Digital holography of optically-trapped aerosol particles

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.

Morphology and motion of single optically trapped aerosol particles from digital holography

“…The experimental setup and procedure to reconstruct the shape of the trapped particles used in this study have already been presented elsewhere 25 . They will only be discussed briefly here.…”

“…bright field and dark field 24 ) are limited because of their fixed imaging plane. This renders the study of particle dynamics virtually impossible because aerosol particles are very mobile even if they are confined in optical traps 25 . Furthermore, these methods can only provide two-dimensional (2D) images of the particle morphology.…”

“…As an alternative, digital holography (DH) provides the particle's 3D morphology [30][31][32] and 3D position 33 from each measured hologram because the reconstruction can be done at any arbitrary imaging plane 34 . DH can also measure the 6D motion of nonspherical objects (3D for the position and 3D for the rotation) suspended in liquid 33,35,36 or in air 25 . In addition, holography is very well suited to image fast moving objects, such as an aerosol particle, because the imaged particle only needs to be quasi-immobile during the duration of the exposure time of each hologram, typically several 10 ns for pulsed lasers 37 and about 10 μs for continuous lasers 38 .…”

Automated droplet size distribution measurements using digital inline holography

“…DIH has been recently demonstrated for the analysis of the motion and morphology of individual aerosol particles (Berg & Holler, 2016;David et al, 2018;Giri et al, 2019). DIH has also been used successfully employed in a diverse array of fields, including the study of social behaviors of flies (Kumar et al, 2016), snowflake size distribution function and morphology (Nemes et al, 2017), atmospheric mixing and cloud formation (Beals et al, 2015), ocean sediment particle size and velocity transport (Graham & Nimmo Smith, 2010), oil droplets in oceans (Li et al, 2017), bubbly wake behind a ventilated supercavity (Shao et al, 2019), and preliminarily (without automated processing), droplets from sprays in compressible flow cross wind (Olinger et al, 2014).…”

Characterization and control of droplets optically trapped in air

“…In addition, these experiment can be coupled to obtain a more thorough characterization of single particles. The BLS 15,23 , Raman scattering 15 , photoacoustics 7,20,24 and digital holography 25,26 experiments have already been discussed in details elsewhere and will not be discussed further here. This paper explains the feedback control mechanism and the polarizationresolved TAOS measurements.…”