Capillary blockages

Capillary blockage occurs when the non-wetting phase is blocked at a pore throat by the interfacial tension (Y ) with the wetting phase (Fig. 11). 

Pulsing should also lead to better grouting practices and other benefits such as accelerated clean-up of potable water aquifers that are contaminated by NAPLs and other materials. In these cases, the suppression of advective instabilities and the aid to overcoming capillary blockages will lead to more efficient displacement, less fingering of flushing agents, and so on. 

In the case of multiparticle blockage, as the suspension flows through the medium, the capillary walls of the pores are gradually covered by a uniform layer of particles. This particle layer continues to build up due to mechanical impaction, particle interception and physical adsorption of particles. As the process continues, the available flow area of the pores decreases. Denoting as the ratio of accumulated cake on the inside pore walls to the volume of filtrate recovered, and applying the Hagen-Poiseuille equation, the rate of filtration (per unit area of filter medium) at the start of the process is ... 

The development of the two-phase flow in a heated capillary at different Peclet number is illustrated in Fig. 9.13. It shows that different mechanisms of two-phase flow formation may occur depending on the value of Peu. At small Pcl the fine bubble formation (on the micro-channel wall) plays a dominant role. Growth of these bubbles leads to a blockage of the micro-channel, to a sharp change of the hydraulic... 

For parenteral emulsions, the formulation scientist must be particularly aware of changes in particle size distribution of the oil phase. Droplet coalescence results in increased droplet size. As a general rule, average droplet size should be less than 1 pm. Droplet sizes of more than 6 pm can cause blockage of capillaries (capillary emboli). 

There are numerous other problems associated with the technique. Such systems need very careful setting up to ensure that the fractions park accurately in the flow cell so as to maximise concentration and hence signal to noise. Other minor irritants can include various plumbing problems, blockages causing capillaries to burst off, wet carpets etc. 

Control of the vaporization process, i.e. the temperature of the capillary, is of crucial importance. Optimum performance is obtained with around 95% vaporization of the liquid stream. Too much heat results in vaporization occurring within the capillary with deposition of analyte and, if operation of the interface under these conditions is continued for any length of time, blockage of the capillary. On the other hand, if insufficient heat is applied to the capillary, vaporization does not occur and liquid flows from it and no spray is obtained. The optimum temperature is dependent on a number of parameters, among the most important being the composition of the mobile phase and its flow rate. Good temperature control is therefore required to obtain the best conditions when gradient elution is employed. 

A relatively wide capillary (about 0.5 mm. I.D.) should be used to avoid blockages toward the end of the distillation. 

There are several types of nebulisers, for example the concentric quartz system (Fig. 3.9) or cross-flow system (Fig. 3.10) the tips of which are made from synthetic material or precious stones. The first system can be subject to blockage with solutions containing as little as 0.1% of dissolved salts due to its small capillary inlet. The second system is less fragile and is resistant to corrosion from solutions containing hydrofluoric acid. 

In general, liposomes due to their physicochemical characteristics, may cause adverse effects i.e., blockage of capillaries, pulmonary embolism etc. The new technological knowledge and industrial improvements on liposomal production as well as the high quality control of the preparation procedure of the liposomal formulations can eliminate most of their undesired side effects. 

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