Static and Dynamic Solid-Water Interfaces: charge regulation, diffusio-osmosis and heterogeneous electrokinetics

The results presented in this thesis consists of three parts, each with a different view of a solid-water interface. In the first part we consider the case where both the interface and the fluid are static, and investigate the interface between muscovite mica and water for varying salt species and concentration. By combining two experimental techniques, we obtain detailed information of both the number and location of the adsorbed ions, and we construct a model of the interface consistent with all data. In the second part we consider again a static interface but with a dynamic fluid, where water flows from one reservoir to another through a microscopic channel. We construct a theoretical model to predict the transport of water, charge and salt through a channel from one reservoir to the other as a function of the reservoir and channel properties, specifically in the context of reverse electrodialysis, where an electric current is generated by mixing salt and fresh water. We furthermore investigate the transport properties of an array of microscopic channels, i.e. a membrane. We elaborate why the current generated by a membrane is very different from a single channel. In the last part we investigate a system with both a dynamic interface and fluid, as experiments have shown that ions adsorbed in the interface are laterally mobile and are chemically exchanged with the fluid. We show how and why this can significantly alter the transport properties of these microscopic channels

Behind Horndeski: structurally robust higher derivative EFTs

Higher derivative scalar interactions can give rise to interesting cosmological scenarios. We present a complete classification of such operators that can yield sizeable effects without introducing ghosts and, at the same time, define an effective field theory robust under the inclusion of quantum corrections. A set of rules to power count consistently the coefficients of the resulting Lagrangian is provided by the presence of an approximate global symmetry. The interactions that we derive in this way contain a subset of the so-called Horndeski and beyond Horndeski theories. Our construction therefore provides a structurally robust context to study their phenomenology. Applications to dark energy/modified gravity and geodesically complete cosmologies are briefly discussed

Thermo-electric response in two-dimensional Dirac systems: the role of particle-hole pairs

Clean two-dimensional Dirac systems have received a lot of attention for being a prime candidate to observe hydrodynamical transport behavior in interacting electronic systems. This is mostly due to recent advances in the preparation of ultrapure samples with sufficiently strong interactions. In this paper, we investigate the role of collective modes in the thermo-electric transport properties of those systems. We find that dynamical particle-hole pairs, plasmons, make a sizeable contribution to the thermal conductivity. While the increase at the Dirac point is moderate, it becomes large towards larger doping. We suspect, that this is a generic feature of ultraclean two-dimensional electronic systems, also applicable to degenerate systems

Connectivity, Not Density, Dictates Percolation in Nematic Liquid Crystals of Slender Nanoparticles

We show by means of continuum theory and simulations that geometric percolation in uniaxial nematics of hard slender particles is fundamentally different from that in isotropic dispersions. In the nematic, percolation depends only very weakly on the density and is, in essence, determined by a distance criterion that defines connectivity. This unexpected finding has its roots in the nontrivial coupling between the density and the degree of orientational order that dictate the mean number of particle contacts. Clusters in the nematic are much longer than wide, suggesting the use of nematics for nanocomposites with strongly anisotropic transport properties

Explicit Construction of Local Hidden Variables for Any Quantum Theory Up to Any Desired Accuracy

The machinery of quantum mechanics is fully capable of describing a single realistic world. Here we discuss the converse: in spite of appearances, and indeed numerous claims to the contrary, any quantum mechanical model can be mimicked, up to any finite accuracy, by a completely classical system of equations. An implication of this observation is that Bell’s theorem is not applicable in the cases considered. This is explained by scrutinising Bell’s assumptions concerning causality, retrocausality, statistical (in-)dependence, and his fear of ‘conspiracy’ (there is no conspiracy in the language used to describe the deterministic models). The most crucial mechanism for the counter intuitive Bell/CHSH violation is the fact that, regardless the settings chosen by Alice and Bob, the initial state of the system should be a realistic one. The potential importance of our construction in model building is discussed

Coupled water, charge and salt transport in heterogeneous nano-fluidic systems

We theoretically study the electrokinetic transport properties of nano-fluidic devices under the influence of a pressure, voltage or salinity gradient. On a microscopic level the behaviour of the device is quantified by the Onsager matrix ${\bf L}$, a generalised conductivity matrix relating the local driving forces and the induced volume, charge and salt flux. Extending ${\bf L}$ from a local to a global linear-response relation is trivial for homogeneous electrokinetic systems, but in this manuscript we derive a generalised conductivity matrix ${\bf G}$ from ${\bf L}$ that applies also to heterogeneous electrokinetic systems. This extension is especially important in the case of an imposed salinity gradient, which gives necessarily rise to heterogeneous devices. Within this formalism we can also incorporate a heterogeneous surface charge due to, for instance, a charge regulating boundary condition, which we show to have a significant impact on the resulting fluxes. The predictions of the Poisson-Nernst-Planck-Stokes theory show good agreement with exact solutions of the governing equations determined using the Finite Element Method under a wide variety of parameters. Having established the validity of the theory, it provides an accessible method to analyse electrokinetic systems in general without the need of extensive numerical methods. As an example, we analyse a Reverse Electrodialysis "blue energy" system, and analyse how the many parameters that characterise such a system affect the generated electrical power and efficiency

Field-theoretic derivation of bubble-wall force

We derive a general quantum field theoretic formula for the force acting on expanding bubbles of a first order phase transition in the early Universe setting. In the thermodynamic limit the force is proportional to the entropy increase across the bubble of active species that exert a force on the bubble interface. When local thermal equilibrium is attained, we find a strong friction force which grows as the Lorentz factor squared, such that the bubbles quickly reach stationary state and cannot run away. We also study an opposite case when scatterings are negligible across the wall (ballistic limit), finding that the force saturates for moderate Lorentz factors thus allowing for a runaway behavior. We apply our formalism to a massive real scalar field, the standard model and its simple portal extension. For completeness, we also present a derivation of the renormalized, one-loop, thermal energy-momentum tensor for the standard model and demonstrate its gauge independence

Self-organization of tip-functionalized elongated colloidal particles

Weakly attractive interactions between the tips of rodlike colloidal particles affect their liquid-crystal phase behavior due to a subtle interplay between enthalpy and entropy. Here we employ molecular dynamics simulations on semiflexible, repulsive bead-spring chains where one of the two end beads attract each other. We calculate the phase diagram as a function of both the volume fraction of the chains and the strength of the attractive potential. We identify a large number of phases that include isotropic, nematic, smectic-A, smectic-B, and crystalline states. For tip attraction energies lower than the thermal energy, our results are qualitatively consistent with experimental findings: We find that an increase of the attraction strength shifts the nematic to smectic-A phase transition to lower volume fractions, with only minor effect on the stability of the other phases. For sufficiently strong tip attraction, the nematic phase disappears completely, in addition leading to the destabilization of the isotropic phase. In order to better understand the underlying physics of these phenomena, we also investigate the clustering of the particles at their attractive tips and the effective molecular field experienced by the particles in the smectic-A phase. Based on these results, we argue that the clustering of the tips only affects the phase stability if lamellar structures ("micelles") are formed. We find that an increase of the attraction strength increases the degree of order in the layered phases. Interestingly, we also find evidence for the existence of an antiferroelectric smectic-A phase transition induced by the interaction between the tips. A simple Maier-Saupe-McMillan model confirms our findings

Effective Field Theory of Black Hole Quasinormal Modes in Scalar-Tensor Theories

The final ringdown phase in a coalescence process is a valuable laboratory to test General Relativity and potentially constrain additional degrees of freedom in the gravitational sector. We introduce here an effective description for perturbations around spherically symmetric spacetimes in the context of scalar-tensor theories, which we apply to study quasi-normal modes for black holes with scalar hair. We derive the equations of motion governing the dynamics of both the polar and the axial modes in terms of the coefficients of the effective theory. Assuming the deviation of the background from Schwarzschild is small, we use the WKB method to introduce the notion of "light ring expansion". This approximation is analogous to the slow-roll expansion used for inflation, and it allows us to express the quasinormal mode spectrum in terms of a small number of parameters. This work is a first step in describing, in a model independent way, how the scalar hair can affect the ringdown stage and leave signatures on the emitted gravitational wave signal. Potential signatures include the shifting of the quasi-normal spectrum, the breaking of isospectrality between polar and axial modes, and the existence of scalar radiation

Effects of disorder in the Fibonacci quasicrystal

We study the properties of the one-dimensional Fibonacci chain, subjected to the placement of on-site impurities. The resulting disruption of quasiperiodicity can be classified in terms of the renormalization path of the site at which the impurity is placed, which greatly reduces the possible amount of disordered behavior that impurities can induce. Moreover, it is found that, to some extent, the addition of multiple weak impurities can be treated by superposing the individual contributions together and ignoring nonlinear effects. This means that a transition regime between quasiperiodic order and disorder exists in which some parts of the system still exhibit quasiperiodicit, while other parts start to be characterized by different localization behaviors of the wave functions. This is manifested through a symmetry in the wave-function amplitude map, expressed in terms of conumbers, and through the inverse participation ratio. For the latter, we find that its average of states can also be grouped in terms of the renormalization path of the site at which the impurity has been placed