Description
Soil evaporation is a globally significant process which returns a fifth of terrestrial precipitation directly to the atmosphere. As soil is a porous medium composed of aggregates that create pores filled with water and air, this large scale process is governed by microscale air-water interface dynamics. An efficient drying of soil is ensured by capillary flows along water films which must be continuous from the top of the soil down to the evaporative front [1]. Evaporation is therefore sensitive to processes that impact the properties of the interface between air and water, such as adsorption of surface active molecules or changes in grain wettability.
Soil is rich in bacterial life, with a typical number of $10^{10}$ bacteria per gram of top soil. These many bacteria are also diverse, with some strains having strong affinity to air-water interfaces. As part of their colonisation of interface, some bacteria produce biosurfactants which alter significantly the surface tension of air-water interfaces. Later, biofilms forming there can confer visco-elastic properties to these interfaces, changing fundamentally their responses to pressure variations [2]. My thesis project investigates how bacterial colonization of air-water interfaces and the subsequent changes of their mechanical properties impact drying dynamics.
To this end, we propose a capillary microfluidic device to model a soil pore with an open air-water interface under evaporative forcing at one end. We control the water pressure in the microfluidic device to simulate a varying evaporative front, and confirm that our design reproduce sudden interface jumps at critical pressures as in Haine's jumps in soil. Initially, the characterization of water loss is necessary to understand the drying dynamics in our model [3]. We characterize the controlled drying rate at the air-water interface that can be achieved in this device by varying the relative humidity of a forcing air-flow inside the channel. As a model soil bacterium, we demonstrate that Bacillus subtilis can significantly modify the interfacial properties that are key to the pinning of the evaporative interface, due to the release of the biosurfactant surfactin [2] into the water phase. In our microfluidic device, such release leads to interface properties such as surface tension varying with time as surfactant progressively accumulates at the interface as a result of evaporation-driven flow and continuous surfactant production. Futhermore, the evaporative flow is favorable to biofilm development, associated to further changes of the mechanical properties of the air-water interface. Our device potential opens the door to probing in-situ the mechanical properties of these modified air-water interfaces under different drying conditions, and can shed light on how bacterial activity modifies their dynamics, with implication for understanding larger scale impact on drying soils.
[1] Or et al., 2013, Vadose Zone Journal
[2] Ron and Rosenberg, 2001, Environmental Microbiology
[3] Dollet et al., 2019, Journal of The Royal Society Interface