Superficial tension.

Characteristic property of liquids is the superficial tension formed on a free surface of a liquid i.e. on border with the gaseous environment. This phenomenon is connected by that on a molecule and, taking place on a free surface (fig. 1), forces of an attraction on the side of environmental molecules of a liquid operate much more strongly, than from the side vapors of liquids or gas on which the liquid borders.

Fig. 1 Fig. 2

If conditionally to choose on a surface of a liquid a piece in length ℓ (fig. 1) forces of a superficial tension can be represented pointers perpendicular to a piece.

The value δ is called superficial tension. It is measured the relation of force of a superficial tension to length of a piece on which force operates.

δ=F/ℓ

The superficial tension can be certain as work which is necessary for making against force of a superficial tension

δ=A/S

The superficial tension of biological liquids can serve in some cases can serve as the diagnostic factor. So, for example, at disease by jaundice a superficial tension wet sharply decreases owing to occurrence to urine of bilious acids. At diabetes and some other diseases the contents of lipase in blood raises. The contents of lipase judge on change of a superficial tension of a solution of threebutilen at addition in it of blood. For definition of a superficial tension in medical practice use a method of falling drops. At the slow expiration of a liquid the drop is formed of an aperture or from a vertical tube. At the moment of tearing of drops force F =2πrδ superficial a tension it is equal to a gravity of drops Q=mg=ρυg, 2πrδ= ρgυ (r -radius of neck of drops, ρ is density of liquids, V -volume of a drop)

δ= ρVg/2π (l)

To measure radius of neck practically it is impossible (it it is possible to make at photographing a drop at the tearing moment), therefore using a method of falling drops, resort to a comparative way. If the superficial tension is known of standard liquid δ₀, for example waters, then formula (1) can be written down.

δ₀=ρ₀ V₀ g /2π (2)

If to take identical volumes of water and a researched liquid V and having counted up quantity of drops n in these volumes, then it is possible to calculate volume of a drop.

V₀= V₁/n_{0 (waters),} V= V₁/n _{(a researched liquid)}

Having substituted these expressions accordingly in the formula (1) and (2) and having taken their relation, we shall receive

δ/ δ₀= ρn₀/ ρ₀n (3)

δ= δ₀ ρn₀/ ρ₀n (4)

The factor of a superficial tension depends upon nature liquids and on temperature (decreases at its increase), but does not depend upon size or the form of a surface. Unit of measurement of superficial tension is N/ m or dyne/cm.

Let's result values of a superficial tension for some liquids at temperature

Table 1

Liquid	δ (N/m)	Liquid	δ (N/m)
Water	0,0725	Mercury	0,470
Bile	0,0480	Whey of blood	0,060
Milk	0,0500	Ether	0,0170
Urine	0,0660	Spirit	0,022

Density of some water solutions (kg/m³) at 20˚С different concentration

Table 2

%	CuSo	Spirit	NaCl	KJ	%	Spirit	KJ
0	999	998,6	998,6	998,6	55	904,3	1630
5	1051	999,8	1034,5	1036,3	60	892,8	1731
15	1167	976,0	1109,0	1120,0	70	869,3
20	1230	969,9	1148,5	1167,9	75	857,4
30		955,1		1273	85	832,7
35		946,3		1332	90	819,7
40		936,7		1397	95	806,0
50		915,5		1545