Polydimethylsiloxane brushes and the search for extraterrestrial life

The low temperature and high pressure tribological properties of polydimethylsiloxane brushes with ice are explored to demonstrate their feasibility as an exterior coating for an off-world cryobot. Successful deposition of the brushes on silicon and glass was confirmed with a contact angle hysteresis < 2° and a surface roughness below 1 nm. The friction factor of the brushes roughly doubled when the temperature was lowered from +20 °C to −20 °C, but it decreased by 55% when the normal force was increased from 0.5 N to 16 N. When sheared, adhered ice slid on the brushes at a shear stress around 21 kPa, and this did not increase with an additional normal pressure of up to 98 kPa. A glass rod coated with the brushes served as a cryobot surrogate and was frozen within cores of −10 °C ice 1–3 cm high. Weight attached to the rod enabled it to cleanly slide completely through the ice cores at the ambient −10 °C, i.e. without melting the ice. Together, these results indicate that polydimethylsiloxane brushes may be a feasible exterior coating for an off-world cryobot that would enable it to slide through the frozen surface of potentially life-harboring bodies such as Europa or Enceladus, avoiding the need to melt the entire cryobot’s exterior.