Abstract
Many processes taking place in atmospheric aerosol particles are accompanied by changes in the particles’ morphology (size and shape), with potentially significant impact on weather and climate. However, the characterization of dynamic information on particle morphology and position over multiple time scales from microseconds to days under atmospherically relevant conditions has proven very challenging. Here we introduce holographic imaging of unsupported aerosol particles in air that are spatially confined by optical traps. Optical trapping in air allows contact-free observation of aerosol particles under relevant conditions and provides access to extended observation times, while the digital in-line holographic microscope provides six-dimensional spatial maps of particle positions and orientations with maximum spatial resolution in the sub-micron range and a temporal resolution of 240 μs. We demonstrate the broad applicability of our approach for a few examples and discuss its prospects for future aerosol studies, including the study of complex, multi-step phase transitions.
Highlights
Many processes taking place in atmospheric aerosol particles are accompanied by changes in the particles’ morphology, with potentially significant impact on weather and climate
The experimental setup consists of a digital in-line holographic microscope combined with a counterpropagating optical tweezer (CPT; 532 nm diode laser)[22,50,51] for spatial confinement of a single particle or multiple aerosol particles in air
The digital in-line holographic microscope combined with optical traps provides high time (240 μs) and spatial (770 nm) resolution imaging of unsupported, submicron aerosol particles
Summary
Many processes taking place in atmospheric aerosol particles are accompanied by changes in the particles’ morphology (size and shape), with potentially significant impact on weather and climate. Three-dimensional (3D) morphologies can be retrieved by imaging particles from different angles, for example, with Fourier ptychography[29,30] or optical diffraction tomography[31], or by scanning the whole particle volume using confocal imaging[32] These methods suffer from reduced temporal resolution in the range of milliseconds or seconds because several measurements are required to acquire the whole 3D information. DH is very well suited to image fast moving objects, such as aerosol particles, because the imaged particles only need to be quasi-immobile during the exposure time of each hologram, which typically is several 10 ns for pulsed lasers[34] and about 10 μs for continuous lasers[39] Even though this has so far only been demonstrated for objects suspended in liquids[40,41], DH can provide movies of the 6D spatial motion of non-spherical objects suspended in air (3D for the position and 3D for the orientation). The performance of the new experimental approach is demonstrated for different cases, which include the fast imaging of the morphology and the translation and rotational motion of a peanut-shaped particle, the determination of optical forces that act on a particle from imaging of the particle’s trajectory, and the complex dynamics of an optically bound particle pair
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