The water's fine

 作者:奚瑁算     |      日期:2019-03-08 01:11:01
By Catherine Zandonella FRIGID ocean depths may become a more hospitable place for scuba divers who have to spend hours underwater at temperatures just above freezing, thanks to the US Navy. For some jobs, divers can wear “hot water suits” that rely on warm water being pumped around a suit via a hose from the surface, but the hose can get in the way. So the US Navy has long been experimenting with special materials to prevent heat loss. Now it looks as though they’re finally getting somewhere. The secret is a layer of foam containing bits of wax inside tiny plastic beads, each just a tenth the width of a human hair. These bits of wax melt and freeze as the temperature changes. While the diver is putting on the drysuit, the wax absorbs warmth from the diver’s body and melts. But when the diver slips into the icy waters, the wax freezes, radiating latent heat back to the diver. The waxes are long hydrocarbon chains that have been chosen because of the temperatures at which they change phase. To provide optimal warmth to the diver, the waxes must melt at a temperature below that of bare skin—about 33 °C. Similarly, the waxes must freeze at temperatures as high as 18 °C to provide enough heat to the diver. So far, the warming effect lasts for about the first half an hour or so of the dive (Ocean Engineering, vol 26, p 547), but the US Navy wants to improve on that, says Lew Nuckols, of the US Naval Academy in Annapolis, the Navy scientist developing the suit. He is looking at various ways of extending the warming period, such as packing more of the waxes into the diving suit liner or weaving the beads into layers of synthetic fabric instead of foam. The beads, which look like fine dust, can be placed inside fabric fibres by mixing them with the liquid polymers used to make artificial fibres. Fabrics such as polypropylene, which is water-resistant, are made by pumping a liquid polymer through a device like a spaghetti-maker. Another way to extend the warming capacity could be using a combination of waxes that freeze at different temperatures, so that heat is released gradually as the temperature drops. The technology for trapping wax inside microcapsules, created by Yvonne Bryant and David Colvin at Triangle Research and Development in North Carolina, was originally used as a high-temperature insulation material in US Air Force aircraft engines. One promising new application is in agriculture: