Additional Complications You might vaguely remember back on our introductory page when we made some assumptions? Maybe? About viscosity? And laminar flow? The sad truth is that we don't always get to make such assumptions, especially if we're talking straws (and pipes which are really just giant straws or fire-hoses which are really just very big bendy straws). For instance, assuming no viscosity rules milkshakes straight out of straw drinking, and we all know that that's a dirty lie. We also neglected to talk about the limitations of facial musculature and the effort of drinking. From our understanding of basic pressure: p = F/A where p is the pressure created by a force F acting on a surface area A, it should seem intuitive that we'll need less 'force' to create a pressure differential for a smaller area. So, intuitively, it should be the case that drinking from a smaller diameter straw should be easier. Because I am a silly person who thought way too much about straws, I tested this little hypothesis. It is the case that your general straw with 7 mm diameter is a bit easier to drink from than a 15 mm diameter milkshake straw. However, the effort required to drink from those dorky little slim coffee stirring straws? Much greater. I'm not the only one who noticed [7]. For us, drinking isn't about getting the first layer to the top of the straw, it's about getting the first mouthful. With significantly small straws, both the amount of puckering of the mouth to accommodate the straw and the tiny cross sectional area of the straw work against us. Furthermore, friction between the liquid and the straw cannot be discounted! The smaller the diameter of the straw, the more friction between the liquid and the straw. Also, the faster a fluid is pushed through a straw, the more turbulent the flow and the more 'friction'. This is especially noticed by firefighters. Well, by the engine operator in particular. In field fire hydraulics, there are losses in pressure (psi) between the discharge pressure at the engine and the nozzle operator at the end of the hose. Different hose diameters are given a constant friction loss coefficient per 50' section of hose, which is multiplied by the gallon per minute flow[8]. In order to facilitate rapid water deployment, it's not unusual for fire departments to create a pump chart with the discharge pressure required for combinations of common lengths, flow rates, and appliances used on the fireground [8]. An additional complication happens with sufficiently tiny straws. Capillary tubes, and other tiny straws, are occasionally used for collecting small samples of blood or other liquids in biology. Liquids are able to 'climb' the tubes due to a combination of the cohesion (attraction of the liquid to itself) and adhesion (attraction of the liquid molecules to the surface of the tube) [9].
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