Ring Method Detachment

In the classical methods used to measure surface tension of liquids, we find that detachment of a solid from a liquid surface provides very accurate results. A method that has been rather widely used involves the determination of the force to detach a ring or loop of wire from the surface of a liquid. This method is based on using a ring (platinum) and measuring the force when it is dipped in the liquid surface. 

The method is one of the many detachment methods, of which the drop-weight and Wilhelmy slide methods are also examples. It is based on the principle that within an accuracy of few percents, the detachment force is given by the surface tension multiplied by the periphery of the surface (liquid surface) detached (from a solid surface of a tubing or ring or plate). This assumption is also found to be acceptable for most experimental purposes. Thus, for a ring. 

Wtotai is the total weight of the ring Wring is the weight of the ring in air Rring is the radius of the ring 

The circumference is 2nRring, and factor 2 is because of the two sides of contact. 

This relation assumes that the contact between the fluid and the ring is geometrically simple. It is also found that this relation is fairly correct (better than 1%) for most working situations. However, it was observed that Equation 1.36 needed correction factor, in much the same way as was done for the drop-weight method. Here, however, there is one additional variable so that the correction factor f now depends on two dimensionless ratios (Adamson and Cast, 1997). 

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There are numerous other methods for measuring surface tension that we do not discuss here. These include (a) the measurement of the maximum pressure beyond which an inert gas bubble formed at the tip of a capillary immersed in a liquid breaks away from the tip (the so-called maximum bubble-pressure method) (b) the so-called drop-weight method, in which drops of a liquid (in a gas or in another liquid) formed at the tip of a capillary are collected and weighed and (c) the ring method, in which the force required to detach a ring or a loop of wire is measured. In all these cases, the measured quantities can be related to the surface tension of the liquid through simple equations. The basic concepts involved in these methods do not differ significantly from what we cover in this chapter. The experimental details may be obtained from Adamson (1990). 

The ring method (du Nuoy tensiometer) can be used to measure the interfacial tension as shown in Figure 4.9. The method measures the force required to detach a platinum wire ring from the interface by pulling on the ring. The mathematical relationship between the pulling force and interfacial tension is given by ... 

The Wilhelmy plate method, as shown in Figure 4.10, is similar to du Nouy s ring method, but it uses a thin mica plate or microscope slide. The plate is suspended from a balance and dips into the liquid. The force, F, required to detach the liquid meniscus surrounding the plate depends on the surface tension or interfacial tension by ... 

Ring method — Method to determine the - interfacial tension in liquid-gas systems introduced by Lecomte du Noiiy [i]. It is based on measuring the force to detach a ring or loop of a wire from the surface of a liquid. The method is similar to the -> Wilhelmyplate method when used in the detachment mode [ii]. See also -> electrocapillarity, -r electrocapillary curve, -> Gibbs-Lippmann equation, - Wilhelmy plate (slide) method, - drop weight method, - Lippmann capillary electrometer. 

Measuring the force (the weight) needed to break away a body from the liquid surface is the basis for many variations of the detachment method. The body may be in the form of a rod with a circular base (Pin detachment method), a rod with a rectangular base (Wilhelmy slide method), a horizontal circular ring (Ring method), etc. The different forms of the detachment method have been used for surface tension measurement of liquids from room temperature up to approximately 1000°C. It is evident that the detachment methods can be used only in cases when the liquid wets the measuring body. 

There are two modifications to the Wilhelmy plate method. In the first modification, the cup carrying the liquid is mobile and is lowered until the previously immersed plate becomes detached from the liquid surface, and the maximum vertical pull, / max on the balance is noted, similarly to the ring method. Then the capillary force, for the zero contact angle, can be given as... 

The ring method, attributed to du Noiiy, has been widely used. It involves the determination of the force necessary to detach a ring or loop of wire from the surface of the liquid, or interface between the liquids. This method is of good precision. A zero or near zero contact angle is necessary. Otherwise, a Teflon or polyethylene ring may be used. 

Most techniques stretch the liquid-air surface at the moment of measurement. For example, the drop weight method [318] and the ring method [319-322] stretch the surface during detachment. However, instruments are now available which measure surface tension without detaching the ring from the liquid (e.g., the Ki-iiss Tensiometer K12). 

Several convenient ways to measure surface tension involve the detachment of a solid from the liquid surface. These include the measurement of the weight in a drop falling from a capillary and the force to detach a ring, wire, or thin plate from the surface of a liquid. In this section we briefly describe these methods and their use. 

A zero or near-zero contact angle is necessary otherwise results will be low. This was found to be the case with surfactant solutions where adsorption on the ring changed its wetting characteristics, and where liquid-liquid interfacial tensions were measured. In such cases a Teflon or polyethylene ring may be used [47]. When used to study monolayers, it may be necessary to know the increase in area at detachment, and some calculations of this are available [48]. Finally, an alternative method obtains y from the slope of the plot of W versus z, the elevation of the ring above the liquid surface [49]. 

The Wilhelmy plate method provides an extremely simple approach that, unlike the ring detachment method, permits the measurement of continuously varying or dynamic surface tensions. If a thin plate (e.g., a microscope slide, a strip of platinum foil, or even a slip of filter paper) is attached to a microbalance and suspended so that its lower edge is just immersed in a liquid, the measured apparent weight Wj, is related to the actual weight of the plate Wp and the surface tension y by the following simple equation ... 

However, the advantage of the former over the latter method consists in that it makes it possible to choose the most convenient form and size of the body (platinum rod, ring, or plate) so as to enable the measurement to be carried out rapidly but without any detriment to its accuracy. The detachment method has found an application in the case of liquids whose surface tensions change with time. 

In this method the force required to detach a ring from a surface or interface is measured either by suspending the ring from the arm of a balance or by using a torsion-wire arrangement (du Noiiy tensiometer). 

The du Noiiy ring surface tension method involves slowly raising a platinum ring through a liquid until it detaches from the surface (Figure 3.8, Lower). The force at the point of detachment is measured using a balance or torsion wire. If F is the force on the ring,... 

There are static and dynamic methods. The static methods measure the tension of practically stationary surfaces which have been formed for an appreciable time, and depend on one of two principles. The most accurate depend on the pressure difference set up on the two sides of a curved surface possessing surface tension (Chap. I, 10), and are often only devices for the determination of hydrostatic pressure at a prescribed curvature of the liquid these include the capillary height method, with its numerous variants, the maximum bubble pressure method, the drop-weight method, and the method of sessile drops. The second principle, less accurate, but very often convenient because of its rapidity, is the formation of a film of the liquid and its extension by means of a support caused to adhere to the liquid temporarily methods in this class include the detachment of a ring or plate from the surface of any liquid, and the measurement of the tension of soap solutions by extending a film. 

For this reason, the surface tension has units of either energy per unit area or force per unit length. The surface tension of a liquid is measured in one of several ways capillary rise, ring detachment, or drop weight. Each method of surface tension measurement is outlined in Figure 9.2. With capillary rise, the fluid is suspended by the surface... 

FIGURE 9.2 Schematic diagram indicating various methods to measure the surface tension of a liquid (a) drop weight, (b) ring detachment, and (c) capillaiy rise. 

Table 1.2. Surface tensions of water in mN m , obtained by various investigations using different techniques. Temperatures in degrees Celsius. Abbreviations for methods CR = capillary rise, WP = Wilhelmy plate, DNR = Du Nouy ring, DM = other detachment method or object in the surface. HD = hanging (pendent) drop, SD = sessile drop, MBP = maximum bubble pressure DW = drop weight.

Abbreviations CR = capillary rise DM = detachment method (including Wilhelmy plate and Du Nouy ring) DW = drop weight ( stalagmometer ) HD = hanging (pendant) drop MGP = maximum gais bubble pressure SD = sessile drop. 

It is a widely used detachment method, in which the force to detach a ring from the surface of the liquid is determined. In the first approximation, the detachment force is supposed to be equal to the surface tension multiplied by the periphery of the surface to be detached. For a ring we can thus write... 

The method is very precise when some experimental conditions are fulfilled. The ring is usually made of platinum wire and should be kept as horizontal as possible. A decline of 1° causes an error of 0.5%. Care must be taken to provide that the ring is in the horizontal position and to avoid any disturbance of the surface when the critical point of detachment is approached. The ring is usually heated to red glow before use to remove surface contaminants. A zero or near zero contact angle is necessary, otherwise wrong values of surface tension could be measured. 

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