Liquids dynamic surface tension measurement

Howell, E. 2001. Dynamic surface tension measurements of liquid solder using oscillating jets of elliptical cross section. Mechanical and Industrial Engineering. University of Illinois at Chicago, Chicago, IL. pp. 75. 

Experimental results for solutions of other Tritons have been reported in Ref 113. From these studies it was concluded that the distribution coefficient for Triton X-45 is significantly higher than for Triton X-405. To visualize the significant differences in dynamic surface tensions measured for the three cases discussed above, the results of experiments with Triton X-45 are reported in Fig. 9. It is obvious that for case 3 the adsorption process is flie fastest as adsorption takes place from both adjacent liquid phases. 

Table 2.2 Dynamic Surface Tension Measurement Methods for Liquids ...

One way to circumvent the problem raised above is to measure the dynamic surface tension, instead of the equilibrium surface tension (described so far). The dynamic surface tension is the surface tension measured at short times after a surface has been formed and hence it is a non-equilibrium property. If an imaginary cut is made through a liquid and the molecules are not allowed to relax into an equilibrium state, the surface tension at the cut section will be the arithmetic average of the surface tensions of the present components. After equilibrium is reached, however, the most surface-active species, the one with the lowest surface tension, will be found in excess at the liquid/air surface. This relaxation of the surface tension, from an arithmetic mean to an equilibrium surface tension, is called the dynamic surface tension. If the surface tension is measured at short times after a surface has been formed, it reflects the surfactant bulk concentration, without the preferential adsorption of the more surface-active species which are present in small amounts. Hence, dynamic surface tension measurements are preferred for determining... 

A recent design of the maximum bubble pressure instrument for measurement of dynamic surface tension allows resolution in the millisecond time frame [119, 120]. This was accomplished by increasing the system volume relative to that of the bubble and by using electric and acoustic sensors to track the bubble formation frequency. Miller and co-workers also assessed the hydrodynamic effects arising at short bubble formation times with experiments on very viscous liquids [121]. They proposed a correction procedure to improve reliability at short times. This technique is applicable to the study of surfactant and polymer adsorption from solution [101, 120]. 

Dynamic surface tension has also been measured by quasielastic light scattering (QELS) from interfacial capillary waves [30]. It was shown that QELS gives the same result for the surface tension as the traditional Wilhelmy plate method down to the molecular area of 70 A. QELS has recently utilized in the study of adsorption dynamics of phospholipids on water-1,2-DCE, water-nitrobenzene and water-tetrachloromethane interfaces [31]. This technique is still in its infancy in liquid-liquid systems and its true power is to be shown in the near future. 

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 ... 

Unlike in three dimensions, where liquids are often considered incompressible, a surfactant monolayer can be expanded or compressed over a wide area range. Thus, the dynamic surface tension experienced during a rate-dependent surface expansion, is the result of the surface dilational viscosity, the surface shear viscosity, and elastic forces. Often, the contributions of shear and/or the dilational viscosities are neglected during stress measurements of surface expansions. Isolating interfacial viscosity effects is difficult because, since the interface is connected to the substrate on either side of it, the interfacial viscosity is coupled to the two bulk viscosities. 

A number of methods are available for the measurement of surface and interfacial tension of liquid systems. Surface tension of liquids is determined by static and dynamic surface tension methods. Static surface tension characterises the surface tension of the liquid in equilibrium and the commonly used measurement methods are Du Notiy ring, Wilhelmy plate, spinning drop and pendant drop. Dynamic surface tension determines the surface tension as a function of time and the bubble pressure method is the most common method used for its determination. 

The dynamic methods depend on the fact that certain vibrations of a liquid cause periodic extensions and contractions of its surface, which are resisted or assisted by the surface tension. Surface tension therefore forms an important part, or the whole, of the restoring force which is concerned in these vibrations, and may be calculated from observations of their periodicity. Dynamic methods include determination of the wave-length of ripples, of the oscillations of jets issuing from non-circular orifices, and of the oscillations of hanging drops. Dynamic methods may measure a different quantity from the static methods, in the case of solutions, as the surface is constantly being renewed in some of these methods, and may not be old enough for adsorption to have reached equilibrium. In the formation of ripples there is so little interchange of material between the surface and interior, and so little renewal of the surface, that the surface tension measured is the static tension ( 12. ... 

The most frequently used parameter to characterize the dynamic properties of liquid adsorption layers is the dynamic surface tension (a time-dependent quantity). Various techniques are available to measure Y y as a function of time (which ranges from a fraction of a millisecond to minutes and hours or even days). 

Sohl C, Miyano K, Ketterson J (1965) Novel technique for dynamic surface tension and viscosity measurements at liquid-gas interfaces. Rev Sci Instrum 49 1464-1469... 

Dynamic properties of interfaces have attracted attention for many years because they help in understanding the behaviour of polymer, surfactant or mixed adsorption layers.6 In particular, interfacial rheology (dilational properties) is crucial for many technological processes (emulsions, flotation, foaming, etc).1 The present work deals with the adsorption of MeC at the air-water interface. Because of its amphiphilic character MeC is able to adsorb at the liquid interface thus lowering the surface tension. Our aim is to quantify how surface active this polymer is, and to determine the rheological properties of the layer. A qualitative and quantitative evaluation of the adsorption process and the dilata-tional surface properties have been realised by dynamic interface tension measurements using a drop tensiometer and an axisymmetric drop shape analysis. 

Steady State Expansion Measurements. The dynamic surface tension in steady state expansion was determined using a modified Langmuir trough equipped with six barriers fixed to an endless belt which were moved caterpillar-wise one after another over the liquid surface.10,24 Surface tension was determined using the Wilhelmy plate technique. Measurements were performed going from the highest to the lowest expansion rate. 

Fainerman, V. B., Makievski, A. V., and Miller, R., The measurement of dynamic surface tensions of highly viscous liquids by the maximum bubble pressure method, Colloid Surf A, 75, 229-235 (1993). 

For a pure liquid in eqmlibrium with its vapor, the number density and orientation of molecules at the surface will be different from that of bulk molecules (Fig. 8.2). When new surface is created, it is reasonable to assume that a finite amount of time will be required for new molecules to diffuse to the surface and to return the system to equilibrium. In that interim, as short as it may be, the measured surface tension of the system will be different from that of the system in equilibrium. The surface tension of such new surface is referred to as the dynamic surface tension. 

Surface tension measurement techniques are divided into methods for solids and liquids. There are two modes for measuring the surface tension of liquids static and dynamic. Values reported in the literature are often static surface tensions of liquids. Tables 2.1-2.3 present a brief description of the common techniques for surface tension measurement of liquid and solid materials. Some of these methods have been described in further detail. 

There is significant overlap in describing techniques for the measurement of equilibrium surface tension with those for dynamic surface tension, as discussed in the following chapter. Many of those dynamic surface tension methods can be used to measure equilibrium tension simply by performing the experiment over sufficiently long times. The time required for equilibration can range widely, from the practically instantaneous equilibration for pure liquids, to many hours or even days for dilute surfactant or polymer solutions. Thus, some of the dynamic techniques, particularly those that can only be used to study short times, such as the oscillating jet method, are not well suited for equilibrium measurements. 

---