Measuring and testing – Liquid analysis or analysis of the suspension of solids in a... – Surface tension
Patent
1994-04-22
1996-01-02
Noland, Thomas P.
Measuring and testing
Liquid analysis or analysis of the suspension of solids in a...
Surface tension
73 6452, G01N 1302
Patent
active
054798167
DESCRIPTION:
BRIEF SUMMARY
The interface tension between two non-miscible liquids, or between a liquid and an ambient fluid, is a physical magnitude that is much used in various scientific and technical fields. It represents the energy brought into play by the forces of intermolecular cohesion on an interface between a liquid and an ambient medium. An ambient medium is a medium, which is also fluid, into which the liquid considered is plunged. The liquid and the ambient medium are non-miscible. In very concrete terms, it is the force that tends to collect the liquids together into drops whose shape would be spherical if they were not deformed by gravity.
Interface tension is expressed in terms of force per unit of length, for example in Newton/m or millinewton/m.
For a given pair constituted by a pure liquid and a pure ambient medium, under fixed conditions of temperature and pressure, the interface tension is a constant magnitude.
However, the presence of certain chemical agents on the interface may modify the structure of the interface and, consequently, the value of the interface tension. These agents are then known as surface-active agents.
The measurement of the interface tensions may then serve to assess the surfactant activity of certain chemical agents or, indirectly, to measure the concentration of these agents in a liquid.
These measurements find numerous applications, notably in the petroleum, chemical, food, cosmetic and other industries.
For example, these measurements are useful for the developing of efficient detergents or emulsifiers; they are useful for the development of products to disperse crude petroleum in the event of oil slicks; and again they are useful for the study of enzymatic physiological reactions.
There is a known way of calculating the value of the interface tension between a liquid and an ambient medium on the basis of the measurements (optical measurements) of the shape and dimensions of a drop of liquid with vertical axial symmetry, plunged into the ambient medium and clinging to the end of a hollow needle used to form the drop.
The drop takes a shape that results from the equilibrium among the forces of gravity, buoyancy, the forces of interface tension and the pressure due to the curvature of the interface.
It can thus be seen that, for two given liquids, the separation of the drop from its own medium depends on the interface tension of the radius of the hollow needle and on the angle of contact. This angle is directly related to the material constituting the needle.
By analyzing the shape and dimensions of the drop, it is possible to determine the value of the interface tension in an absolute way. There is indeed a relationship between the value of the interface tension and the geometrical parameters characterizing the shape of the drop. This relationship is defined by an equation known as the Laplace equation.
This is what is called the calculation of the interface tension by the "pendant drop" method. There is also a known way of carrying out this calculation using the shape of a drop laid on a horizontal solid surface.
However, the Laplace equation does not allow for an analytical approach to the obtaining of the value of the interface tension. It can therefore be resolved only in an indirect way.
Two major types of approaches have been used in the past to obtain a value of the interface tension on the basis of the knowledge of the shape of the drop. The first method, which is speedy but has little precision, consists in determining the position of two characteristic points of contour in order to deduce an approximate value of the interface tension therefrom. The second method, which is more precise but slower, consists in using all the points of the contour of the drop and in carrying out successive approximations until a value of tension is found for which the theoretical shape of the drop is as close as possible to the real shape.
The standard approaches used to determine the interface tension by the method of the pendant drop or the standing drop are described in the various documents. F
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Bois Andre
Grimaldi Michel
Nury Sylvie
Richou Jacques
Riviere Claude
Noland Thomas P.
Universite de Toulon et du Var, Laboratoire d'Optoelectronique
Wiggins J. David
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