Fluid Physics: Fluid and interface physics

Fluid physics

54th ESA Parabolic Flight Campaign

23 May 2011

E. Cammeraat ⁽¹⁾, S.J. de Vet ⁽¹⁾, L. Mulder ⁽¹⁾, C. Giannopapa ⁽²⁾

-

-

Ever returned from your holiday and found out that your plants have been deprived of water for too long? In a last desperate effort to save their lives you might consider pouring some water on the soil, only to find out the water is not absorbed and the majority probably ends up next to the pot. This form of water repellency is a very common problem for soils in nature. Although it is inconvenient for your plants at most, the effects on landscape scales
can be devastating. The common view is held that organic compounds of plants and fungi coat sand particles with a minute layer of organic molecules. Adding warmth or drought stress in the form of e.g. forest fires, poor land management or effects resulting from climate change allows this coating to become a water repelling layer. Soil scientists understand these effects very well on the scales of landscapes, sand heaps and the involved chemistry.
However, little is known about how the large scale properties of water repellency are reflected at the intermediate scale of individual sand particles. This scale has often been difficult to measure yet it is possibly the most important, as it ties the smallest scales of molecules to the large scale processes observed in landscapes. The aim of the ARID experiment is to advance water repellency research within this niche by studying particle-scale interactions in microgravity.

The water repellent coating of a sand grain consist of amphiphilic molecules; organic residues from plants and fungi which poses a charged polar head, and an uncharged a-polar tail. In moist soil conditions, the coatings will have the polar heads sticking out and water can easily adhere to these surfaces. Under drought stress a remarkable process occurs as the soil particles become water repellent when the molecular coating ‘flips’ and the a-polar tails are oriented to the exterior. The same soil particles, yet two totally different states depending on environmental conditions. Although never measured directly, is has been assumed for decades that this dynamic process is causing water repellency.
The ARID experiment is an innovative approach to studying the properties of the coating orientation at the level of an individual sand particle. In the experiment the hypothesised orientation and associated charge properties of the coating’s state are utilised by studying collisions between individual sand particles. When charged (non-water repellent) particles collide, they will slightly repel each other and these collisions thus differ from those with
‘neutral’ grains (water repellent or stripped from their coating). The only suitable research environment for observing such collisions, free of interfering processes, is in microgravity.
Kinetics of these collisions are used to make the link between the chemical and large scale soil properties. The collisions are started by giving the test tubes a starting velocity using a special launch platform. Sand and sand-analogue particles with a normal coating, water repellent coating and stripped from a coating will be used and cover different particles sizes and vegetation. Their water repellent properties are measured pre-flight under a macroscope to measure the contact angle with a water droplet. Once the particles are free-floating and colliding in microgravity, three high-speed camera’s will record the collisions with high precision in 3D. Post-flight data processing then allows the use of the microgravity data as a bridge, linking chemical and field studies in one broad approach to studying the mechanisms of soil water repellency.

Soil water repellency is a growing problem in many European countries within the context of our changing global climate. This environmental challenge has a deeply-rooted societal component. In the Mediterranean areas the combination of changing precipitation patterns resulting from climate change lead to higher drought stress and intensified precipitation.
During torrent-events agricultural areas are easily eroded by mudflows and other forms of erosion when the large amount of water cannot infiltrate the soil. This increased overland flow of water also increases natural hazards for nature, wildlife and the local population.
Soil water repellency does not only affect Mediterranean areas in Spain, France, Italy and Greece, but it is common too in northern countries such as The Netherlands and Denmark.
Increased erosion resulting from water repellency in coastal defence landforms such as the Dutch dune systems are of particular relevance for the security and livelihood in these low countries.
Means for combating and mitigating water repellency are being studied world-wide. Understanding the broad-scale dimension of water repellency allows more effective strategies to be developed. The results of the ARID experiment fit well within the larger context and contributes to the linking and understanding of large scale effects of water repellency in relation to the small microscale of the coating chemistry. New insights gained by the application of the microgravity experiment data can improve our understanding of soil water repellency
and may influence the development of better mitigation techniques and land management strategies.

-

Olivier Minster (e-mail: olivier.minster@esa.int)