Although our technique includes a washing step to remove the density gradient medium (OptiPrep™), some of the OptiPrep™ could still remain adsorbed to the INSs and influence their ice nucleation ability. To determine if this may be important in our experiments, we measured the ice nucleation ability of both INSs used in our experiments (K-feldspar and Snomax) in the presence of 1% OptiPrep™. This concentration is most likely an upper limit to the concentrations expected after the washing step in our experiments based on the amount of dilution in the washing steps. Corrections were made for the freezing point depression caused by OptiPrep™ using Eq. S2. For a concentration of 1% v/v the calculated freezing point depression was small (0.0036 °C).