Fig. 11 shows a hot drinks mug. The surface of the mug changes colour when a hot liquid is poured into it. The product is manufactured in large quantities u...

(a) The type of smart material used on the surface of the mug is thermochromic pigment (thermochromic ink) [1]. This is a smart material because it changes its properties in response to an external stimulus (temperature) and reverses when the stimulus is removed.

(b) Thermochromic pigment changes colour in response to a change in temperature [1]. When a hot liquid such as tea or coffee is poured into the mug, the heat transfers through the mug wall and raises the temperature of the pigment. This causes a reversible chemical or physical change in the pigment's molecular structure, making it appear a different colour. When the liquid cools and the mug returns to room temperature, the pigment reverts to its original colour [1]. This reversibility distinguishes it from a permanent colour change.

(c) One other smart material and a product it is used in:

• Shape memory alloy (SMA), such as Nitinol - used in dental braces [1+1]. The wire is deformed at room temperature to fit over the patient's teeth. Body heat causes the alloy to return to its pre-set shape, applying a gentle, constant force that gradually straightens the teeth. Other valid products include spectacle frames and medical stents.

Other acceptable answers include photochromic pigment (used in sunglasses that darken in UV light) or quantum tunnelling composite (QTC), used in pressure sensors and membrane switches.