Bessel beam (BB) optical traps have become widely used to confine single and multiple aerosol particles across a broad range of sizes, from a few microns to < 200 nm in radius. The radiation pressure force exerted by the core of a single, zeroth-order BB incident on a particle can be balanced by a counter-propagating gas flow, allowing a single particle to be trapped indefinitely. The pseudo non-diffracting nature of BBs enables particles to be confined over macroscopic distances along the BB core propagation length; the position of the particle along this length can be finely controlled by variation of the BB laser power. This latter property is exploited to optimize the particle position at the center of the TEM<sub>00</sub> mode of a high finesse optical cavity, allowing cavity ring-down spectroscopy (CRDS) to be performed on single aerosol particles and their optical extinction cross section, σ<sub>ext</sub>, measured. Further, the variation in the light from the illuminating BB elastically scattered by the particle is recorded as a function of scattering angle. Such intensity distributions are fitted to Lorenz-Mie theory to determine the particle radius. The trends in σ<sub>ext</sub> with particle radius are modelled using cavity standing wave Mie simulations and a particle’s varying refractive index with changing relative humidity is determined. We demonstrate σ<sub>ext</sub> measurements on individual sub-micrometer aerosol particles and determine the lowest limit in particle size that can be probed by this technique. The BB-CRDS method will play a key role in reducing the uncertainty associated with atmospheric aerosol radiative forcing, which remains among the largest uncertainties in climate modelling.
To resolve some of the significant uncertainties in the impact of aerosols on global climate, new tools are required to probe light scattering and absorption by aerosol particles. Ideally, such tools should allow direct measurements on individual particles over extended periods of time, providing data to better constrain the optical properties of aerosol, how they depend on the environmental conditions (relative humidity and temperature) and how they change with time. Here, we present a new technique using a combination of a Bessel beam to manipulate individual particles and cavity ringdown spectroscopy for ultrasensitive measurements of the optical extinction. We show that particles can be spatially separated along the propagation direction of a Bessel beam according to their size and refractive index when confined by a Bessel beam core and a counter-propagating gas flow, referred to as optical chromatography. The time-dependent position of a particle is shown to be a consequence of the differing size dependencies of the forces arising from Stokes drag and radiation pressure. We also show that particles captured in a Bessel beam can be moved in and out of an optical cavity formed by two highly reflective mirrors. The time constant for the ringdown in light coupled within the cavity can then be used to measure the optical cross-section of the individual particle with high accuracy. An individual particle can be captured indefinitely and its change in optical cross-section measured with change in environmental conditions.
Using optical tweezers for micro-rheological investigations of a surrounding fluid has been routinely demonstrated. In this work, we will demonstrate that rheological measurements of the bulk and surface properties of aerosol particles can be made directly using optical tweezers, providing important insights into the phase behavior of materials in confined environments and the rate of molecular diffusion in viscous phases. The use of holographic optical tweezers to manipulate aerosol particles has become standard practice in recent years, providing an invaluable tool to investigate particle dynamics, including evaporation/ condensation kinetics, chemical aging and phase transformation. When combined with non-linear Raman spectroscopy, the size and refractive index of a particle can be determined with unprecedented accuracy <+/- 0.05%). Active control of the relative positions of pairs of particles can allow studies of the coalescence of particles, providing a unique opportunity to investigate the bulk and surface properties that govern the hydrodynamic relaxation in particle shape. In particular, we will show how the viscosity and surface tension of particles can be measured directly in the under-damped regime at low viscosity. In the over-damped regime, we will show that viscosity measurements can extend close to the glass transition, allowing measurements over an impressive dynamic range of 12 orders of magnitude in relaxation timescale and viscosity. Indeed, prior to the coalescence event, we will show how the Brownian trajectories of trapped particles can yield important and unique insights into the interactions of aerosol particles.
The significant increase in the air pollution, and the impact on climate change due to the burning of fossil fuel has led to the research of alternative energies. Bio-ethanol obtained from a variety of feedstocks can provide a feasible solution. Mixing bio-ethanol with gasoline leads to a reduction in CO emission and in NOx emissions compared with the use of gasoline alone. However, adding ethanol leads to a change in the fuel evaporation. Here we present a preliminary investigation of evaporation times of single ethanol-gasoline droplets. In particular, we investigated the different evaporation rate of the droplets depending on the variation in the percentage of ethanol inside them. Two different techniques have been used to trap the droplets. One makes use of a 532nm optical tweezers set up, the other of an electrodynamics balance (EDB). The droplets decreasing size was measured using video analysis and elastic light scattering respectively. In the first case measurements were conducted at 293.15 K and ambient humidity. In the second case at 280.5 K and a controlled environment has been preserved by flowing nitrogen into the chamber. Binary phase droplets with a higher percentage of ethanol resulted in longer droplet lifetimes. Our work also highlights the advantages and disadvantages of each technique for such studies. In particular it is challenging to trap droplets with low ethanol content (such as pure gasoline) by the use of EDB. Conversely such droplets are trivial to trap using optical tweezers.
The use of optical tweezers for the analysis of aerosols is valuable for understanding the dynamics of atmospherically
relevant particles. However to be able to make accurate measurements that can be directly tied to real-world phenomena
it is important that we understand the influence of the optical trap on those processes. One process that is seemingly
straightforward to study with these techniques is binary droplet coalescence, either using dual beam traps, or by particle
collision with a single trapped droplet. This binary coalescence is also of interest in many other processes that make use
of dense aerosol sprays such as spray drying and the use of inhalers for drug delivery in conditions such as asthma or hay
fever. In this presentation we discuss the use of high speed (~5000 frames per second) video microscopy to track the
dynamics of particles as they approach and interact with a trapped aqueous droplet and develop this analysis further by
considering elastic light scattering from droplets as they undergo coalescence. We find that we are able to characterize
the re-equilibration time of droplets of the same phase after they interact and that the trajectories taken by airborne
particles influenced by an optical trap are often quite complex. We also examine the role of parameters such as the salt
concentration of the aqueous solutions used and the influence of laser wavelength.
Holographic aerosol optical tweezers can be used to trap arrays of aerosol particles allowing detailed studies of
particle properties and processes at the single particle level. Recent observations have suggested that secondary
organic aerosol may exist as ultra-viscous liquids or glassy states at low relative humidity, potentially a
significant factor in influencing their role in the atmosphere and their activation to form cloud droplets. A
decrease in relative humidity surrounding a particle leads to an increased concentration of solute in the droplet
as the droplet returns to equilibrium and, thus, an increase in the bulk viscosity. We demonstrate that the
timescales for condensation and evaporation processes correlate with particle viscosity, showing significant
inhibition in mass transfer kinetics using ternary sucrose/sodium chloride/water droplets as a proxy to
atmospheric multi-component aerosol. We go on to study the fundamental process of aerosol coagulation in
aerosol particle arrays, observing the relaxation of non-spherical composite particles formed on coalescence.
We demonstrate the use of bright-field imaging and elastic light scattering to make measurements of the
timescale for the process of binary coalescence contrasting the rheological properties of aqueous sucrose and
sodium chloride aerosol over a range of relative humidities.
Aerosols play a crucial role in many areas of science, ranging from atmospheric chemistry and physics, to
drug delivery to the lungs, combustion science and spray drying. The development of new methods to
characterise the properties and dynamics of aerosol particles is of crucial importance if the complex role that
particles play is to be more fully understood. Optical tweezers provide a valuable new tool to address
fundamental questions in aerosol science. Single or multiple particles 1-15 μm in diameter can be
manipulated over indefinite timescales using optical tweezing. Linear and non-linear Raman and fluorescence
spectroscopies can be used to probe a particle's composition and size. In this paper we will report on the latest
developments in the use of holographic optical trapping (HOT) to study aerosols. Although widely used to
trap and manipulate arrays of particles in the condensed phase, the application of HOT to aerosols is still in its
infancy. We will explore the opportunities provided by the formation of complex optical landscapes for
controlling aerosol flow, for comparing the properties of multiple particles, for performing the first ever
digital microfluidic operations in the aerosol phase and for examining interparticle interactions that can lead
to coalescence/coagulation. Although aerosol coagulation is the primary process driving the evolution of
particle size distributions, it remains very poorly understood. Using HOT, we can resolve the time-dependent
motion of trapped particles and the light scattering from particles during the coalescence process.
Two counter-propagating Bessel beams are used to create an optical trap to confine polydisperse aerosol droplets. A
single arm can be used to optically guide droplets over macroscopic distances. Two opposing beams create a trapping
region to optically confine particles over distances of 4mm. Droplets are optically trapped in the surrounding rings and
the central core and are characterised using light scattering techniques. The elastically scattered fringe spacing from the
532nm trapping beam and from a 633nm probe beam are used to independently size droplets using Mie theory, as well as
assessing the size from glare spots.
Aerosols play a crucial role in many areas of science, ranging from atmospheric chemistry and physics, to
pharmaceutical aerosols and drug delivery to the lungs, to combustion science and spray drying. The
development of new methods for characterising the properties and dynamics of aerosol particles is of crucial
importance if the complex role that particles play is to be more fully understood. Optical tweezers provide a
valuable new tool to address fundamental questions in aerosol science. Single or multiple particles 1-15 μm in
diameter can be manipulated for indefinite timescales. Linear and non-linear Raman and fluorescence
spectroscopies can be used to probe particle composition, phase, component mixing state, and size. In
particular, size can be determined with nanometre accuracy, allowing accurate measurements of the
thermodynamic properties of aerosols, the kinetics of particle transformation and of light absorption. Further,
the simultaneous manipulation of multiple particles in parallel optical traps provides a method for performing
comparative measurements on particles of different composition. We will present some latest work in which
optical tweezers are used to characterise aerosol dynamics, demonstrating that optical tweezers can find
application in studies of hygroscopicity, the mixing state of different chemical components, including the
phase separation of immiscible phases, and the kinetics of chemical transformation.
Aerosol droplets are guided over mm distances using single beam optical traps. The micron-sized particles are confined in two dimensions and guided along the direction of beam propagation. Both Gaussian and Bessel beam geometries are compared for water, ethanol and dodecane droplets. The observed trapping of multiple droplets in 1-D arrays will also be discussed.
Aerosol droplets are trapped and manipulated with a single-beam gradient-force optical trap for timescales of hours. By coupling the optical trap with cavity enhanced Raman scattering, the size of the trapped droplet can be determined with nanometre accuracy and high time resolution. This allows the evolution in droplet size and composition to be monitored during the growth or evaporation of a single trapped droplet, providing a method for characterising the factors that govern aerosol droplet size. The simultaneous trapping of two or more aerosol droplets in parallel optical traps can permit studies of aerosol coagulation.