A positive temperature coefficient is the term which has been used to indicate that an increase in solubility occurs as the temperature is raised, whereas a negative coefficient indicates a decrease in solubility with rise in temperature.

Effect of Temperature

The classical Poisson-Boltzmann (PB) theory of electrolytes assumes a dilute solution of point charges with mean-field electrostatic forces. Even for very dilute solutions, however, it predicts absurdly large ion concentrations (exceeding close packing) for surface potentials of only a few tenths of a volt, which are often exceeded, e.g. in microfluidic pumps and electrochemical sensors. Since the 1950s, several modifications of the PB equation have been proposed to account for the finite size of ions in equilibrium, but in this two-part series, we consider steric effects on diffuse charge dynamics (in the absence of electro-osmotic flow). In this first part, we review the literature and analyze two simple models for the charging of a thin double layer, which must form a condensed layer of close-packed ions near the surface at high voltage. A surprising prediction is that the differential capacitance typically varies non-monotonically with the applied voltage, and thus so does the response time of an electrolytic system. In PB theory, the capacitance blows up exponentially with voltage, but steric effects actually cause it to decrease above a threshold voltage where ions become crowded near the surface. Other nonlinear effects in PB theory are also strongly suppressed by steric effects: The net salt adsorption by the double layers in response to the applied voltage is greatly reduced, and so is the tangential "surface conduction" in the diffuse layer, to the point that it can often be neglected compared to bulk conduction (small Dukhin number).

Steric effects in the dynamics of electrolytes at large applied voltages: I. Double-layer charging

A general description of the mathematical and numerical formulations used in modern numerical reactive transport codes relevant for subsurface environmental simulations is presented. The formulations are followed by short descriptions of commonly used and available subsurface simulators that consider continuum representations of flow, transport, and reactions in porous media. These formulations are applicable to most of the subsurface environmental benchmark problems included in this special issue. The list of codes described briefly here includes PHREEQC, HPx, PHT3D, OpenGeoSys (OGS), HYTEC, ORCHESTRA, TOUGHREACT, eSTOMP, HYDROGEOCHEM, CrunchFlow, MIN3P, and PFLOTRAN. The descriptions include a high-level list of capabilities for each of the codes, along with a selective list of applications that highlight their capabilities and historical development.

/pdf/reactive-transport-codes-for-subsurface-environmental-245ig8civz.pdf

Reactive transport codes for subsurface environmental simulation

Spontaneous imbibition of wetting liquids in porous media is a ubiquitous natural phenomenon which has received much attention in a wide variety of fields over several decades. Many traditional and recently presented capillary-driven flow models are derived based on Hagen–Poiseuille (H–P) flow in cylindrical capillaries. However, some limitations of these models have motivated modifications by taking into account different geometrical factors. In this work, a more generalized spontaneous imbibition model is developed by considering the different sizes and shapes of pores, the tortuosity of imbibition streamlines in random porous media, and the initial wetting-phase saturation. The interrelationships of accumulated imbibition weight, imbibition rate and gas recovery and the properties of the porous media, wetting liquids, and their interactions are derived analytically. A theoretical analysis and comparison denote that the presented equations can generalize several traditional and newly developed models fr...

https://eps.utk.edu/faculty/perfect/Papers/Cai%20et%20al.%20(2014).pdf

Generalized Modeling of Spontaneous Imbibition Based on Hagen− Poiseuille Flow in Tortuous Capillaries with Variably Shaped Apertures

Despite having a more advanced water management system than most Middle Eastern countries, similar to the other countries in the region, Iran is experiencing a serious water crisis. The government blames the current crisis on the changing climate, frequent droughts, and international sanctions, believing that water shortages are periodic. However, the dramatic water security issues of Iran are rooted in decades of disintegrated planning and managerial myopia. Iran has suffered from a symptom-based management paradigm, which mainly focuses on curing the problem symptoms rather than addressing the main causes. This paper reviews the current status of water resources in Iran and recognizes three major causes for the current water crisis: (1) rapid population growth and inappropriate spatial population distribution; (2) inefficient agriculture sector; and (3) mismanagement and thirst for development. The country is faced with serious challenges in the water sector, including but not limited to rising water demand and shortage, declining groundwater levels, deteriorating water quality, and increasing ecosystem losses. If immediate actions are not taken to address these issues, the situation could become more tragic in the near future. The paper suggests some crisis exit strategies that need to be immediately adopted to secure sustainable water resources, if Iran does not want to lose its international reputation for significant success in water resources management over thousands of years in an arid area of the world.

/pdf/water-management-in-iran-what-is-causing-the-looming-crisis-4fbm5zwqwu.pdf

Water management in Iran: what is causing the looming crisis?

Pore-scale insights into transport and mixing in steady-state two-phase flow in porous media

When Irvin Fatt wrote his classical paper on pore-network modeling (Fatt 1956), he would
probably not have thought that this field would become one of the largest fields of research in
the porous media discipline. Pore-scale modeling has found its way as an expanding field of
research for understanding the physics of flow and transport in porous media. In addition, it
is becoming a valuable tool for prediction of petrophysical properties as part of the so-called
Digital Rock Physics approaches, thus supplementing and replacing expensive and time consuming
laboratory experiments. The recent popularity of pore-level modeling can also be
attributed to advances in visualization of the pore space, to very high image resolution, and
to the steady increase in computing power. This has made it possible to deal with a multitude
of processes in the pore space and interactions with the solid phase (van Dijke and Piri 2007).
The focus of this special issue of Transport in Porous Media is to provide an overview of
some recent developments of various techniques for pore-scale modeling of multiphase flow
and reactive transport

/pdf/pore-scale-modeling-of-multiphase-flow-and-transport-256duiv10b.pdf

Pore-Scale Modeling of Multiphase Flow and Transport: Achievements and Perspectives

There are abundant examples of natural, engineering and industrial applications, in which “solute transport” and “mixing” in porous media occur under multiphase flow conditions. Current state-of-the-art understanding and modelling of such processes are established based on flawed and non-representative models. Moreover, there is no direct experimental result to show the true hydrodynamics of transport and mixing under multiphase flow conditions while the saturation topology is being kept constant for a number of flow rates. With the use of a custom-made microscope, and under well-controlled flow boundary conditions, we visualized directly the transport of a tracer in a Reservoir-on-Chip (RoC) micromodel filled with two immiscible fluids. This study provides novel insights into the saturation-dependency of transport and mixing in porous media. To our knowledge, this is the first reported pore-scale experiment in which the saturation topology, relative permeability, and tortuosity were kept constant and transport was studied under different dynamic conditions in a wide range of saturation. The critical role of two-phase hydrodynamic properties on non-Fickian transport and saturation-dependency of dispersion are discussed, which highlight the major flaws in parametrization of existing models.

/pdf/hydro-dynamic-solute-transport-under-two-phase-flow-2cee5s5dnc.pdf

Vahid Joekar-Niasar

Papers

Pore-scale insights into transport and mixing in steady-state two-phase flow in porous media

Pore-Scale Modeling of Multiphase Flow and Transport: Achievements and Perspectives

Hydro-dynamic Solute Transport under Two-Phase Flow Conditions

Non-equilibrium in multiphase multicomponent flow in porous media: An evaporation example

Saturation-Dependence of Non-Fickian Transport in Porous Media