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Analysis of Mie scattering from optically trapped particles
Thesis (PhD)--Stellenbosch University, 2026.
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| Format: | Thesis |
| Language: | English |
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Stellenbosch : Stellenbosch University
2026
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| _version_ | 1867613803792826368 |
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| access_status_str | Open Access |
| author | Erasmus, Anneke |
| author2 | Neethling, P. H. |
| author_browse | Erasmus, Anneke Neethling, P. H. |
| author_facet | Neethling, P. H. Erasmus, Anneke |
| author_sort | Erasmus, Anneke |
| collection | Thesis |
| dc_rights_str_mv | Stellenbosch University |
| description | Thesis (PhD)--Stellenbosch University, 2026. |
| format | Thesis |
| id | oai:scholar.sun.ac.za:10019.1/135817 |
| institution | Stellenbosch University (South Africa) |
| language | English |
| last_indexed | 2026-06-10T12:41:57.021Z |
| license_str | Other — see source repository |
| provenance_str_mv | Harvested via OAI-PMH from SUNScholar — Stellenbosch University Repository |
| publishDate | 2026 |
| publishDateRange | 2026 |
| publishDateSort | 2026 |
| publisher | Stellenbosch : Stellenbosch University |
| publisherStr | Stellenbosch : Stellenbosch University |
| record_format | dspace |
| source_str | SUNScholar — Stellenbosch University Repository |
| spelling | oai:scholar.sun.ac.za:10019.1/135817 Analysis of Mie scattering from optically trapped particles Erasmus, Anneke Neethling, P. H. Bosman, G. W. Stellenbosch University. Faculty of Science. Dept. of Physics. Thesis (PhD)--Stellenbosch University, 2026. Erasmus, A. 2026. Analysis of Mie scattering from optically trapped particles. Unpublished doctoral dissertation. Stellenbosch: Stellenbosch University [online]. Available: https://scholar.sun.ac.za/items/2f0ad922-005d-45e6-b857-753b8e4aec9a Understanding the dynamics of aerosols and mass transfer is important in atmospheric processes such as droplet evaporation and growth. Properties of aerosol droplets can be inferred from bulk sample studies, however, that does not take the surface-to-volume ratio into account. In order to obtain more accurate results, the physical properties of individual droplets need to be measured. Two morphological properties highlighted here are the size and dispersion relation of a NaCl-water droplet, which are strongly influenced by its temperature, controlled here by varying the amount of laser light absorbed by a droplet. In this dissertation, we constructed a counter-propagating aerosol optical trap with the ability to measure Mie scattering to accurately determine the droplet’s diameter and refractive index simultaneously. By changing the trap laser power and hence the amount of light absorbed, we could systematically change the droplet diameter and refractive index and measure these changes. The size and refractive index of a trapped spherical particle were determined simultaneously by analysing the scattering from the particle using Mie theory across a broad spectral band. Mie scattering theory provides a solution to the Maxwell’s equations and describes the scattering of electromagnetic waves from a spherical particle with diameters in the order of the wavelength or larger. The incident light couples into the particle, and due to total internal reflection, whispering gallery modes are formed at specific wavelengths. The resonating wavelengths within the particle appear as peaks in the spectrum of the scattered light. The measured scattering was simulated by iteratively varying the size and refractive index of the particle until the simulated peak positions were in good agreement with the measurement. In this way, the two parameters were determined simultaneously. The use of multiple peak positions simultaneously reduced uncertainties in the analysis. For benchmarking purposes, preliminary measurements were conducted to demonstrate the application of the technique to a known sample of polystyrene beads individually trapped in water using optical tweezers. The scattering of broadband light from the trapped bead was analysed and the diameter of each bead was determined. The refractive index of the polystyrene beads was determined over the broad wavelength range of the light source, resulting in a dispersion relation of the bead. This preliminary study gave insights into the analysis and requirements of the system. To study changes in aerosol droplets, a counter-propagating optical trap was constructed to isolate individual saline solution droplets with diameters between 2 µm and 11 µm. Two counter-propagating laser beams (975 nm) were focused with two 100X magnification, high numerical aperture (NA = 0.7), long working distance air objectives into a sample chamber. Once a NaCl-water droplet was trapped, it could be held in the trap in excess of five hours. The size of the droplets was measured with uncertainties on the order of nanometres, which is far smaller than the optical resolution of the system. The refractive index was measured with an uncertainty on the third decimal. An aerosol droplet’s size and refractive index at equilibrium with its environment depend on the relative humidity and temperature. For optical aerosol traps, it is important to note the droplet’s response to the trap laser power. The sensitivity of our system allowed us to measure reversible and controllable morphological changes in an individual aerosol droplet. These changes were induced by changing the trap laser power thereby changing the amount of laser light absorption, causing visible spectral shifts in the scattered light. Analysis of the spectra at various laser powers showed changes in the droplet diameter as small as 16 ± 4 nm for a droplet of diameter 3.947 ± 0.002 µm for a change of 1 mW. As the droplet’s diameter changed, so did the refractive index. This reflects a change in the NaCl concentration as water was evaporated or condensed onto the droplet. In addition, the dynamics of the droplet undergoing these size changes as the laser power is varied were resolved. These dynamics are reflected in shifts of the spectral peaks as the droplet equilibrates, probed with an acquisition period of 160 ms. This showed different dynamics of the droplet for heating and cooling as it reached equilibrium, and both non-resonant heating and the possibility of resonant heating are discussed. The sensitivity of our system highlights the influence of the trap laser on aerosol droplets. In this work, we showed that not only does a change in the optical trap laser power of 1 mW induce measurable nanoscale changes to the trapped droplet’s diameter, but it also affects the solute concentration. These changes in the droplet, as the droplet reaches equilibrium, were probed dynamically. These results contribute and expand on existing literature where laser heating of optically trapped aerosols were studied using Mie scattering. The work in this dissertation describes a powerful platform to probe droplets and their dynamics, providing key information for aerosols studies. Doctoral 2026-04-10T13:38:49Z 2026-04-10T13:38:49Z 2026-03 Thesis https://scholar.sun.ac.za/handle/10019.1/135817 en Stellenbosch University 102 pages : ill. application/pdf Stellenbosch : Stellenbosch University |
| spellingShingle | Erasmus, Anneke Analysis of Mie scattering from optically trapped particles |
| title | Analysis of Mie scattering from optically trapped particles |
| title_full | Analysis of Mie scattering from optically trapped particles |
| title_fullStr | Analysis of Mie scattering from optically trapped particles |
| title_full_unstemmed | Analysis of Mie scattering from optically trapped particles |
| title_short | Analysis of Mie scattering from optically trapped particles |
| title_sort | analysis of mie scattering from optically trapped particles |
| url | https://scholar.sun.ac.za/handle/10019.1/135817 |
| work_keys_str_mv | AT erasmusanneke analysisofmiescatteringfromopticallytrappedparticles |