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AWS Viscometry – Principles and Applications
Wein, Ondřej
Quantitative identification of Apparent wall slip (AWS) effect, accompanying the flow of colloidal dispersions in confined geometries, requires a specific modification of common viscometric methods. Essential distinction between common and AWS viscometry is outlined – including theoretical background, calibration, data acquisition, and downstream data treatment – and demonstrated for a class of rotational viscometers.
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Voltage-step Transient in Redox Systems III. Effect of the Current Primary Distribution via Nernst Model
Wein, Ondřej
In addition to the common transient diffusion (Cottrell), there are further transport results that cannot be neglected: Faradic resistance at surface of the working electrode, and Ohmic resistance in the bulk of streaming electrolyte solution between the working and counter electrodes. These additional resistances are analyzed here for the voltage-step transient (VST) process using the Nernst model of a two-electrode cell (no reference electrode). The Faradic resistance is considered for a single redox couple O + ne- = R according to Butler-Volmer electrode kinetics. The effect of Ohmic and Faradic resistances in VST is accounted for using two models: the recently updated 1D approximation and a new model that accepts primary current distribution for a circular electrode. It is shown that the 1D approximation introduces unacceptable errors.
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Voltage-step Transient in Redox Systems II. 1D Approximation (revisited)
Wein, Ondřej
Using the well-known Cottrell asymptote, voltage-step transient experiment could provide data on bulk concentration and diffusion coefficient of working depolarizer, essential in the liming-current techniques. However, the Cottrell asymptote is not directly applicable in an early stage of the voltage-step transient process,. There are three additional transport resistances that cannot be neglected at extremely high initial currents: Faradaic resistance at surface of working electrode, Ohmic losses in its intimate neighborhood, and the galvanometric constraint in the outer circuit (galvanometer, current follower, potentiostat). Non-linear effect of these additional transport resistances on the transient current is calculated in a 1D approximation, i.e. assuming uniform accessibility of the working electrode, for finite voltage steps.
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Effect of Surface Quality in Viscometry of Aqueous Polymer Solutions
Večeř, M. ; Wein, Ondřej
The proposed paper is focused on experimental viscometric study of AWS effect of aqueous polymer solutions: 1.5% wt solution of Na salt of Carboxymethylcellulose, 0.4%wt of Welan Gum and 1% wt of Polyacrylamide was chosen as the tested liquids. Four viscometric sensors (type KK) with different quality of surface (Stainless steel, Titanium, Alloy of Aluminum and Magnesium, as well as Stainless steel with surface roughened by sandblasting) have been used. It is documented that the extent of AWS effect strongly depends both on the sensor material and type of solution.
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Edge Effects in Voltage-step Transient I. Ohmic Losses in 1D Approximation (revisited)
Wein, Ondřej
Ohmic potential drop between the working and reference electrodes plays an important role in the early stage of the voltage-step transient process. Non-linear effect of the ohmic drop on the transient currents has been calculated for large voltage steps, assuming reversible behavior (negligible surface overpotential) of a redox couple O + ne = R. This report extends an older study by Wein (1991). In particular, it adds a series of analytic approximations and gives more accurate numerical solving.
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