SMART MATERIALS—MATERIALS THAT ADAPT TO CHANGING CONDITIONS - Chemistry: Notable Research and Discoveries - page 109

served by different wing shapes, a problem that the peregrine falcon's
flexible wings solve. Fixed-wing aircraft are not as flexible. And take-
offs and landings, which are critical moments in a flight where many ac-
cidents occur, offer entirely different problems—take-offs require much
lift, but landings do not.
Any single solution to a problem requiring opposite properties is
bound to be a compromise—adequate for both conditions but optimal
for neither. Most aircraft wings have attached surfaces, which when
moved back and forth lengthen or shorten the wing area or adjust some
other property. These movements allow pilots to control the plane, but
the moving parts provide only limited flexibility, and they add a lot of
weight and complexity to the craft, resulting in a serious loss of ease and
efficiency. A few bold aviation engineers have designed and built air-
planes with movable wings, such as a military jet called the F-14 Tom-
cat, which can sweep its wings back and forth. Such designs offer a little
more flexibility, but they are poor imitators of the peregrine falcon.
The ideal solution is a material that can change its shape or prop-
erties as needed. A single, adaptable material or structure requires no
compromises.
AMATErIAl'SI.Q.—
rESPonSIvEnESSToCHAngE
Most materials undergo changes when certain features of their environ-
ment change. One of the most common changes is called thermal ex-
pansion—most substances increase in volume as the temperature rises.
(Heat causes a substance's atoms and molecules to increase their mo-
tion, which results in an increase in the substance's size.)
Thermal expansion is a reaction to the environment, but it is not very
useful for engineering purposes. The expansion is typically small—for
example, a 1,000-foot (305-m) steel beam will get only about 3.5 inches
(8.75 cm) longer when the temperature increases from 50°F (10°C) to
100°F (37.8°C). Even though it is not a drastic change, engineers must
take it into account, otherwise structures such as bridges and railway
tracks might fail when the temperature changes too much.
Changes like thermal expansion are generally more of a nuisance
than anything else. Although thermal expansion is convenient in mak-
ing certain thermometers, in most cases it is not a useful adaptation.