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We present a new algorithm for the analysis of whispering gallery modes (WGMs) found in the cavity enhanced Raman spectra retrieved from optically tweezed droplets. Our algorithm improves the computational scaling when analyzing core–shell droplets ( i.e. phase-separated or biphasic droplets) in the aerosol optical tweezers (AOT), making it computationally practical to analyze spectra collected at a few Hz over hours-long experiments. This enables the determination of the size and refractive index of both the core and shell phases with high accuracy, at 0.5 Hz time resolution. Phase-separated core–shell droplets are common morphologies in a wide variety of biophysical, colloidal, and aerosolized chemical systems, and have recently become a major focus in understanding the atmospheric chemistry of particulate matter. Our new approach reduces the number of parameters directly searched for, decreasing computational demands. We assess the accuracy of the diameters and refractive indices retrieved from a homogeneous or core–shell droplet. We demonstrate the performance of the new algorithm using experimental data from a droplet of aqueous glycerol coated by squalane. We demonstrate that a shell formation causes adjacent WGMs to split from each other in their wavenumber position through the addition of a secondary organic aerosol shell around a NaCl(aq) droplet. Our new algorithm paves the way for more in-depth physiochemical experiments into liquid–liquid phase separation and their consequences for interfacial chemistry—a topic with growing experimental needs for understanding the dynamics and chemistry of atmospheric aerosol particles, and in biochemical systems.
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Core–shell droplets and microcapsules formed through liquid–liquid phase separation of a colloid–polymer mixture
Microcapsules allow for the controlled containment, transport, and release of cargoes ranging from pharmaceuticals to fragrances. Given the interest from a variety of industries in microcapsules and other core–shell structures, a multitude of fabrication strategies exist. Here, we report on a method relying on a mixture of temperature-responsive microgel particles, poly( N -isopropylacrylamide) (pNIPAM), and a polymer which undergo fluid–fluid phase separation. At room temperature this mixture separates into colloid-rich (liquid) and colloid-poor (gas) fluids. By heating the sample above a critical temperature where the microgel particles shrink dramatically and develop a more deeply attractive interparticle potential, the droplets of the colloid-rich phase become gel-like. As the temperature is lowered back to room temperature, these droplets of gelled colloidal particles reliquefy and phase separation within the droplet occurs. This phase separation leads to colloid-poor droplets within the colloid-rich droplets surrounded by a continuous colloid-poor phase. The gas/liquid/gas all-aqueous double emulsion lasts only a few minutes before a majority of the inner droplets escape. However, the colloid-rich shell of the core–shell droplets can solidify with the addition of salt. That this method creates core–shell structures with a shell composed of stimuli-sensitive microgel colloidal particles using only aqueous components makes it attractive for encapsulating biological materials and making capsules that respond to changes in, for example, temperature, salt concentration, or pH.
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- Award ID(s):
- 1919429
- PAR ID:
- 10342682
- Date Published:
- Journal Name:
- Soft Matter
- Volume:
- 17
- Issue:
- 36
- ISSN:
- 1744-683X
- Page Range / eLocation ID:
- 8300 to 8307
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D1SM01091C
