Direct observation through membrane

When yeast and latex particles were used as a model solution for DOTM experiments, it was found that the foulants were more likely to accumulate around existing depositions due to variations in the local pore size distribution [14]. The foulant deposition on the membrane could also be observed with direct visual observation (DVO), equipment similar to the DOTM setup, with the difference that the microscope focused on the feed side [64]. Both of these setups allowed the quantification of fouling surface coverage as a function of filtration time. Patchy foulant deposition was viewed after 5 min of filtration with 82% of surface covered with yeast [64] (see Chapter 2). 

The foulant movements for either particles or bacteria can be summarized into three consecutive behavioral movements [14, 30]  

Foulants moved along the membrane without stopping (rolling or sliding). 

Foulants then stopped on the membrane surface but most of them soon were moved away (momentary deposition). 

Due to their simpler physiochemical properties, the deposition rates of model particles, which were observed with a direct observation technique were much 

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A major advance vas made by Tony Fane and co-workers at the University of New South Wales when they developed a method to perform visual observation from the permeate side of the membrane. This technique, called direct observation through membrane (DOTM), was first described by Hodgson, PUlay and Fane [9] and then discussed more fully by Li et al. [10]. The key idea is to use a membrane which becomes transparent when wetted, and then observe partide deposition by placing a microscope on the permeate or back side of the membrane. Because of the relatively short focal length, compared to looking into the center of a membrane molecule from the side, high magnification may be used to observe individual particles as small as 1-10 pm in diameter. 

Li H, Fane AG, Coster HGL, and Vigneswaran S, An assessment of depolarisation models of crossflow microfiltration by direct observation through the membrane, J. Memb. Sci. 2000 172 135-147. 

Critical Coagulation Concentration Chromatographable Organic Carbon Chloral Hydrate Forming Potential Disinfection By-Product Diethylaminoethyl Diffusion Limited Aggregation Dissolved O anic Carbon Dissolved Organic Matter Direct Observation through the Membrane Technique Diffusive Reflectance Fourier Transform Infrared Spectroscopy Electronic Conductive Carbon Black Electron Dispersive Spectra Ethylene Diamine Tetra Acetic Acid Fulvic Acid... 

DOTM Direct observation through the membrane n Viscosity... 

Figure 2.3 Schematic of a crossflow microfiltration device employing direct observation through the membrane. (From Li et al. [10]).

A significant advance in optical characterization was made with the introduction of an approach now known as direct observation through the membrane (DOTM) [5, 6]. The microscope objective is positioned on the permeate side and focused through the permeate channel and the membrane onto the membrane surface on the feed side. Thus the membranes used must be transparent. Although this is a major limitation, significant findings have been made, as discussed elsewhere. 

For porous membranes, the selectivity of membranes is governed by the size of membrane pores and pore size distribution. There is no standard technique for determining the membrane pore size, and its distribution as every existing techniqne has its advantages and limitations. The commonly used techniques for membrane pore characterization involve the displacement or phase transition of a fluid filling the pores. In addition, membrane snrface pores can be directly observed through microscopic techniques. Instead of pore size and pore size distribution, it is also common to determine the solute rejection of membrane, which is sometimes more indicative of actual membrane performance. These commonly used techniques are briefly discussed as follows and summarized in Table 15.3. 

Figure 18.34. Components of the Proton-Conducting Unit of ATP Synthase. The c subunit consists of two a helices that span the membrane. An aspartic acid residue in the second helix lies on the center of the membrane. The structure of the a subunit has not yet been directly observed, but it appears to include two half-channels that allow protons to enter and pass partway but not completely through the membrane.

We have given perhaps undue attention to the mobile carrier mechanism because at one time it was assumed that the Na and K transport in excitable cell membranes occurred precisely via this mechanism. In 1965, Chandler and Meves undertook an experiment to assess the aforementioned specifics of the high-frequency conductance. A nerve fiber was placed in a solution containing no Na or K ions. This precluded direct current through the membrane. However, if there had been any mobile charged carriers in the membrane, the authors would have detected current on application of a variable field. The authors did not observe a detectable current under these conditions, from which it could be deduced that the transport systems of excitable membrane are structured as ion channels whose conductance is controlled by electric field. 

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