The physical properties of aerosol define how they respond to changes in conditions, and describes the equilibrium state and the dynamics associated with approaching equilibrium. The phase morphology of aerosol describes the state of the condensed phase of the aerosol – whether particles are liquid, solid or exhibit a more complex phase morphology. Water is the main factor in the atmosphere that regulates the phase of the aerosol, however other factors such as temperature and pressure can have a more significant influence in industrial processes. For liquid droplets, the equilibrium state is defined by the vapor pressure, chemical activity (or simply concentration of constituent chemical species) and surface tension. The rate at which aerosol gain or lose mass by condensation or evaporation is determined by the equilibrium state and a number of kinetic factors describing the rate of transport of material and heat in the bulk, across the interface and in the gas phase.
The factors which influence the physical state of an aerosol are summarized here. In the atmosphere, understanding the physical state of aerosol and their dynamic response to changes in environmental conditions is important in order to accurately predict their time-dependent influence on the climate. For instance, the interaction of aerosol particles with water influences their activation to cloud droplets.
A full understanding of the thermodynamic equilibrium properties and the kinetics of heat and mass transfer in aerosol will lead to better estimates of cloud droplet number concentrations and, ultimately, the influence of clouds on climate. In pharmaceutics, quantifying the rate and magnitude of water uptake following inhalation of a medicinal aerosol is important to be able to predict the deposition pattern within the lungs. In spray drying, where the rate of evaporation influences the morphology of the dry particles, understanding the heat and mass transport dynamics leads to an improved understanding of the factors which influence the properties of spray-dried powders.
Figures © 2014 James Davies