Strong Electronic Correlation Effects in Coherent Multidimensional Nonlinear Optical Spectroscopy

We discuss a many−body theory of the coherent ultrafast nonlinear optical response of systems with a strongly correlated electronic ground state that responds unadiabatically to photoexcitation. We introduce a truncation of quantum kinetic density matrix equations of motion that does not rely on an expansion in terms of the interactions and thus applies to strongly correlated systems. For this we expand in terms of the optical field, separate out contributions to the time−evolved many−body state due to correlated and uncorrelated multiple optical transitions, and use “Hubbard operator” density matrices to describe the exact dynamics of the individual contributions within a subspace of strongly coupled states, including “pure dephasing”. Our purpose is to develop a quantum mechanical tool capable of exploring how, by coherently photoexciting selected modes, one can trigger nonlinear dynamics of strongly coupled degrees of freedom. Such dynamics could lead to photoinduced phase transitions. We apply our theory to the nonlinear response of a two−dimensional electron gas (2DEG) in a magnetic field. We coherently photoexcite the two lowest Landau level (LL) excitations using three time−delayed optical pulses. We identify some striking temporal and spectral features due to dynamical coupling of the two LLs facilitated by inter−Landau−level magnetoplasmon and magnetoroton excitations and compare to three−pulse four−wave−mixing (FWM) experiments. We show that these features depend sensitively on the dynamics of four−particle correlations between an electron−hole pair and a magnetoplasmon/magnetoroton, reminiscent of exciton−exciton correlations in undoped semiconductors. Our results shed light into unexplored coherent dynamics and relaxation of the quantum Hall system (QHS) and can provide new insight into non−equilibrium co−operative phenomena in strongly correlated systems

Chaordic event co‐creation and tourism destination image: Strategic carnival shifts in the post‐pandemic era

Major street events, such as carnivals, offer a unique opportunity for destination experience value co‐creation by participants which relates directly to the destination image. This study uses service‐dominant logic (SDL) to consider the effects of event co‐creation on destination image from the point of view of a participatory process rather than from an outcome perspective. Drawing from a sample of 400 street event participants in the Patras Carnival in Greece, it examines the complexity aspects of co‐creational experience and its influence upon the destination image of the host city. Those aspects are examined by means of fuzzy‐set qualitative comparative analysis. The findings revealed three sufficient configurations (co‐creational involvement and satisfaction; co‐creational event image; experience‐satisfaction nexus) that could affect the destination image of the host destination. The paper contributes to the theoretical body of experiential co‐creational approaches to destination image with clear managerial implications for both event organizers and destination managers

Grain-scale modelling of swelling granular materials; application to super absorbent polymers

Swelling is an important process in many natural materials and industrial products, such as swelling clays, paper, and Super Absorbent Polymer (SAP) particles in hygienic products. SAP particles are capable to absorb large amounts of fluid. Each grain of SAP can absorb water 30 to 1000 times its initial mass, depending on the water composition. To gain insight in the swelling behaviour of a bed of SAP particles, we have developed a grain-scale model and have tested it by comparing it to experiments. The grain-scale model is based on a combination of the Discrete Element Method (DEM) and the Pore Finite Volume (PFV) method, which we have extended to account for the swelling of individual SAP particles. Using this model, we can simulate the behaviour of individual particles inside a water-saturated bed of swelling SAP particles while taking into account the hydro-mechanical effect arising from the presence of pore water. The model input includes physical parameters such as particle stiffness and friction angle, which were found in the literature, as well as particle size distribution and diffusion coefficients, which were measured experimentally. A swelling rate equation was developed to simulate the swelling of individual particles based on water diffusion into a spherical particle. We performed experiments to measure the rise of the surface of a bed of initially dry SAP particles, which were put inside a glass beaker that contained sufficient amount of water for the SAP particles to swell and to remain saturated at all times. We used our model to simulate the swelling of that SAP particle bed as a function of time. Simulations show that the numerical model is in accordance with the experimental data. We have also verified the model with Terzaghi's analytical solution for a small swelling event. Finally, a sensitivity analysis was performed to study the effects of main grain-scale parameters on the larger-scale behaviour of a bed of particles

Direct simulations of two-phase flow experiments of different geometry complexities using Volume-of-Fluid (VOF) method

Two-phase flow in three porous media with different geometry complexities are simulated using the Volume-of-Fluid (VOF) method. The evolution of the flow pattern, as well as the dynamics involved are simulated and compared to experiments. For a simple geometry and smooth solid surface, like single capillary rise experiment, VOF simulation gives results which are in good agreement with the experiments. For a micromodel, with a relatively simple geometry, we can predict the flow pattern while we cannot effectively capture the dynamics of the process in terms of the temporal evolution of flow. With an increase in the geometry complexity in another micromodel, we fail to predict both the flow pattern and the flow dynamics. The reasons for this failure are discussed: interface modeling, pinning of contact line, 3D effects and the sensitivity of the system to initial and boundary conditions. More work regarding benchmarking of pore-scale methods in combination with experiments with different geometry complexities is needed. Also, possibilities and the potential to make better use of the porous media structure data from advanced visualization methods should be addressed

Direct simulations of two-phase flow experiments of different geometry complexities using Volume-of-Fluid (VOF) method

Two-phase flow in three porous media with different geometry complexities are simulated using the Volume-of-Fluid (VOF) method. The evolution of the flow pattern, as well as the dynamics involved are simulated and compared to experiments. For a simple geometry and smooth solid surface, like single capillary rise experiment, VOF simulation gives results which are in good agreement with the experiments. For a micromodel, with a relatively simple geometry, we can predict the flow pattern while we cannot effectively capture the dynamics of the process in terms of the temporal evolution of flow. With an increase in the geometry complexity in another micromodel, we fail to predict both the flow pattern and the flow dynamics. The reasons for this failure are discussed: interface modeling, pinning of contact line, 3D effects and the sensitivity of the system to initial and boundary conditions. More work regarding benchmarking of pore-scale methods in combination with experiments with different geometry complexities is needed. Also, possibilities and the potential to make better use of the porous media structure data from advanced visualization methods should be addressed

Micromodel study of two-phase flow under transient conditions: Quantifying effects of specific interfacial area

Recent computational studies of two-phase flow suggest that the role of fluid-fluid interfaces should be explicitly included in the capillarity equation as well as equations of motion of phases. The aim of this study has been to perform experiments where transient movement of interfaces can be monitored and to determine interfacial variables and quantities under transient conditions. We have performed two-phase flow experiments in a transparent micromodel. Specific interfacial area is defined, and calculated from experimental data, as the ratio of the total area of interfaces between two phases per unit volume of the porous medium. Recent studies have shown that all drainage and imbibition data points for capillary pressure, saturation, and specific interfacial area fall on a unique surface. But, up to now, almost all micromodel studies of two-phase flow have dealt with quasi-static or steady state flow conditions. Thus, only equilibrium properties have been studied. We present the first study of two-phase flow in an elongated PDMS micromodel under transient conditions with high temporal and spatial resolutions. We have established that different relationships between capillary pressure, saturation, and specific interfacial area are obtained under steady state and transient conditions. The difference between the surfaces depends on the capillary number. Furthermore, we use our experimental results to obtain average (macroscale) velocity of fluid-fluid interfaces and the rate of change of specific interfacial area as a function of time and space. Both terms depend on saturation nonlinearly but show a linear dependence on the rate of change of saturation. We also determine macroscale material coefficients that appear in the equation of motion of fluid-fluid interfaces. This is the first time that these parameters are determined experimentally. Key Points Specific interfacial area depends on dynamic conditions Interfacial velocity and production term show similar trends Further investigation of the dynamic conditions and of all interfaces is neede

Study of Multi-phase Flow in Porous Media: Comparison of SPH Simulations with Micro-model Experiments

We present simulations and experiments of drainage processes in a micro-model. A direct numerical simulation is introduced which is capable of describing wetting phenomena on the pore scale. A numerical smoothed particle hydrodynamics model was developed and used to simulate the two-phase flow of immiscible fluids. The experiments were performed in a micro-model which allows the visualization of interface propagation in detail. We compare the experiments and simulations of a quasistatic drainage process and pure dynamic drainage processes. For both, simulation and experiment, the interfacial area and the pressure at the inflow and outflow are tracked. The capillary pressure during the dynamic drainage process was determined by image analysis