Speaker
Description
Evaporation of soil is a key hydrological process which returns $20 \%$ of terrestrial precipitation directly to the atmosphere. This large-scale phenomenon is governed at the microscale by capillary flows along water films. Indeed, continuity of these films between the top of the soil and the evaporative front deep inside the soil is essential for efficient drying. Since the fate of these water films depends on the physico-chemical properties of the soil (surface tension of the water phase, contact angle of water phase with grains), evaporation is sensitive to processes which impact interfacial properties between air and water.
The many bacteria in soil – with typical number $\sim 10^{10}$ bacteria per gram of top soil – release into their environment molecules with affinity to air-water interfaces, in particular biosurfactants which can modify surface tension at these interfaces. This raises the question of whether bacterial growth in soil can significantly modify drying dynamics, and thereby opens the door to new strategies for water preservation by modification of the soil microbiome.
As a first approach to this question, we focus on a model soil pore, built as a capillary microfluidic system. This novel device presents an open air-water interface under evaporative forcing at one end, pinned to a sharp ridge, while at the other end pressure is set to emulate a controlled water table depth. The geometry is designed to promote a sudden jump of the interface following depinning, similarly to interfacial dynamics in soil pores. We investigate how the deformation and potential depinning of the air-water interface in this device are modified in presence of bacterial surfactants. We demonstrate that growing Bacillus subtilis, a model soil bacterium, can significantly alter interfacial properties that are key to the pinning of the evaporative interface, by releasing into the water phase the biosurfactant surfactin. From this characterization, we build a mathematical model to provide insight into the expected dynamics of the air-water interface in our experimental device as flow proceeds. These dynamics are controlled by the accumulation of surfactants at the interface due to a coupling between evaporation-driven flow towards the interface and on-going surfactant production by bacteria. Our model allows us to qualitatively predict if and when a jump of the interface will be triggered, that is when a critical surfactant concentration – which itself depends on the geometry and the imposed pressure – is reached at the interface. These experimental and theoretical developments pave the way to further investigation of the impact of bacterial biosurfactants on drying soils.
| References | Or et al., Vadose Zone Journal, 1-16 (2013); Ron et al., Environmental Microbiology, 229-236 (2001) |
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| Country | France |
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