Pull-force methods

The du Nouy ring and Wiiheimy plate methods (Fig. 3.4) are two most frequently used techniques of measuring surface tension at the liquid—air interface or mterfacial tension at a liquid—liquid interface. Only the du Nouy method can be applied to measure interfacial tension. Both of these techniques are based on pulling an object with a well-defined geometry off the surface of liquids and measuring the pull force. These techniques are also known as pull-force methods. In the Wiiheimy method, a plate is the pull object, whereas in the du Nouy technique a ring is used. These techniques are ascribed to two scientists who conducted some of the earliest research in the area of surface tension measurement. In 1863, Wilhelmy described measurement of capillary constants in a paper, without a detailed calculation of surface tension. Lecomte du Nouy illustrated the... 

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 ... 

Pull-out and/or pull through methods have also been developed for a number of other objects. Mostly they have been designed to avoid some of the physical drawbacks of the plate and ring methods, but they do not remove the chemical problems of insufficient wetting or high De. In fact, as long ago as 1880 Gay Lussac measured the force to be exerted on a horizontal plate to pull it free from the surface of a liquid ). 

Surface tension was measured at 25.0°C + 0.5°C with a Kruss processor tensiometer K21 (Kruss GmbH, Germany) using Wilhetmy s plate method. A square platinum plate was cleaned, washed with twice distilled water and heated in a reductive flame to purge aU impurities. This cleaning procedure was repeated before every measurement. During the measurement, the plate is dipped into the liquid. The tensiometer measures the pulling force of the liquid on the plate and, with known plate size, calculates the surface tension. 

A stand-alone static method is the popular Wilhelmy plate method (Figure 1.18). In this method, a completely wetted platinum plate is brought into contact with a liquid surface, and a pull force is applied to the plate. Equilibrium is achieved when that force, corrected by the buoyancy force acting on the immersed part of the plate, is balanced by the surface tension, that is, F + dbHpg = 2(d + b)a. The force F is measured with a sensitive dynamometer, which typically forms the core of modem surface tension meters. 

The first major paper on mold release was published in 1946 by Ziegler. It contained a thorough analysis of the influence of mold material on release properties. Even more important, the paper contained a proposal for a new method of mold release testing, which gave the fundamental tool for rational analysis of the problem. More than twenty of the so-called mold lubricants were tested by the newly developed pull-out force method. In addition, the effect of lubricant on product clarity was observed - both results being considered in the final selection. In the best cases, the decrease of up to 80% of pull-out force was obtained with these tested lubricants . Plasticizers and pigments were also found to facilitate mold release properties in combination with mold release agents. 

The majority of HDD bore paths consist of a series of straight sections and bends. To estimate the total pull forces, the above equations must be applied to each straight section and bend, and totaled. The following is a method developed by Larry Slavin for estimating the loads along a bore path with no horizontal bends. This method is an estimate only. The designer or engineer must determine their suitability for any application... 

The carried out investigation is an attempt to combine the speckle photography with the pull-out method applied to the SFRC, From the obtained results the distribution of the fibre strain and stress as well as the effective fibre-matrix bond distribution at each level of loading have been determined along the fibre. To confirm the high accuracy of the results the compliance between values of pull-out load measured by means of a load cell and from force distribution in the fibre along its length (Fig.7) is presented in Fig.12. 

Du Nouy ring Traditional method for the measurement of surface and interfacial tension measurement. Maximum pulling force on a ring by the surface is measured. Wetting properties of liquids have no influence on this technique. 

Methods for determining fiber-to-fiber friction have been developed (29—31). The friction coefficient can also be measured in terms of the force required to pull entwined fibers apart (32—34) or the force necessary to remove a single fiber from a mass of fibers under pressure (35). Another test involves an apparatus wherein one or a series of parallel fibers are mounted across a small bridge similar to a violin bridge. This is then pressed against a surface that may be another fiber or some other material, and the fibers alternately sHp and stick as they sHde across each other (36,37). 

Standard Test Method for Adhesion Between Steel Tire Cords and Rubber. Steel cords are vulcanised into a block of mbber and the force necessary to pull the cords linearly out of the mbber is measured as adhesive force. ASTM method D2229-93a can be used for evaluating mbber compound performance with respect to adhesion to steel cord. The property measured by this test method indicates whether the adhesion of the steel cord to the mbber is greater than the cohesion of the mbber, ie, complete mbber coverage of the steel cord or less than the cohesion of mbber (lack of mbber coverage). 

In this method [89], a single fiber is taken and partially embedded in a drop of uncured resin placed on a holder. The resin is then cured with the fiber held upright. The holder, with resin and fiber, is held in a grip attached to the crosshead and then pulled out from the resin. The force pulling the fiber out of the resin is balanced by shear stress at the resin-fiber interface holding the fiber in place. The maximum shear stress occurs as the embedded length tends to zero and is given by ... 

One point, which is often disregarded when nsing AFM, is that accurate cantilever stiffness calibration is essential, in order to calculate accurate pull-off forces from measured displacements. Althongh many researchers take values quoted by cantilever manufacturers, which are usually calculated from approximate dimensions, more accurate methods include direct measurement with known springs [31], thermal resonant frequency curve fitting [32], temporary addition of known masses [33], and finite element analysis [34]. 

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