Mass Transport via Thermoplasmonics
Doctoral thesis, 2020
Elevated temperatures increase the Brownian motion of small particles. Moreover, if large temperature gradients are present, then a process known as thermophoresis is likely to occur. Thermophoresis tends to cause a local depletion of Brownian particles around a hot region. From the context of “conventional” plasmonic applications (like molecular sensing), these thermally driven mass transport mechanisms are adverse side effects since they reduce the interaction rate between the plasmonic system and the analyte. An investigation of thermal effects in plasmonic optical tweezers showed that the increased Brownian motion essentially negated the optical tweezing effect, resulting in an overall insensitivity between the resonance condition of the antenna and the particle confinement when evaluated in terms of the local temperature increase. Additionally, a significant thermophoretic depletion of analytes occurred, extending tens of microns from the plasmonic structure. This depletion acts in opposition to the plasmonically enhanced optical forces, which are restricted to a region of only a few hundred nanometres.
However, thermoplasmonic effects can also be used for advantageous means. Once example is by driving thermocapillary flows directed towards the plasmonic system, thereby facilitating the efficient accumulation of analytes. One method of employing this effect is to superheat a plasmonic particle to a high enough temperature such that a bubble is nucleated. Once a bubble is formed, thermocapillary effects at the bubble interface drive fluid motion with a flow profile similar to that of a Stokeslet. This fluid flow can be utilized for analyte accumulation near the plasmonic structure. In addition to the thermocapillary induced flow, it was found that even more intense flow speeds were achieved immediately upon nucleation due to the mechanical action of the bubble. This transient peak in flow speed was approximately an order of magnitude faster than the subsequent persistent (thermocapillary) flow. By designing the plasmonic nanoparticle so that the Laplace pressure restricted the ultimate bubble size, these bubbles could be kept small enough to permit high modulation rates and maximize the relative effect of the peak transient flow.
Chalmers, Physics, Nano and Biophysics
Jones S., Andrén D., Antosiewicz T. J., Stilgoe A., Rubinsztein-Dunlop H., & Käll M., Strong Transient Flows Generated by Thermoplasmonic Bubble Nucleation
Photothermal Heating of Plasmonic Nanoantennas: Influence on Trapped Particle Dynamics and Colloid Distribution
ACS Photonics,; Vol. 5(2018)p. 2878-2887
Ultrafast Modulation of Thermoplasmonic Nanobubbles in Water
Nano Letters,; Vol. 19(2019)p. 8294-8302
Losses are a ubiquitous feature of our universe. They are encoded in the fundamental laws of thermodynamics. In our everyday lives we experience these losses as friction, air resistance, etc. Generally speaking, losses are the effects that occur in any physical process that cause a decrease in efficiency and result in the generation of heat.
Plasmonics is no different.
Plasmonics is the study of how light interacts with metallic nanostructures (e.g. gold nanoparticles). Small gold structures can act as optical antennas and allow scientists to probe nanoscopic environments. However, because of losses, these nanoantennas can also generate a significant amount of heat.
Thermoplasmonics concerns itself with studying the effects of heat generated by plasmonic interactions. Historically this heat generation has often been viewed as an unwelcome side-effect, and thus it is imperative to study these processes to understand ways to design systems to mitigate heat generation. However, there are also new and exciting ways that these losses can be utilized for entirely new purposes.
This thesis explores both the detrimental and beneficial aspects of thermoplasmonics. In particular, the adverse effects of thermally enhanced Brownian motion and thermophoretic depletion are examined in the context of conventional plasmonic systems. Additionally, the use of thermally nucleated bubbles is explored as a potential means of efficient mass transport in microfluidic systems.
Areas of Advance
Nanoscience and Nanotechnology (SO 2010-2017, EI 2018-)
Chalmers Materials Analysis Laboratory
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4862
Chalmers University of Technology
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Opponent: Prof. Jochen Feldmann, Ludwig-Maximilians-Universität, München, Germany