The heterogeneous dynamics exhibited by supercooled liquids near the glass transition temperature (𝑇_𝑔) has been a topic of much research over the past several decades. In particular, the advent of single molecule (𝖲𝖬) methods has permitted great insight into the extent of both spatial and temporal heterogeneities in these systems, information which is either difficult or impossible to access via ensemble approaches. Despite this, the related phenomenon of rotational-translation decoupling, whereby the translational motion observed in supercooled systems is enhanced relative to Debye-Stokes-Einstein predictions, is difficult to study with 𝖲𝖬 approaches. This is due to the very low localization uncertainty required to accurately report the extremely slow translational motion in supercooled systems near 𝑇_𝑔. In this thesis, a new approach for quantifying rotational dynamics in supercooled liquids is introduced which leverages fluorescence intensity fluctuations due to out-of-plane fluorophore rotations.
Unlike linear dichroism (LD) measurements, the most common experiment used to access rotational dynamics, this technique does not require a polarizing optical element, thus improving localization precision in the acquired images. This intensity fluctuation-based approach is shown to report comparable rotational correlation timescales (𝝉_𝘤) and information on dynamic heterogeneity to that typically extracted via LD measurements. On a probe-by-probe basis, rotational correlation times obtained from simultaneous measurement of LD (𝝉_𝘤,𝘓𝘋) and intensity fluctuations (𝝉_𝘤,𝘐 ) are found to be only moderately well-correlated. We postulate that this is a consequence of dynamic heterogeneity due to temporal dynamic exchange, the process in which a probe (and its surroundings) undergoes sudden changes in dynamics.
This hypothesis is explored through simulations, which reveal that the Pearson R correlation coefficients associated comparing log 𝝉_𝘤,𝘐 and log 𝝉_𝘤,𝘓𝘋 increases as the time between dynamic exchange increases. The information obtained from such simulations is then used to estimate the exchange timescales from experimental data. When examined in concert with experimentally measured degrees of relaxation non-exponentiality - generally considered a metric of heterogeneity in an interrogated supercooled liquid – this permits access to previously inaccessible information regarding the breadth of the distribution of underlying timescales experienced by these supercooled systems. In addition to this work focused on rotational dynamics, we also aim to further clarify information contained in 𝖲𝖬 experiments characterizing translational dynamics, towards the goal of full understanding of rotational-translational decoupling.
Here, two widefield fluorescence imaging setups are optimized to minimize localization uncertainty, and differences in how localization uncertainties manifest in perceived translational motion near 𝑇_𝑔 are examined. The setup with greater localization uncertainty reports faster translational dynamics compared to the other optical setup, suggesting significant influence of the localization noise floor on perceived dynamics and highlighting the importance of maximizing the signal to noise ratio of 𝖲𝖬 experiments aiming to study the underlying cause of rotational-translational decoupling.
| Identifer | oai:union.ndltd.org:columbia.edu/oai:academiccommons.columbia.edu:10.7916/y0wa-9q13 |
| Date | January 2024 |
| Creators | Meacham, Alec Robert |
| Source Sets | Columbia University |
| Language | English |
| Detected Language | English |
| Type | Theses |
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