Ruth Signorell

1. Electrically charged droplets are ubiquitous in natural environments and are often generated in the laboratory or during industrial processes. In the Earth’s atmosphere, microscopic droplets rea...

Aromatic molecules at water droplet surfaces: photoelectron imaging quantifies how interfacial hydration reshapes electronic structure and how the vast surface-to-volume ratio of submicron droplets alters surfactant partitioning.

Absorbing aerosols, such as brown carbon (BrC) and absorbing secondary organic aerosols (SOA), have attracted broad interest due to their importance for climate and human health. The pronounced time-dependence of light absorption during aging renders the precise estimation of their impact on global warming difficult. Single particle studies of such aerosols would be very useful to better understand their aging in the atmosphere through processes such as photochemistry. Previously proposed optical traps cannot continuously trap particles whose absorption state changes from strongly absorbing to non-absorbing or vice versa. Some of the traps presented can isolate absorbing and non-absorbing particles, but require mechanical alignment of the trap depending on the strength of particle absorption. However, mechanical realignment is not compatible with continuous trapping and observation. Here, we introduce a universal optical trap which does not require mechanical realignment. The versatility of the trap relies on four trapping beams - either vortex Laguerre-Gaussian (LG) or fundamental Gaussian beams - which are modulated with a spatial light modulator (SLM). The performance of the trap is demonstrated by trapping different types of absorbing and non-absorbing particles. We also show that the trap can be used to observe the photochemical reaction of aqueous droplets containing fulvic acid, a common component of BrC. Digital holography measurements demonstrate that the confinement of the particles in the trap can be controlled by changing the orbital angular momentum (OAM) of the LG beams. Spectroscopic measurements, such as fluorescence and Raman scattering, are shown to be possible in all configurations of the proposed trap.

Carbon dioxide (CO ₂ ) accelarates gas-phase butane nucleation in a binary gas mixture.

Understanding gas-particle mass transport is essential for predicting aerosol behavior in the atmosphere and in industrial processes. The mass accommodation coefficient, α_M, is a key parameter describing this exchange. Defined as the probability of a gas-phase molecule adhering to a particle upon collision, α_M presents 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 α_M, 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 10³ elementary charges─exceeding typical atmospheric aerosol charge levels─yet our results show that α_M is 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.

Urea is a key molecule in the search for the origin of life and a basic chemical produced in large quantities by industry. Its formation from ammonia and carbon dioxide requires either high pressures and temperatures or, under milder conditions, catalysts or additional reagents. In this study, we observed the spontaneous formation of urea under ambient conditions from ammonia and carbon dioxide in the surface layer of aqueous droplets. Single, optically trapped droplets were probed by using Raman bands as markers. We found the surface layer to act like a microscopic flow reactor, with chemical gradients providing access to unconventional reaction pathways. This observation revealed a general mechanistic scheme for distinctive droplet chemistry. Interfacial chemistry is a possible nonenergetic route for urea formation under prebiotic conditions.

Many atmospheric aerosol particles have inhomogeneous chemical compositions, resulting in pronounced differences in their optical properties compared with those of homogeneous particles. It has been proposed to approximate the optical properties of inhomogeneous particles by those of spherical core–shell particles because this includes aspects of the particle inhomogeneities while still being computationally feasible to be implemented in radiative forcing models. Core–shell particles are also receiving a lot of interest to better understand processes creating inhomogeneous particles, such as liquid–liquid phase transition, coagulation, and condensation. However, there are still no systematic studies of the influence of volume and the refractive index of core and shell on scattering and absorption of light by core–shell particles. It also remains unclear under which conditions absorption and scattering by core–shell particles can be approximated by (equivalent) homogeneous particles. In this study, we present systematic simulation results for absorption and scattering of light by core–shell particles. Absorption cross-sections of core–shell particles depend mainly on the volumes and the imaginary part of the refractive index of core and shell, while the real part of the refractive index has only a moderate influence on absorption. Scattering cross-sections of core–shell particles depend strongly on all particle properties and vary substantially. The study highlights general trends found for the absorption and scattering cross-section as a function of core and shell properties. General conditions are formulated that allow one to assess when core–shell particles have optical properties similar to those of homogeneous particles. The results also provide a guideline for using optical properties of core–shell particles in the analysis of light scattering measurements and for radiative forcing and climate models.

Electron scattering cross sections are essential for the quantitative description of electron-induced processes in nanoconfined aqueous systems. Here we report on energy- and size-dependent cross sections for water clusters with diameters between ∼ 1.0 and 2.5 nm and electron energies below ∼ 32 eV. The cross sections of the clusters are up to a factor of 2 larger than those of water, suggesting a reduced dielectric screening in nanoconfinement. The cross sections show no significant cluster size dependence. A slight increase of cross sections with increasing electron energy is attributed to the increasing contribution from electronic scattering channels. As the simplest form of nanoconfined water, nanoscale water clusters represent an important model system. Published by the American Physical Society 2025

Urea is a key molecule in the search for the origin of life and a basic chemical produced in large quantities by industry. Its formation from ammonia and carbon dioxide requires either high pressures and temperatures or, under milder conditions, catalysts or additional reagents. Here we report the spontaneous formation of urea under ambient conditions from ammonia and carbon dioxide in the surface layer of aqueous droplets. Single optically-trapped droplets were probed using Raman bands as markers. We found the surface layer to act like a microscopic flow reactor with chemical gradients providing access to unconventional reaction pathways. This reveals a general mechanistic scheme for unique droplet chemistry. Interfacial chemistry is a possible non-energetic route for urea formation under prebiotic conditions.