Electrochemical shear probe

An electrochemical probe can be used to measure the shear forces at the surface of a membrane in the presence of bubbles. The system consists of a cathode that is mounted flush to the outside of a Teflon tube as a test fiber and a reference anode. The cathode is called the shear probe and is generally made of platinum wire (0.5 mm diameter) which protrudes onto a test fiber. The electrochemical reaction between cathode and anode is provided under the influence of a constant electric potential of 250 mV. The obtained signal in voltage drop is then conditioned through an amplifier and low-pass filter. The relationship between mass transfer at the probe and the limiting diffusion current is used to convert the voltage values into shear forces [77, 78]. 

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Electrochemical shear probe Measurements membrane surface shear rate in the presence of air sparging. Online measurement with high speed camera allows the observation of bubble shapes No visualization of fouling layer [77]... 

For measurement by electrochemical methods, probes have to be developed. For example, knowledge of the magnitude of the frictional forces between the solids and the gas-liquid mixture is very important for design of bioreactors. The growth of biomass on solid surfaces may be sensitive to shear stress. In fluidized bed bioreactors the suspended carriers are used for microbial immobilization or enzyme encapsulation. The knowledge of shear stress is important because some micro-organisms and cells attached to microcarriers are sensitive to excessive friction. The aim is to develop and verify a fast and inexpensive method for measuring the frictional forces in multiphase reactors beds. 

The electrochemical quartz crystal microbalance is a versatile technique for studying several aspects of electroactive polymer film dynamics. For rigid films, it is a sensitive probe of mobile species (ion and solvent) population changes within the film in response to redox switching. For non-rigid films, it can be used to determine film shear moduli. In the former case, one simply follows changes in crystal resonant frequency. In the latter case, the frequency dependence of resonator admittance in the... 

The Stanton tube is a rectangular-shaped pitot tube located very close to the boundary wall, and the mean velocity measured from this pitot tube pressure difference is directly related to the shear stress. The Preston tube is similar to the concept of the Stanton tube using a pitot static tube close to the surface, and the difference between the stagnation pressure at the center of the tube from the static pressure is related to the shear stress. The electrochemical or mass transfer probe is flush mounted with the wall, and the concentration at the wall element is maintained constant. The measurement of mass transfer rate between the fluid and the wall element is used for determination of the wall shear stress. One of the limitations of the mass transfer probe is that at very high flow rates, the mass transfer rate becomes large and it may not be possible to maintain the wall concentration constant. A detailed discussion on the above three techniques can be found in Hanratty and Campbell [1]. 

In addition, electrochemical sensors are difficult to place in-line mainly for hygienic problems the best way is to adapt them for on-line monitoring with the aid of a sampling probe (Figure 15.2) durable in design and construction to withstand the temperatures, pressures, viscosities, shear forces, and chemicals typical of the process under analysis. 

Yamada, H., Y. Ikuta, T. Koike, and T. Matsue, Eabrication of a shear force-based ion-selective capillary probe for scanning electrochemical microscopy, Chem. Lett., Vol. 37, 2008 pp. 392-393. 

There is synergy between efforts to monitor shear distribution using electrochemical probes and foulant monitoring, as poor hydrodynamics eventually lead to foulant build-up. In situ monitoring of fouling in industrial MBR systems still remains elusive however, a combination of small optical and shear sensing probes may provide long-term solutions. 

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