Soft and Switchable Pearls—Capsules that Exhibit Pearlescence on Heating

A pearl-like luster (i.e., pearlescence) is seen in many natural materials like nacre and also in some commercial products such as paints and cosmetics. This optical phenomenon is attributed to the diffraction of light from plate-like particles in the material, with the particle size or spacing being comparable to the wavelength of light. Here, for the first time, we demonstrate the onset of pearlescence in soft capsules that contain no plate-like particles. The capsules (~ mm in size) are transparent and non-pearlescent at room temperature. The thin (~ 500 µm) outer shell of the capsules is a hydrogel of N-isopropylacylamide (NIPA), which shrinks when heated above its lower critical solution temperature (LCST) of ~ 32°C. When heated above this LCST, the transparent capsule spontaneously turns pearlescent. The effect is reversible, with the transparent state being recovered upon cooling. This is the <b><i>first example of</i> <i>reversible pearlescence</i></b> in any material. Spectral reflectance measurements show that the pearlescence of the capsules is comparable to that of real pearls. Interestingly, bulk hydrogels of NIPA do not turn pearlescent on heating ¾ they simply become turbid. Thus, pearlescence is observed only in NIPA-shelled capsules, and we show that it requires both a thin NIPA shell and a water-rich core capable of swelling. When heated above its LCST, the NIPA shell further shrinks, and we postulate that NIPA-rich domains phase-separate within this shell. The size or spacing of these domains is likely to be responsible for the pearlescence. Our work sheds fresh insight into the nature of pearlescence, on how it can be tuned, and on how it can be introduced into various soft materials.