Transient area changes

When the shape of a particle oscillates, the surface area changes with time. This situation has been modeled by neglecting the motion adjacent to the surface due to the terminal velocity of the particle, i.e., by considering the particle to be oscillating but stationary, with material transferred by transient molecular diffusion over a time equal to the period of oscillation. For Sc 1 the thin concentration boundary layer assumptions are invoked (see Chapter 1). 

There is an extra oscillation module, based on direct measurements of the capillary pressure, which operates from 1 to 150 Hz. There is also an additional accessory for the PAT1 for low-frequency oscillations. The range of surface and interfacial tension is 1 to 1000 mN/m with a resolution of 0.1 mN/m. The instrument allows for transient relaxation measurements, using perturbations such as ramp, square pulse, or trapezoidal area changes. 

The kinetics of the adsorption process taking place at the surface of a growing drop or bubble is important for the interpretation of data from drop volume or maximum bubble pressure experiments. The same problem has to be solved in any other experiment based on growing drops or bubbles, such as bubble and drop pressure measurements with continuous, harmonic or transient area changes (for example Passerone et al. 1991, Liggieri et al. 1991, Horozov et al. 1993, Miller at al. 1993, MacLeod Radke 1993, Ravera et al. 1993, Nagarajan Wasan 1993). 

The two types of relaxations methods, based on harmonic and transient area changes respectively, will be described in this paragraph first from the theoretical and then from the experimental point of view. It will be shown that the exchange of matter functions are generally applicable to both types of relaxations, as pointed out by Loglio et al. (1991b). Finally, experimental details of several methods and examples of results from literature, for surfactants as well as biopolymers, will be discussed. 

Fig. 6.1. Schematic of different transient area changes (solid lines) and the corresponding interfacial tension responses (dotted lines) (a) - step type, (b) - ramp type, (c) - square pulse, (d) -trapezoidal change...

It was shown by Loglio et al. (1991a) that the most useful disturbance for interfacial relaxation experiments is the trapezoidal area change. For time regimes realised in most of the transient relaxation experiments the trapezoidal area change can be approximated adequately by a square pulse. For the square pulse area change we obtain ... 

The software driven apparatus allows different types of area changes step and ramp type, square pulse and trapezoidal as well as sinusoidal area deformations. The construction ensures that area changes are almost isotropic. Area changes used in transient and harmonic relaxation experiments are of the order of 1 to 5%. The surface tension response measured via the Wilhelmy balance has an accuracy of better than 0.1 mN/m. 

A recently developed modification of the pendent drop method gives definite area changes of the drop surface, which can be used to initiate transient relaxation processes (Miller et al. 1993a, b). A metering system consisting of two syringes (cf Fig. 6.7) is used to form a drop... 

The whole theoretical treatment of the derivation of interfacial response functions was discussed recently by Miller et al. [160]. It was shown by Loglio et al. [144, 161, 162] that exchange of matter functions derived for harmonic disturbances can be applied to transient ones. As the result for a diffusion-controlled exchange of matter, using the theory of Lucassen and van den Tempel [157], the following functions result for a trapezoidal area change [162]... 

Dilational rheological experiments are based on area changes by keeping the shape of the interface constant. Models for the exchange of matter, which sets in after a compression or expansion of the interface, are generally applicable to both harmonic and transient types of relaxations (178). Stress-relaxation experiments may yield results different from those obtained from measurements on small disturbances as the composition of the surface layer can vary (179). Overviews on experimental and theoretical aspects of dilational rheology were given recently in Refs 180—182. 

Here (0 is the oscillation frequency, and the parameter cOb is the characteristic frequency, which is inverse proportinal to the diffusion relaxation time Xd given in Eq. (35). This characteristic frequency exists also for any transient relaxation processes. The interfacial response functions for a number of transient relaxations were discussed recently by Loglio et al. (2001). Among these, the trapezoidal area change is the most general perturbation which contains area changes such as the step or ramp type and the square pulse as particular cases. 

The interfacial tension response to transient and harmonic area perturbations yields the dilational rheological parameters of the interfacial layer dilational elasticity and exchange of matter function. The data interpretation with the diffusion-controlled adsorption mechanism based on various adsorption isotherms is demonstrated by a number of experiments, obtained for model surfactants and proteins and also technical surfactants. The application of the Fourier transformation is demonstrated for the analysis of harmonic area changes. The experiments shown are performed at the water/air and water/oil interface and underline the large capacity of the tensiometer. 

Although no direct comparison with other commercial products are given we can state that the instrument PATl discussed here has the best features in respect to interfacial rheology studies. It provides a comfortable function generator for any type of transient and harmonic relaxation studies and also the theoretical tools to analyse trapezoidal and sinusoidal relaxation experiments. The on-line control of the interfacial area changes is very accurate and the oscillations performed in the range between 0.01 and 0.2 Hz are ideally smooth sinusoidal functions in contrast to experiments performed with other instruments. 

Electroporation. When bacteria are exposed to an electric field a number of physical and biochemical changes occur. The bacterial membrane becomes polarized at low electric field. When the membrane potential reaches a critical value of 200—300 mV, areas of reversible local disorganization and transient breakdown occur resulting in a permeable membrane. This results in both molecular influx and efflux. The nature of the membrane disturbance is not clearly understood but bacteria, yeast, and fungi are capable of DNA uptake (see Yeasts). This method, called electroporation, has been used to transform a variety of bacterial and yeast strains that are recalcitrant to other methods (2). Apparatus for electroporation is commercially available, and constant improvements in the design are being made. 

The analysis of steady-state and transient reactor behavior requires the calculation of reaction rates of neutrons with various materials. If the number density of neutrons at a point is n and their characteristic speed is v, a flux effective area of a nucleus as a cross section O, and a target atom number density N, a macroscopic cross section E = Na can be defined, and the reaction rate per unit volume is R = 0S. This relation may be appHed to the processes of neutron scattering, absorption, and fission in balance equations lea ding to predictions of or to the determination of flux distribution. The consumption of nuclear fuels is governed by time-dependent differential equations analogous to those of Bateman for radioactive decay chains. The rate of change in number of atoms N owing to absorption is as follows ... 

A potential pitfall with stop-time experiments comes with temporal instability of responses. When a steady-state sustained response is observed with time, then a linear portion of the production of reporter can be found (see Figure 5.15b). However, if there is desensitization or any other process that makes the temporal responsiveness of the system change the area under the curve will not assume the linear character seen with sustained equilibrium reactions. For example, Figure 5.16 shows a case where the production of cyclic AMP with time is transient. Under these circumstances, the area under the curve does not assume linearity. Moreover, if the desensitization is linked to the strength of signal (i.e., becomes more prominent at higher stimulations) the dose-response relationship may be lost. Figure 5.16 shows a stop-time reaction dose-response curve to a temporally stable system and a temporally unstable system where the desensitization is linked to the... 

In this paper we report the effect of varying loads on a small size DMFC stack (10 cells with 9 cm active-area each). The transient responses of the stack voltage have been investigated upon variable current load conditions to obtain the information on the dynamic characteristics of the stack. Also, the transient responses of the stack current upon changing fuel flow rates have been monitored to obtain the optimal operating conditions for the staek. 

In general, intramolecular isomerization in coordinatively unsaturated species would be expected to occur much faster than bimolecular processes. Some isomerizations, like those occurring with W(CO)4CS (47) are anticipated to be very fast, because they are associated with electronic relaxation. Assuming reasonable values for activation energies and A-factors, one predicts that, in solution, many isomerizations will have half-lives at room temperature in the range 10 7 to 10 6 seconds. The principal means of identifying transients in uv-visible flash photolysis is decay kinetics and their variation with reaction conditions. Such identification will be difficult if not impossible with unimolecular isomerization, particularly since uv-visible absorptions are not very sensitive to structural changes (see Section I,B). These restrictions do not apply to time-resolved IR measurements, which should have wide applications in this area. 

In the present communication we report on the effects of voltage, temperature and electrode surface area on the transient and steady-state behavior of the system. The change in the rate of C2Hi 0 production can exceed the rate of 0 pumping by a factor of 400 and is proportional to the anodic overvoltage both at steady state and during transients. 

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