Size ratio parameter molecular significance

Surface active poly(ethylene glycol) ethers have been found to exert a aippressive effect on tuberculosis The greatest activity is observed wdien the number of ethylene oxide urtits is IS to 20 and the activity is completely abolished in those polymers where the number of imits ranges between 25 and 30. Conforth and coworkers have also found that when the number of ethylene oxide units was 45—70 the polymers, instead of displaying chemotherapeutic activity, significantly enhanced the infection, the so-caUed protuberculous effect. Thus, as the lipophflic to hydro-phflic ratio decreases, activity passes from antituberculosis to inactive protuberculous. These observations demonstrate the importance of two physical parameters in chemotherapy, namely surface activity and molecular size. 

In the case of flow in microscale pathways, some important parameters in macroscale fluid mechanics remain important, but the features of the flow are very different. The influences of viscosity and surface effects are much more significant compared with fluid flow in traditional pipelines or channels. That is, the ratio of the inertial force to the viscous force (Reynolds number) is in general smaller than unity, and the ratio of the inertial force to surface tension (Weber number) is also small, so that accurate microfluidic metering is somewhat more difficult than metering for macroscopic flows. For gas flows, the effect of the Knudsen number, defined as the ratio of the molecular mean free path to the channel size, is high and cannot be ignored for microscale channels, and hence the fluid metering discussed here will be for liquids only. The Reynolds number Re, the Weber number We, and the Knudsen number Kn are expressed as follows ... 

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