddle of liquid on a surface whose contact angle is 180° is given by:
  where
  h is the depth of the puddle in centimeters or meters.
  γ is the surface tension of the liquid in dynes per centimeter or newtons per meter.
  g is the acceleration due to gravity and is equal to 980 cm/s² or 9.8 m/s²
  ρ is the density of the liquid in grams per cubic centimeter or kilograms per cubic meter
  In reality, the thicknesses of the puddles will be slightly less than what is predicted by the above formula because very few surfaces have a contact angle of 180° with any liquid. When the contact angle is less than 180°, the thickness is given by:
  For mercury on glass, γHg = 487 dyn/cm, ρHg = 13.5 g/cm³ and θ = 140°, which gives hHg = 0.36 cm. For water on paraffin at 25 °C, γ = 72 dyn/cm, ρ = 1.0 g/cm³, and θ = 107° which gives hH₂O = 0.44 cm.
  
  The formula also predicts that when the contact angle is 0°, the liquid will spread out into a micro-thin layer over the surface. Such a surface is said to be fully wettable by the liquid.
caption: Illustration of how lower contact angle leads to reduction of puddle depth