Surface tension dynamic

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. 

Stretched surface - high dynamic surface tension 

FIGURE 8.2. Dynamic smface tension in pure liquids (a) for a liquid of isotropic molecular shape, dynamic surface tension effects are controlled by the rate of diffusion of molecules from the bulk to the new surface (b) in polar or anisotropic liquids, the situation may be further complicated by the question of molecular orientation at the surface. 

Qualitatively, it is assumed that the time required to attain equilibrium after formation of new surface is related to the time for diffusion of liquid molecules to the surface—that is, to the self-diffusion constant. Diffusion times are usually on the order of 10 cm s which translates to times of milliseconds for the attainment of equilibrium. The accurate measurement of surface tensions over such short time frames is difficult at best, so that a great deal is still in question concerning the thermodynamics and kinetics of such fresh surfaces. 

An additional comphcation in evaluating dynamic surface tensions may arise in terms of molecular orientation at the surface. For a symmetric molecule, orientation will not be a problem however, for many systems, especially 

---

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]. 

Hsu and Berger [43] used the maximum bubble pressure method (MBP) to study the dynamic surface tension and surface dilational viscosity of various surfactants including AOS and have correlated their findings to time-related applications such as penetration and wetting. A recent discussion of the MBP method is given by Henderson et al. [44 and references cited therein]. 

As can be seen in Table 6.5, ONB in APG solution of concentration C = 100 ppm took place at significantly higher surface temperatures. It should be noted that the ONB in surfactant solutions may not be solely associated with static surface tension Sher and Hetsroni (2002). Other parameters such as heat flux, mass flux, kind of surfactant, surface materials, surface treatments, surface roughness, dynamic surface tension and contact angle need to be considered as well. 

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. 

Cross, B., Scher, H.B., Eds. "Pesticide Formulations" Chapter 13. Berger, P., et al. "Dynamic Surface Tensions of Spray Tank Adjuvants," ACS 1988, Washington, DC. 

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. 

The few examples of deliberate investigation of dynamic processes as reflected by compression/expansion hysteresis have involved monolayers of fatty acids (Munden and Swarbrick, 1973 Munden et al., 1969), lecithins (Bienkowski and Skolnick, 1974 Cook and Webb, 1966), polymer films (Townsend and Buck, 1988) and monolayers of fatty acids and their sodium sulfate salts on aqueous subphases of alkanolamines (Rosano et al., 1971). A few of these studies determined the amount of hysteresis as a function of the rate of compression and expansion. However, no quantitative analysis of the results was attempted. Historically, dynamic surface tension has been used to study the dynamic response of lung phosphatidylcholine surfactant monolayers to a sinusoidal compression/expansion rate in order to mimic the mechanical contraction and expansion of the lungs. 

Until very recently, there has been little or no experimental protocol for obtaining quantitative dynamic surface tension data on monolayer films. In most cases, the experimental set-up has consisted of a simple Langmuir film balance equipped with a variable-speed motor to drive the moving barrier. Hysteresis data were then obtained at a number of compression/expansion rates and compared qualitatively. This experimental set-up was improved considerably by Johnson (Arnett et al., 1988a), who modified a special... 

Fig. 9 Schematic of the modified dynamic surface tension apparatus.

The dynamic surface tension properties of the heterochiral and homochiral DPPC films are also independent of stereochemistry. Figure 14 shows the hysteresis loops of five successive compression/expansion cycles obtained on the modified Cahn DST apparatus. Although all five compression/expansion cycles of the DPPC films are not coincident, the areas and shapes of the... 

Shorter chain analogs of DPPC were also investigated in order to determine if the lack of stereo-differentiation in monolayer properties could be due to DPPC s higher gel point or complicating steric effects. Figure 15 shows the compression/expansion isotherms of DPPC as compared with racemic and enantiomeric dimyristoylphosphatidyl choline (DMPC) and dilauroyl phosphatidyl choline (DLPC). Again no stereodifferentiation in monolayer properties was observed as reflected by 11/A isotherms or dynamic surface tension. 

Enantiomeric discrimination and its relation to film component reorganization upon compression can also be observed in dynamic surface tension hysteresis loops. Figure 26 shows the WjA isotherms generated upon five successive compression/expansion cycles (from II = 0 to lOdyncm-1) of racemic and enantiomeric films containing 17 mole percent palmitic acid. The hysteresis loops, obtained on the apparatus described in Section 2 (p. 63), show that the first compression/expansion cycle of the racemic system is repeated in each successive cycle. Upon expansion of the film from the maximum surface pressure back to Odyncm-1, the racemic film returns to its original state without detectable reorganization of the components. However, the... 

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. 

On-line Determination of Dynamic Surface Tension by the Bubble-pressure Method... 

After the accomplishment of the above mentioned experiment on the nonlinear viscoelasticity of the DPPE thin film, we have tried to construct a new instrument for the measurement of the dynamic surface tension. We have noticed that, the blades used to change the surface area in the commercial instrument, did not show genuine triangle or sinusoidal trajectory but rather mathematically undefined. With our newly designed instrument, the time change of the surface area can be controlled according to a chosen function with the aid of a micro-computer. 

Figure 16 shows the experimental arrangement for the measurement of the surface pressure. The trough (200 mm long, 50 mm wide and 10 mm deep) was coated with Teflon. The subphase temperature was controlled within 0.1 C by means of a jacket connected to a thermostated water circulator, and the environmental air temperature was kept at 18 °C. The surface tension was measured with a Wilhelmy plate of platinum(24.5 x 10.0 x 0.15 mm). The surface pressure monitored by an electronic balance was successively stored in a micro- computer, and then Fourier transformed to a frequency domain. The surface area was changed successively in a sinusoidal manner, between 37.5 A2/molecule and 62.5 A2/molecule. We have chosen an unsaturated phospholipid(l,2-dioleoyl-3-sn-phosphatidyI-choline DOPC) as a curious sample to measure the dynamic surface tension with this novel instrument, as the unsaturated lipids play an important role in biomembranes and, moreover, such a "fluid" lipid was expected to exhibit marked dynamic, nonlinear characteristics. The spreading solution was 0.133 mM chloroform solution of DOPC. The chloroform was purified with three consecutive distillations. 

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 ... 

Figure 4. Automatic dynamic surface tension balance showing Wilhelmy plate suspended in surface. Photograph supplied by Cahn Instruments, Inc.

THE. 12. R. Defay et I. Prigogine, Tension superficielle dynamique d une surface parfaite (Dynamical surface tension of a perfect surface), Bull. Cl. Sci. Acad. Roy. Belg. 32, 400-421 (1946). 

THE. 13. R. Defay et I. Prigogine, Theorie thermodynamique de la tension superficielle dynamique (Thermodynamic theory of dynamic surface tension), J. Chim. Phys. Physico-chim. Biol. 43, 217-234... 

SOL.5. 1. Prigogine et R. Defay, Tension superficieUe dynamique des solutions regulieres (Dynamic surface tension of regular solutions), J. Chim. Phys. 46, 367-372 (1949). 

The method has been employed by Dorsey Phys. Rev. v. 213, 1897) and Griinmach Ann. derPhysik, xxxviil. 1018,1912) for the determination of the surface tensions of a number of salt solutions. It will be noted that a is dependent on the cube of the wave length, a factor militating against the general adoption of this method for the accurate determination of the surface tension. Again it is a difficult matter to decide how far such a method really yields a true value for the dynamic surface tension. 

More closely approaching the conditions desirable for determining the true dynamic surface tension is the vibrating jet method. 

In the case of soluble substances of low capillary activity the evaluation of = dimensional pressure must be regarded as the difference in surface tension between that of the liquid without the Gibbs film and one in which the Gibbs film is established only as a first approximation may the ideal dynamic surface tension of a solution, i.e. of a solution without a Gibbs film, be regarded as equal to the surface tension of the solvent. 

Anderson NH, HaU DJ, Wastern NH (1983) The role of dynamic surface tension in spray droplet retention. Proc 10th Int Congr Plant Protect 2 576-581... 

Raney K, Benton W, Miller CA (1987) Optimum detergency conditions with nonionic siufactants II. Effect of hydrophobic additives. J Colloid Interface Sci 119 539-549 Rosen MJ, Wu Y (2001) Superspreading of trisiloxane surfactant mixtures on hydrophobic siufaces 1. Interfacial adsorption of aqueous trisiloxane surfactant -M-alkyl pyrrolidinone mixtures on polyethylene. Langmuir 17 7296-7305 Stevens PJG, Kimberely MO, Mimphy DS, Policello GA (1993) Adhesion of spray droplets to foliage - the role of dynamic surface tension and advantages of organosil-icone surfactants. Pesticide Sci 38 237-245... 

A Low-Cost Dynamic Surface Tension Meter with a LabVIEW Interface and Its Usefulness in Understanding Foam Formation 102... 

---