Surface tension time dependence

There are a number of techniques available to measure the surface or interfacial tension of liquid systems, which together cover a wide range of time. In many cases, several methods are required in order to receive the complete surface tension time dependence of a surfactant system. One of the important points in this respect is that the data obtained from different experimental techniques have to be recalculated such that a common time scale results, i.e. one has to calculate the effective surface age from the experimental time, which is typically determined by the condition of the methods. For example, the maximum bubble pressure... 

A combined apparatus for measuring surface tension by the capillary tube method, viscosity, and density, seems handy for work of moderate accuracy carried out by experimenters in a hurry. In measuring the surface tensions of liquid mixtures or solutions, evaporation must be completely prevented, otherwise the surface tension will depend on time. ... 

The decrease in the surface tension at constant adsorption, occurring in agreement with the Gibbs equation, is solely due to the increase in chemical potential of the adsorbed substance caused by the increased concentration of the latter in solution. As is commonly known, the increase in the chemical potential in a stable two-component system always corresponds to the concentration increase. For the present case it translates into the increase of surface concentration, and consequently, of the adsorption. Therefore, in the concentration region where the surface tension linearly depends on the log of concentration, a slow but finite, increase in adsorption not detected experimentally should occur. At the same time a sharp increase in the chemical potential of the surfactant molecules in the adsorption layer... 

We can summarise that the problem of diffusion-controlled adsorption kinetics of reorientable surfactant molecules is formulated and solved even for the case when the reorientation process within the surface layer requires some time. It was shown that this non-instantaneous reorientation can result in either acceleration or deceleration of the surface tension decrease, depending on the adsorption characteristics of the different molecular states, and on the actual surface lifetime [227] faster (for medium n values) or slower (large n values) decrease of y is caused by an oversaturation of the surface layer by the state possessing maximum molar area. 

For example, the solid can swell in contact with a certain liquid or even interact by chemical interfacial reactions it can also be partially dissolved. In the case of polymer surfaces, the molecular reorientation in the surface region under the influence of the liquid phase is assumed to be a major cause of hysteresis. This reorientation or restructuring is thermodynamically favoured at the polymer-air interface, the polar groups are buried away from the air phase, thus causing a lower solid-vapour interfacial tension. In contact with a sessile water drop, the polar groups turn over to achieve a lower solid-liquid interfacial tension. Time-dependent changes in contact angles can also be observed (33). 

The time required for surface tension reduction depends on diffusion processes involved in surfactant adsorption. Kinetic models for surfactant adsorption divide the adsorption process into two steps [67]. The first step is the transport of the surfactant to the subsurface, driven by a concentration gradient or hydrody-... 

Fig. 11-13. Apparatus for measuring the time dependence of interfacial tension (from Ref. 34). The air and aspirator connections allow for establishing the desired level of ftesh surface. IV denotes the Wilhelmy slide, suspended from a Cahn electrobalance with a recorder output.

In a foam where the films ate iaterconnected the related time-dependent Marangoni effect is mote relevant. A similar restoring force to expansion results because of transient decreases ia surface concentration (iacteases ia surface tension) caused by the finite rate of surfactant adsorption at the surface. 

The exponential dependencies in Eq. (14-195) represent averages of values reported by a number of studies with particular weight given to Lefebvre [Atomization and Sprays, Hemisphere, New York, (1989)]. Since viscosity can vary over a much broader range than surface tension, it has much more leverage on drop size. For example, it is common to find an oil with 1000 times the viscosity of water, while most liquids fall within a factor of 3 of its surface tension. Liquid density is generally even closer to that of water, and since the data are not clear mat a liquid density correction is needed, none is shown in Eq. 

A is the area of the surface. In a foam, where the surfaces are interconnected, the time-dependent Marangoni effect is important. A restoring force corresponding to the Gibbs elasticity will appear, because only a finite rate of absorption of the surface-active agent, which decreases the surface tension, can take place on the expansion and contraction of a foam. Thus the Marangoni effect is a kinetic effect. 

The dynamic behavior of fluid interfaces is usually described in terms of surface rheology. Monolayer-covered interfaces may display dramatically different rheological behavior from that of the clean liquid interface. These time-dependent properties vary with the extent of intermolecular association within the monolayer at a given thermodynamic state, which in turn may be related directly to molecular size, shape, and charge (Manheimer and Schechter, 1970). Two of these time-dependent rheological properties are discussed here surface shear viscosity and dynamic surface tension. 

The dynamic surface tension of a monolayer may be defined as the response of a film in an initial state of static quasi-equilibrium to a sudden change in surface area. If the area of the film-covered interface is altered at a rapid rate, the monolayer may not readjust to its original conformation quickly enough to maintain the quasi-equilibrium surface pressure. It is for this reason that properly reported II/A isotherms for most monolayers are repeated at several compression/expansion rates. The reasons for this lag in equilibration time are complex combinations of shear and dilational viscosities, elasticity, and isothermal compressibility (Manheimer and Schechter, 1970 Margoni, 1871 Lucassen-Reynders et al., 1974). Furthermore, consideration of dynamic surface tension in insoluble monolayers assumes that the monolayer is indeed insoluble and stable throughout the perturbation if not, a myriad of contributions from monolayer collapse to monomer dissolution may complicate the situation further. Although theoretical models of dynamic surface tension effects have been presented, there have been very few attempts at experimental investigation of these time-dependent phenomena in spread monolayer films. 

The difference between the static or equilibrium and dynamic surface tension is often observed in the compression/expansion hysteresis present in most monolayer Yl/A isotherms (Fig. 8). In such cases, the compression isotherm is not coincident with the expansion one. For an insoluble monolayer, hysteresis may result from very rapid compression, collapse of the film to a surfactant bulk phase during compression, or compression of the film through a first or second order monolayer phase transition. In addition, any combination of these effects may be responsible for the observed hysteresis. Perhaps understandably, there has been no firm quantitative model for time-dependent relaxation effects in monolayers. However, if the basic monolayer properties such as ESP, stability limit, and composition are known, a qualitative description of the dynamic surface tension, or hysteresis, may be obtained. 

Droplet Formation in Water Atomization. In water atomization of melts, liquid metal stream may be shattered by impact of water droplets, rather than by shear mechanism. When water droplets at high velocities strike the liquid metal stream, some liquid metal fragments are knocked out by the exploding steam packets originated from the water droplets and subsequently contract into spheroidal droplets under the effect of surface tension if spheroidization time is less than solidification time. It is assumed that each water droplet may be able to knock out one or more metal droplet. However, the actual number of metal droplets produced by each water droplet may vary, depending on operation conditions, material properties, and atomizer designs. 

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