8 research outputs found
A Simple Analytical Model of Evaporation in the Presence of Roots
Root systems can influence the dynamics of evapotranspiration of water out of
a porous medium. The coupling of evapotranspiration remains a key aspect
affecting overall root behavior. Predicting the evapotranspiration curve in the
presence of roots helps keep track of the amount of water that remains in the
porous medium. Using a controlled visual set-up of a 2D model soil system
consisting of monodisperse glass beads, we first perform experiments on actual
roots grown in partially saturated systems under different relative humidity
conditions. We record parameters such as the total mass loss in the medium and
the resulting position of the receding fronts and use these experimental
results to develop a simple analytical model that predicts the position of the
evaporating front as a function of time as well as the total amount of water
that is lost from the medium due to the combined effects of evaporation and
transpiration. The model is based on fundamental principles of evaporation flux
and includes empirical assumptions on the quantity of stoma in the leaves and
the transition time between regime 1 and regime 2. The model also underscores
the importance of a much prolonged root life as long as the root is exposed to
a partially saturated zone composed of a mixture of air and water. Comparison
between the model and experimental results shows good prediction of the
position of the evaporating front as well as the total mass loss from
evapotranspiration in the presence of real root systems. These results provide
additional understanding of both complex evaporation phenomenon and its
influence on root mechanisms.Comment: 10 pages, 6 figure
Kinetics of Gravity-Driven Water Channels Under Steady Rainfall
We investigate the formation of fingered flow in dry granular media under
simulated rainfall using a quasi-2D experimental set-up composed of a random
close packing of mono-disperse glass beads. Using controlled experiments, we
analyze the finger instabilities that develop from the wetting front as a
function of fundamental granular (particle size) and fluid properties
(rainfall, viscosity).These finger instabilities act as precursors for water
channels, which serve as outlets for water drainage. We look into the
characteristics of the homogeneous wetting front and channel size as well as
estimate relevant time scales involved in the instability formation and the
velocity of the channel finger tip. We compare our experimental results with
that of the well-known prediction developed by Parlange and Hill [1976]. This
model is based on linear stability analysis of the growth of perturbations
arising at the interface between two immiscible fluids. Results show that in
terms of morphology, experiments agree with the proposed model. However, in
terms of kinetics we nevertheless account for another term that describes the
homogenization of the wetting front. This result shows that the manner we
introduce the fluid to a porous medium can also influence the formation of
finger instabilities.Comment: 13 pages, 7 figure
Microfluidics as a tool to assess and induce emulsion destabilization
Microfluidic technology enables judicious control of the process parameters on a small length scale, which in turn allows speeding up the destabilization of emulsion droplets interface in microfluidic devices. In this light, microfluidic channels can be used as an efficient tool to assess emulsion stability and to observe the behavior of the droplets immediately after their formation, enabling to determine whether or not they are prone to re-coalescence. Observation of the droplets after emulsifier adsorption also allows the investigation of emulsion stability over time. Both evaluations would contribute to determine emulsion stability aiming at specific applications in food and pharmaceutical industries. Furthermore, emulsion coalescence can also be performed under extremely controlled conditions within the microfluidic devices in order to explore emulsion droplets as micro-reactors (for regulated biological and chemical assays). Such microfluidic procedures can be performed either in confined environments or under dynamic flow conditions. Under confined environments, droplets are observed in fixed positions simulating different environmental conditions. On the other hand, with the scrutiny of emulsions under dynamic flow processes, it is possible to determine the behavior of the droplets when subjected to shear forces, comparable to those experienced in conventional emulsification techniques or even in pumping operations. Given the above, this paper reviews different microfluidic techniques (such as changing channel geometry or wettability) hitherto used to destabilize emulsions, mainly focusing on the specificities of each study, whether the droplets are destabilized in confined or dynamic flow processes. Thereby, by going deeper into this review, readers will be able to identify different strategies for emulsion destabilization (in order to understand stabilizing mechanisms or even to apply these droplets as micro-reactors), as this paper shows the particularities of the most recent studies and elucidates the current state-of-the-art of this microfluidic-related application
Preferential Root Tropism Induced by Structural Inhomogeneities in 2D Wet Granular Media
11 pages, 6 FiguresWe investigate certain aspects of the physical mechanisms of root growth in a granular medium and how these roots adapt to changes in water distribution induced by the presence of structural inhomogeneities in the form of solid intrusions. Physical intrusions such as a square rod added into the 2D granular medium modify water distribution by maintaining robust capillary action, pumping water from the more saturated areas at the bottom of the cell towards the less saturated areas near the top of the cell while the rest of the medium is slowly devoid of water via evaporation. This water redistribution induces `preferential tropism' of roots, guiding the roots and permitting them to grow deeper into more saturated regions in the soil. This further allows more efficient access to available water in the deeper sections of the medium thereby resulting to increased plant lifetim
Particle Deposition Kinetics of Colloidal Suspensions in Microchannels at High Ionic Strength
Despite its considerable
practical importance, the deposition of
real Brownian particles transported in a channel by a liquid, at small
Reynolds numbers, has never been described at a comprehensive level.
Here, by coupling microfluidic experiments, theory, and numerics,
we succeed in unravelling the problem for the case of straight channels
at high salinity. We discover a broad regime of deposition (the van
der Waals regime) in which particle–wall van der Waals interactions
govern the deposition mechanism. We determine the range of existence
of the regime, for which we calculate the concentration profiles,
retention profiles, and deposition kinetics analytically. The retention
profiles decay as the inverse of the square root of the distance from
the entry, and the deposition kinetics are given by the expression S≈(A2.1kTξL)1/2, where <i>S</i> is a dimensionless
deposition function, <i>A</i> is the Hamaker constant, and
ξ<sub>L</sub> is a dimensionless parameter characterizing fluid
flow properties. These findings are well supported by numerics. Experimentally,
we find that the retention profiles behave as <i>x</i><sup>–0.5±0.1</sup> (where <i>x</i> is the distance
from the channel entry) over three decades in scale, as predicted
theoretically. By varying the flow conditions (speed, geometry, surface
properties, and concentration) so as to cover four decades in ξ<sub>L</sub> and taking the Hamaker constant as a free parameter, we accurately
confirm the theoretical expression for the deposition kinetics. Operating
in the van der Waals regime enables control of the deposition rates
via surface chemistry. From a surface science perspective, working
in the van der Waals regime enables us to measure the Hamaker constants
of thousands of particles in a few minutes, a task that would take
a much longer time to perform with standard AFM