Prior to the imaging experiments, the temperature-controlled cell was purged with nitrogen for a minimum of 15 min and then sealed from the surrounding atmosphere. We also performed additional experiments with the particles exposed to ambient air during heating and found that the results were not sensitive to the type of atmosphere. Imaging experiments were carried out using a temperature ramp rate of 2 °C min -1 . Alternatively, the temperature was quickly increased (2 °C s -1 ) to a set temperature and then held at the set temperature for a certain period of time.