Potentials liquid film driving

The liquid film has a varying thickness and is alternately exposed to the gas and to the liquid with different concentrations. However, the film damps the effect of varying concentration, and the concentration at the wall is almost constant. The time constant for diffusion in the liquid film is bj/2D = 0.1 sec. (Eq. 32), and the contact time for the gas bubble and the liquid slug is 0.02 sec. Thus the wall concentration will be almost constant, and the mass transfers directly from the gas bubble and through the liquid slug can be added using the same driving potential. 

An interesting idea has been to prepare the photosensitive electrode on site having the liquid play the dual role of a medium for anodic film growth on a metal electrode and a potential-determining redox electrolyte in the electrochemical solar cell. Such integration of the preparation process with PEC realization was demonstrated initially by Miller and Heller [86], who showed that photosensitive sulfide layers could be grown on bismuth and cadmium electrodes in solutions of sodium polysulfide and then used in situ as photoanodes driving the... 

The film (individual) coefficients of mass transfer can be defined similarly to the film coefficient of heat transfer. A few different driving potentials are used today to define the film coefficients of mass transfer. Some investigators use the mole fraction or molar ratio, but often the concentration difference AC (kg or kmol m ) is used to define the liquid phase coefficient (m while the partial pressure difference A/i (atm) is used to define the gas film coefficient (kmolh m 2 atm ). However, using and A gp of different dimensions is not very convenient. In this book, except for Chapter 15, we shall use the gas phase coefficient (m h" ) and the liquid phase coefficient ki (m h ), both of which are based on the molar concentration difference AC (kmol m ). With such practice, the mass transfer coefficients for both phases have the same simple dimension (L T" ). Conversion between k and is easy, as can be seen from Example 2.4. 

In the experiment on the helium film, the exposed surfaces of the beaker are in equilibrium with the saturated vapour of the liquid of the bath and will, therefore, be covered by an adsorbed layer of helium. At temperatures below the superfluid is able to flow in the adsorbed layer or film which, therefore, acts as a sort of syphon. The driving force for the motion is the difference in gravitational potential energy of the liquid in the beaker and in the bath, which leads to a difference in the specific Helmholtz functions. 

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