Heterogeneous nucleation studied in a long working-distance optical trap
The phase state of an atmospheric particle is critical in determining its optical properties, ability to serve as cloud-condensation nuclei, heterogeneous chemical reactivity, and atmospheric lifetime. The ability to predict when a particle will exist as a liquid, solid, or amorphous (semi)-solid is therefore essential for accurate global climate and air quality models. In the atmosphere, nucleation of a solid or crystalline phase from a liquid precursor is complicated by the wide variety of chemical components that exist and the ability for multiple particles to interact.
In the Tolbert lab, we have recently developed a Bessel-beam based long working-distance optical trap to characterize a variety of nucleation mechanisms. In particular, we are currently investigating contact nucleation. Contact nucleation occurs when a foreign particle, generally a solid, collides with a liquid particle and induces nucleation of the crystalline phase. A variety of optical techniques are used to characterize the phase transition (Figure 1). An example of the contact-induced nucleation of a levitated aqueous ammonium sulfate droplet into crystalline ammonium sulfate is shown in Figure 2.
Figure 1. Examples of how a liquid (below) and solid (above) appear in a) 532-nm far-field elastically scattered light b) 632.8-nm far-field elastically scattered light, c) bright-field/near-field 632.8-nm scatter combination and d) bright-field imaging.
Figure 2. A sequence of images showing the contact-induced nucleation of solid ammonium sulfate. The far-field elastically scattered laser light from the trapping laser is used to image both the levitated droplet and contacting particle.