Michael Gleichweit

Last Name

Gleichweit

First Name

Michael

ORCID

0000-0001-8562-5023

Organisational unit

03961 - Signorell, Ruth / Signorell, Ruth

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Publications 1 - 10 of 11

Photoacoustics of single aerosol droplets immobilised by counter-propagating optical tweezers
Diveky, Matus; Gleichweit, Michael; Roy, Sandra; et al. (2020)
Optical Trapping and Optical Micromanipulation XVII ~ Proceedings Volume c, Optical Trapping and Optical Micromanipulation XVII; 1146316 (2020)
In this contribution we discuss the influence of relative humidity on photoacoustic measurements from both an experimental and theoretical perspective. We present a refined model of the photoacoustic (PA) signal that accounts for elevated particle temperatures and different levels of relative humidity. We use this new model together with the photoacoustic data collected with our photothermal single-particle spectrometer (PSPS) to retrieve the mass accommodation coefficients of water on organic aerosol particles. The single-particle nature of our experiments is achieved by employing counter-propagating tweezers. Furthermore, we investigate the influence of relative humidity on the eigenfrequency of the PA cell.
Electric Charge Effect on the Water Mass Transfer across Mixed Aqueous–Organic Droplet Interfaces
Azizbaig Mohajer, Mercede; Gleichweit, Michael; David, Grégory; et al. (2025)
The Journal of Physical Chemistry A
Understanding gas-particle mass transport is essential for predicting aerosol behavior in the atmosphere and in industrial processes. The mass accommodation coefficient, αM, is a key parameter describing this exchange. Defined as the probability of a gas-phase molecule adhering to a particle upon collision, αMpresents a highly surface-sensitive property. Aqueous atmospheric aerosol droplets carry electric charges, which accumulate near the surface; however, their influence on the gas-particle mass transport remains elusive. To access the influence of charge on αM, we combined an aerosol charging method with photothermal single-particle spectroscopy, enabling direct water mass exchange measurements on the surface of single aerosol droplets. We investigated charged and neutral aqueous glycerol and tetraethylene glycol droplets across a wide range of concentrations and temperatures. The micrometre-sized droplets carried approximately 10³elementary charges─exceeding typical atmospheric aerosol charge levels─yet our results show that αMis independent of the droplet charge and instead is dominated by composition and temperature. Theoretical estimates of the charge-dipole and dipole–dipole interaction energies corroborate this finding, highlighting that under atmospherically relevant conditions, electric charge plays a negligible role in the mass accommodation process.
Coupling between modulated Mie scattering and photoacoustic signal generation in optically trapped, single aerosol particles
Gleichweit, Michael; Azizbaig Mohajer, Mercede; Borgeaud, Dominique; et al. (2022)
Proceedings of SPIE ~ Optical Trapping and Optical Micromanipulation XIX
Photoacoustic spectroscopy and photothermal spectroscopy are two common methods to probe aerosol particle absorption coefficients and can be performed both on aerosol ensembles and on the single particle level. With photothermal spectroscopy typically changes in the particle’s light scattering pattern upon heating or cooling are observed with photo-diodes or cameras. In photoacoustic spectroscopy, the acoustic response to periodic light absorption is recorded e.g. with a microphone. Although both methods are closely related through their excitation process, the detection pathways are quintessentially different. In our single particle optical trapping setup, however, we observe a previously unnoticeable, unidirectional coupling between modulated Mie scattering (result of the photothermal effect) and photoacoustic spectroscopy. The coupling manifests itself via differently shaped, sudden features in the acoustic signal. Our analysis suggests a non-trivial interaction between light scattering of single, optically trapped particles and the photoacoustic signal generation based on interactions of light with the acoustic resonator’s walls. Measurements over several trapping powers and photoacoustic excitation powers support this conclusion. How the coupling manifests itself, such as shape and strength, can be conclusively explained by the structure of the particle’s momentary phase function (scattering intensity) calculated by classical Mie theory. This allows us to formulate conditions to either utilise or minimise the coupling effects in future experiments.
A Multilayer Heat and Mass Transfer Model for Photoacoustics of Aerosol Particles (MHM-PA)
Corral Arroyo, Pablo; Gleichweit, Michael; Diveky, Matus; et al. (2023)
Aerosol Science and Technology
A diffusion multi-layer model is introduced which allows time-dependent simulations of heat and mass transfer for photoacoustic experiments on aerosol particles (MHM-PA model). It provides spatial and temporal information on the particle temperature, composition and size during photoacoustic modulation cycles, and thus enables accurate simulations of the photoacoustic amplitude and phase at different modulation frequencies. Simulations for micrometer-sized, mixed water-tetraethylene glycol droplets at different relative humidity and at different modulation frequencies between 4 and 100 kHz reveal a substantial loss of the information content of the photoacoustic signal above ∼30 kHz. The photoacoustic amplitude reduces by two orders of magnitude, causing sensitivity issues. The photoacoustic phase converges to the same constant value for all droplet sizes and humidities, meaning that useful information on heat and mass transfer is essentially lost in the phase.
Shining new light on the kinetics of water uptake by organic aerosol particles
Diveky, Matus; Gleichweit, Michael; Roy, Sandra; et al. (2021)
The Journal of Physical Chemistry A
The uptake of water vapor by various organic aerosols is important in a number of applications ranging from medical delivery of pharmaceutical aerosols to cloud formation in the atmosphere. The coefficient that describes the probability that the impinging gas-phase molecule sticks to the surface of interest is called the mass accommodation coefficient, αM. Despite the importance of this coefficient for the description of water uptake kinetics, accurate values are still lacking for many systems. In this Feature Article, we present various experimental techniques that have been evoked in the literature to study the interfacial transport of water and discuss the corresponding strengths and limitations. This includes our recently developed technique called photothermal single-particle spectroscopy (PSPS). The PSPS technique allows for a retrieval of αM values from three independent, yet simultaneous measurements operating close to equilibrium, providing a robust assessment of interfacial mass transport. We review the currently available data for αM for water on various organics and discuss the few studies that address the temperature and relative humidity dependence of αM for water on organics. The knowledge of the latter, for example, is crucial to assess the water uptake kinetics of organic aerosols in the Earth's atmosphere. Finally, we argue that PSPS might also be a viable method to better restrict the αM value for water on liquid water.
Hygroscopic growth of single atmospheric sea salt aerosol particles from mass measurement in an optical trap
Reich, Oliver; Gleichweit, Michael; David, Grégory; et al. (2023)
Environmental Science: Atmospheres
Sea salt aerosol is among the most abundant aerosol species in Earth's atmosphere, and its hygroscopicity is an important parameter to quantify its interaction with solar radiation. Conflicting values for the hygroscopic growth have been reported in the literature, which decreases the accuracy with which their impact on Earth's climate can be modelled. Here we report new values of the hygroscopic growth for a selection of salt compositions representative of atmospheric sea salt. These values are obtained from single optically trapped aqueous droplets with dry radii between 0.3 and 2 mu m, using a recently developed method for single particle mass measurement in an optical trap. We compare our results to earlier studies and propose a way to reconcile the apparent discrepancies found in the literature. Within our studies, we also observe the crystallization of CaSO4 center dot 2H(2)O (Gypsum) during the drying of optically trapped sea salt droplets at significantly larger relative humidity of 65-68% than the main efflorescence relative humidity at 50%. This preceding transition occurred in the absence of any contact of the particle with a surface.
Water transfer between the gas phase and a single aerosol droplet
Gleichweit, Michael (2024)
Aerosol particles and droplets are constantly interacting with water vapour in our atmosphere. Intuitive examples are the formation of clouds, haze or fog, but this interaction also has far-reaching implications on the microscopic scale. Very small aqueous droplets can have a significant impact on the Earth's climate, but they also have various technical or pharmaceutical applications. The goal of this thesis is to gain a fundamental understanding of the two most important aspects of water transfer between the ambient gas phase and a single aerosol droplet. The first aspect is the mass accommodation of water on aqueous organic aerosol droplets. It is characterised by the mass accommodation coefficient αM, defined as the probability of a molecule in the gas phase to enter the condensed phase after colliding with the droplet surface. It is a crucial parameter for understanding the growth dynamics of particles or droplets that are similar in size or smaller than the mean free path of the surrounding gas phase. Knowledge of αM is important, for example, for the modelling of cloud formation or the drug delivery to the lungs. Despite its importance, the question of how temperature and humidity influence αM has received little attention so far. The second aspect is the hygroscopic growth of aerosol droplets. It can be characterised by the hygroscopic mass growth factor m*, defined as the ratio between the droplet's mass at a specific relative humidity and the mass of the dry droplet. Particularly interesting systems are aerosols containing sea salt, which are the most abundant type of aerosol found over oceans and in maritime areas. Despite ongoing research efforts, m* for aqueous sea salt droplets has not yet been quantified with sufficient confidence. This limits the accuracy of climate models, for instance, in quantifying both direct and indirect radiative forcing, or in calculating the critical supersaturation of a droplet. To address these open questions, we employed two recently developed single-particle methods to fully characterise individual droplets and to avoid ensemble averaging effects. On the one hand, photothermal single-particle spectroscopy (PSPS), a highly sensitive absorption spectroscopy technique, allowed us to study the water mass transfer in aqueous organic droplets with an emphasis on αM of water on aqueous triethylene glycol. On the other hand, our recently developed optical balance made it possible to determine new values for m* of different sea salt mixtures. Both methods enabled us to study droplets similar in size to the mean free path of the ambient gas phase. We observed that αM is highly sensitive not only to changes in temperature, but also in water concentration. The dependence of αM on the droplet composition proved to be unexpectedly complex. With our studies of m* for different sea salts, we were able to reconcile inconsistencies regarding existing literature values. Our analysis further emphasises the importance of the detailed composition of an aerosol droplet. For example, low-concentration ions can induce important phase transitions within the droplets which may not occur in simplified mixtures or may be overlooked by ensemble averaging. Yet another important point to consider when studying water mass transfer is the role of electric charges on the droplets. In this thesis, a new instrument is presented for determining the charge of single optically trapped droplets, referred to as the optical electrometer. The first results obtained for charged aqueous sea salt droplets are promising and provide a solid foundation for future research on the impact of charges on αM or m*.
Direct influence of aerosol particles on cavity enhanced spectroscopy: Modeling and first experimental results
Stollberger, Felix W.; Gleichweit, Michael; David, Gregory; et al. (2024)
Aerosol Science and Technology
This article presents the first experimental analysis of the influence of single, micrometer-sized aerosol particles on the reflectivity of a resonant Fabry-Perot etalon (FPE). The results are presented along with a theoretical model capable of describing and predicting the particle influence on the transfer function of the etalon. The presence of the particle in the cavity alters the etalon reflectivity in the operating point up to several percent. Based on our findings, we show that the etalon reflectivity is mainly affected by light extinction by the particle. Phase effects due to interference of forward scattered light could not be observed experimentally in the investigated size range. However, extrapolations on the presented theoretical model show a significant dependence of the phase contribution on the collection angle of the etalon. Comparable magnitudes of the phase and extinction contribution are reached if the etalon geometry is slightly altered. This could be exploited to obtain the phase shift of the coherently forward scattered light. Cavity geometries can also be adapted to reduce the direct particle effect if it is undesired. In addition, the change in the transfer function of the etalon is analyzed and discussed, as it is especially relevant for measurement techniques such as photothermal interferometry.
Unexpected concentration dependence of the mass accommodation coefficient of water on aqueous triethylene glycol droplets
Gleichweit, Michael; Azizbaig Mohajer, Mercede; Borgeaud dit Avocat, Dominique P.; et al. (2024)
Physical Chemistry Chemical Physics
The mass accommodation coefficient alpha M of water on aqueous triethylene glycol droplets was determined for water mole fractions in the range xmol = 0.1-0.93 and temperatures between 21 and 26 degrees C from modulated Mie scattering measurement on single optically-trapped droplets in combination with a kinetic multilayer model. alpha M reaches minimum values around 0.005 at a critical water concentration of xmol = 0.38, and increases with decreasing water content to a value of approximate to 0.1 for almost pure triethylene glycol droplets, essentially independent of the temperature. Above xmol = 0.38, alpha M first increases with increasing water content and then stabilises at a value of approximate to 0.1 at the lowest temperatures, while at the highest temperature its value remains around 0.005. We analysed the unexpected concentration and temperature dependence with a previously proposed two-step model for mass accommodation which provides concentration and temperature-dependent activation enthalpies and entropies. We suggest that the unexpected minimum in alpha M at intermediate water concentrations might arise from a more or less saturated hydrogen-bond network that forms at the droplet surface.
Multi-frequency photothermal interferometry of single aerosol particles
Stollberger, Felix W.; Gleichweit, Michael; Signorell, Ruth; et al. (2025)
Journal of Aerosol Science
The frequency dependence of photothermal and photoacoustic signals provides information on evaporation, condensation, and heat transfer processes in aerosol particles. Performing such measurements at the single particle level increases accuracy and provides access to various particle properties. Previously, this was not possible due to the resonant acoustic signal amplification required in photoacoustics, which restricted usable modulation frequencies to a single value. In this study, we introduce the use of multi-frequency photothermal interferometry (nω-PTI) on single, optically trapped particles and experimentally investigate the frequency dependence of the photothermal signal. The observed signal and its dependence on the optical and thermophysical properties of the particle and the interferometer probe beam are analyzed by an accompanying theoretical model. Our measurements prove the applicability of the presented method and indicate a stronger frequency dependence of the photothermal amplitude from single particles than previously observed in bulk measurements. Furthermore, we were able to decouple the contributions from the particle temperature and the thermal wave propagation and examine their frequency dependencies individually. Finally, we analyzed the direct influence of the particle on the measured signal and showed the potential of frequency-resolved photothermal measurements to study thermophysical parameters or optical properties at the single particle level in the Knudsen transition regime.

Publications 1 - 10 of 11

Michael Gleichweit

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