85th European Study Group with Industry

16th–20th April 2012, University of East Anglia, Norwich

Liquid interactions with porous media and the environmental fate of toxic materials

A problem brought to the 85th European Study Group with Industry by the Defence Science & Technology Laboratory.

Problem Description

Toxic liquid chemicals released into the environment may pose an immediate risk to human health through contact or related vapour hazards. However, they can also interact with surfaces and remain in situ, potentially presenting a subsequent hazard. Understanding the fate of these materials in different environments is important in order to understand how long they may persist and the likely impact they may have. Understanding the hazard may allow it to be avoided or mitigated.

Experimental studies can provide information on persistence and behaviour for individual liquid-substrate pairings. However, there is a need to understand how the fundamental physical and chemical properties of substrates and chemicals determine their behaviour so that predictions can be made when extrapolating to other materials. This information is also important in selecting simulants with lower toxicity that can be used more easily in experimental tests.

Study Group Report

The study group investigated interactions between liquid droplets and porous media across a range of different time scales. Splashing and the subsequent re-entrainment of micro-droplets into the atmosphere was identified as one possible mechanism though which the area effect of a contamination could be significantly increased. The study group looked at experimentally determined splashing thresholds for droplet impacts with impermeable substrates, to determine initial predictions of whether or not a given droplet will splash.

In cases where splashing occurs the droplet inertia is the most significant effect driving the initial phase of the liquid infiltration into a porous media and the study group developed a model to investigate this behaviour. For longer time scales the study group determined that capillary suction played the most significant role in spreading the liquid within the porous medium. Models for the evolution of the partial saturation within a porous medium based on Richards' equation were investigated.

Over even longer time scales evaporation converts the liquid back into a potentially hazardous vapour. The study group started to incorporate evaporation into models of liquid infiltration in a porous medium in order to describe this phenomenon. Recommendations for future theoretical, numerical and experimental modelling are also provided.