Optical images of catechol SOA material suspended on a tungsten needle (side view) were recorded while the temperature was increased at 2 °C min -1 (Fig. and Video S1). At approximately 40 °C, the cross-sectional area of the material began to increase. At a temperature of 80 °C, the cross-sectional area had increased by a factor of ~4 (Fig. ). Above 80 °C, the crosssectional area suddenly decreased and then increased several times thereafter. The particles also became less opaque when the cross-sectional area increased. We attribute this cycle of increasing and decreasing cross-sectional area to the boiling of SOA material, with the gradual size increase being due to trapped bubbles growing inside the particle, and the sudden decrease in size being caused by bubbles escaping from the particle. For catechol SOA, the onset of boiling was approximately 40 °C. Side-view heating experiments where the temperature was first increased rapidly and then held at 36 °C were also performed for catechol SOA (Fig. and Video S2). The cross-sectional areas of the SOA particles also increased in these experiments, consistent with the boiling behavior in Fig. . Fig. and Video S2 suggest boiling of the catechol SOA occurred even at the mild temperature of 36 °C. Additionally, in all our experiments, bubble-shaped inclusions were clearly visible in the SOA, consistent with the boiling of SOA material.