Self-Lifting NaCl Crystals

Abstract

We show that macroscopic crystals of NaCl that form from evaporating drops of aqueous salt solutions can spontaneously lift themselves up and away from a hydrophobic surface. At the end of the evaporation process, tiny crystals of NaCl grow onto larger ones and form “legs” that push the large crystals away from the surface. The temperature dependence of the lifting speed is found to exhibit Arrhenius behavior with an activation energy similar to that of crystals growing in solution: the crystal growth itself determines the lifting speed that can be up to half a centimeter per minute. We show that surface hydrophobicity is a necessary but not a sufficient condition to obtain this “self-lifting” behavior.

Crystal critters: Self-ejection of crystals from heated, superhydrophobic surfaces

“…Liftoff and ejection on smooth materials have been previously shown in cases where the crystal does not pin to the underlying substrate during the initial phases of evaporation (21). Pinning during evaporative crystallization from drops is largely a function of interfacial properties (11,21), which will also dictate whether vertical growth occurs. As with the critter effect, the vertical lift on smooth hydrophobic surfaces toward the end of evaporation can be attributed to capillary bridges formed between crystals and the substrate (21).…”

“…As with the critter effect, the vertical lift on smooth hydrophobic surfaces toward the end of evaporation can be attributed to capillary bridges formed between crystals and the substrate ( 21 ). Similarly, the rate of vertical ejection on smooth surfaces increases with increased substrate temperature; an effect that has previously been attributed to a temperature-dependent crystal growth ( 21 ). However, in the present experiments on smooth surfaces, we do not observe a discernable t lift at which water markedly dewets from the substrate and triggers leg growth (as shown in Fig.…”

“…Below substrate temperatures of ~50°C, we did not observe either the dewetting event or the subsequent leg growth associated with crystal critter formation. Note, however, that vertical ejection of crystals has been observed at lower temperatures in a related effect on smooth surfaces (see note S1 for more on smooth surfaces) ( 21 ). The apparent temperature limit for critter formation may be related to the enhanced stability of the air/vapor layer within superhydrophobic textures at elevated temperatures ( 25 , 30 – 32 ) and hints that the importance of the air layer in triggering the dewetting event leads to critter growth.…”

“…Pinning during evaporative crystallization from drops is largely a function of interfacial properties ( 11 , 21 ), which will also dictate whether vertical growth occurs. As with the critter effect, the vertical lift on smooth hydrophobic surfaces toward the end of evaporation can be attributed to capillary bridges formed between crystals and the substrate ( 21 ). Similarly, the rate of vertical ejection on smooth surfaces increases with increased substrate temperature; an effect that has previously been attributed to a temperature-dependent crystal growth ( 21 ).…”

“…Here, we report an unexpected and unusual phenomenon in which crystal structures formed from evaporating drops of water saturated with sodium chloride self-eject from heated, superhydrophobic surfaces ( 19 , 20 ). This self-ejection occurs via the growth of crystalline “legs” during the end phase of evaporation, which cause the entire crystal structure to eject from the surface ( 21 ). We term the resulting structures composed of the salt globe and legs “crystal critters” due to the eerie motions produced during self-ejection and to the resemblance of the crystal structures to biological forms ( 19 , 20 ).…”