Observe the evaporation of a drop of table salt solution on a warm hydrophobic surface. After the water evaporates, a variety of characteristic crystal shapes remain. Research and explain this phenomenon.

Background reading from IYPT reference kit

*Crystal Critters (youtube, American Physical Society, 25.11.2019), https://youtu.be/VV5oc28QP7o *S. A. McBride, H.-L. Girard, and K. K. Varanasi. Crystal critters: Self-ejection of crystals from heated, superhydrophobic surfaces. Sci. Adv. 7 eabe6960 (2021), https://doi.org/10.1126/sciadv.abe6960 *S. A. McBride, H.-L. Girard, and K. K. Varanasi. Crystal critters. Phys. Rev. Fluids 5, 11, 110508 (2020), https://doi.org/10.1103/PhysRevFluids.5.110508

*H. Salim, P. Kolpakov, D. Bonn, and N. Shahidzadeh. Self-lifting NaCl crystals. J. Phys. Chem.

TYPT Supplementary References

Crystal Critters

YouTube, American Physical Society, 25.11.2019 https://youtu.be/VV5oc28QP7o

A quick review given by the authors discovering this phenomenon. Look up this YouTube film first before moving to next step.

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

S. A. McBride, H. L. Girard, and K. K. Varanasi . Sci. Adv. 7 eabe6960 (2021)

• Crystals self eject from heated, nanotextured superhydrophobic materials during evaporation of saline water drops.
• These crystal structures (crystal critters) have minimal contact with the substrate.
• This phenomenon is caused by cooperative effects of crystallization, evaporative flows, and nanoscale effects.
• The temperature dependence of the critter effect can be predicted using principles of mass conservation.
• Self propulsion can be generated via temperature gradients, which promote asymmetric growth.

• B) SEM images showing nanotexture of hydrophobic nanograss surface. Scale bars, 3μm .
• C) Growth of crystal critters from a 5 μl drop with time at a substrate temperature of 90 ℃. Scale bar, 0.5 mm.

Proposed theoretical model