Optical manipulation of aerosol particle arrays

Abstract

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.

Exploring the complexity of aerosol particle properties and processes using single particle techniques

“…32,86,87 The optical trap can be loaded from a standard ultrasonic nebuliser or using droplet-on-demand generators. 88 The gradient trap is limited to particles that are approximately spherical in shape, although crystalline, solid, and aspherical particles can be still studied. 48,86,[89][90][91] Arrays of aerosol droplets can be manipulated in real time using holographic optical tweezers.…”

Probing the micro-rheological properties of aerosol particles using optical tweezers

“…Loading AOTs from medical nebulizers requires large quantities of the bulk solution (a few ml), considerably larger than the quantity of exemplar secondary organic aerosol (SOA) that can be typically generated in chamber studies (a few tens of µl). This precludes the possibility of measuring the viscosity of truly representative SOA at this time although the loading of optical traps with droplet-ondemand generators, requiring extremely small samples, has been demonstrated [181]. Instead, a range of binary and ternary mixtures of organic and inorganic solutes have been used to benchmark the technique.…”