arXiv subjects

Dimitrios Krommydas

Publications and source records attributed to Dimitrios Krommydas.

11 recordsLinked to original sources

arXiv2026

Controlling Turbulent Flows in Compressible Active Nematics

Motivated by experiments on light-patterned, quasi-2D active suspensions that exhibit large density variations, we develop a continuum theory of compressible active nematics--suspensions of apolar rods whose orientation is invariant under pi rotations. Under spatially patterned activity, the extensile isotropic active pressure expels material from high-activity regions and accumulates it in low-activity ones; an exactly solvable 1D reduction shows that the resulting density contrast is governed by a single dimensionless parameter, linear in the compressibility. Using compressibility as a tuning knob, we then steer active turbulence from high- to low-activity regions and, at sharp activity interfaces, stabilize an analytically tractable dynamical steady state: a one-dimensional chain of vortices held by soft, activity-induced confinement. Our results establish compressibility as a control parameter for density variations and turbulent flow in active nematic suspensions.

cond-mat.soft↗

arXiv2025

Exponential distance relation (aka Titius-Bode rule) in extra solar planetary systems

In this paper we present phenomenological evidence for the validity of an exponential distance relation (also known as generalized Titius-Bode law) in the 32 planetary systems (31 extra solar, plus our Solar System) containing at least 5 planets each (known up to July 2023). We produce the semi-log fittings of the data, and we check them against the statistical indicators of $R^2$ and $Median$. Then we compare them with the data of 4000 artificial planetary systems created at random. In this way, a possible origin by chance of the Titius-Bode rule (TBR) is reasonably excluded. We also point out that in some systems the fittings can be definitely improved by the insertion of new planets into specific positions. We discuss the Harmonic Resonances method and fittings, and compare them with the Titius-Bode fittings. Moreover, for some specific systems, we compare the Titius-Bode fitting against a polynomial fitting ($r\sim n^2$). Further comparisons with previous relevant works are reported in the last section. It emerges that TBR describes 25 out of the 32 planetary systems ($78\%$) with a $R^2\geq 0.95$. Further, it results to be the most economical (in terms of free parameters) and best fitting law for the description of spacing among planetary orbits. This analysis allows us to conclude that an exponential distance relation can reasonably be considered as ``valid'', or strongly corroborated, also in extra solar planetary systems.

astro-ph.EP↗

arXiv2025

Collective migration and topological phase transitions in confluent epithelia

Collective epithelial migration leverages on topological rearrangements of the intercellular junctions, which allow cells to intercalate without loosing confluency. In silico studies have provided a clear indication that this process could occur via a two-step phase transition, where a hierarchy of topological excitations progressively transforms an epithelial layer from a crystalline solid to an isotropic liquid, via an intermediate hexatic liquid crystal phase. Yet, the fundamental mechanism behind this process and its implications for collective cell behavior are presently unknown. In this article, we show that the onset of collective cell migration in cell-resolved models of epithelial layers takes place via an activity-driven melting transition, characterized by an exponentially-divergent correlation length across the solid/hexatic phase boundary. Using a combination of numerical simulations and Renormalization Group analysis, we show that the availability of topologically distinct rearrangements - known as T1 and T2 processes - and of a non-thermal route to melting, renders the transition significantly more versatile and tunable than in two-dimensional passive matter. Specifically, the relative frequency of T1 and T2 processes and of the "bare" stiffness of the cell layer affect the divergence of positional correlations within a well-defined spectrum of critical behaviors. Suppressing T1 processes, changes the nature of the transition by preventing collective migration in favor of a cellular analog of surface sublimation.

cond-mat.soft↗

arXiv2023

Quasi long-ranged order in two-dimensional active liquid crystals

Quasi-long ranged order is the hallmark of two-dimensional liquid crystals. At equilibrium, this property implies that the correlation function of the local orientational order parameter decays with distance as a power law: i.e. $\sim|\boldsymbol{r}|^{-η_{p}}$, with $η_{p}$ a temperature-dependent exponent. While in general non-universal, $η_{p}=1/4$ universally at the Berezinskii-Kosterlitz-Thouless transition, where orientational order is lost because of the unbinding of disclinations. Motivated by recent experimental findings of liquid crystal order in confluent cell monolayers, here we demonstrate that, in {\em active} liquid crystals, the notion of quasi-long ranged order fundamentally differs from its equilibrium counterpart and is ultimately dictated by the interplay between translational and orientational dynamics. As a consequence, the exponent $η_{p}$ is allowed to vary in the range $0<η_{p}\le 2$, with the upper bound corresponding to the isotropic phase. Our theoretical predictions are supported by a survey of recent experimental data, reflecting a wide variety of different realization of orientational order in two dimensions.

cond-mat.soft↗

arXiv2023

Collective epithelial migration mediated by the unbinding of hexatic defects

Collective cell migration in epithelia relies on cell intercalation: a local remodelling of the cellular network that allows neighbouring cells to swap their positions. Unlike foams and passive cellular fluid, in epithelial intercalation these rearrangements crucially depend on activity. During these processes, the local geometry of the network and the contractile forces generated therein conspire to produce a burst of remodelling events, which collectively give rise to a vortical flow at the mesoscopic length scale. In this article we formulate a continuum theory of the mechanism driving this process, built upon recent advances towards understanding the hexatic (i.e. $6-$fold ordered) structure of epithelial layers. Using a combination of active hydrodynamics and cell-resolved numerical simulations, we demonstrate that cell intercalation takes place via the unbinding of topological defects, naturally initiated by fluctuations and whose late-times dynamics is governed by the interplay between passive attractive forces and active self-propulsion. Our approach sheds light on the structure of the cellular forces driving collective migration in epithelia and provides an explanation of the observed extensile activity of in vitro epithelial layers.

cond-mat.soft↗

arXiv2023

Hydrodynamic Enhancement of $p$-atic Defect Dynamics

We investigate numerically and analytically the effects of hydrodynamics on the dynamics of topological defects in $p-$atic liquid crystals, i.e. two-dimensional liquid crystals with $p-$fold rotational symmetry. Importantly, we find that hydrodynamics fuels a generic passive self-propulsion mechanism for defects of winding number $s=(p-1)/p$ and arbitrary $p$. Strikingly, we discover that hydrodynamics always accelerates the annihilation dynamics of pairs of $\pm 1/p$ defects, and that, contrary to expectations, this effect increases with $p$. Our Letter paves the way towards understanding cell intercalation and other remodelling events in epithelial layers.

cond-mat.soft↗

arXiv2022

Epithelia are multiscale active liquid crystals

Biological processes such as embryogenesis, wound healing and cancer progression, crucially rely on the ability of epithelial cells to coordinate their mechanical activity over length scales order of magnitudes larger than the typical cellular size. While regulated by various signalling pathways, it has recently become evident that this behavior can additionally hinge on a minimal toolkit of physical mechanisms, of which liquid crystal order is the most prominent example. Yet, experimental and theoretical studies have given so far inconsistent results in this respect: whereas nematic order is often invoked in the interpretation of experimental data, computational models have instead suggested that hexatic order could serve as a linchpin for collective migration in confluent cell layers. In this article we resolve this dilemma. Using a combination of in vitro experiments on Madin-Darby canine kidney cells (MDCK), numerical simulations and analytical work, we demonstrate that both nematic and hexatic order are, in fact, present in epithelial layers, with the former being dominant at large length scales and the latter at small length scales. In MDCK GII cells on uncoated glass, these different types of liquid crystal order crossover at a length scale of the order of ten cell sizes. Our work sheds light on the emergent organization of living matter, provides a new framework for deciphering the structure of epithelia, and paves the way toward a comprehensive and predictive mesoscopic theory of tissues.

cond-mat.soft↗

arXiv2021

The Return of the Singularities: Applications of the Smeared Null Energy Condition

The classic singularity theorems of General Relativity rely on energy conditions that can be violated in semiclassical gravity. Here, we provide motivation for an energy condition obeyed by semiclassical gravity: the smeared null energy condition (SNEC), a proposed bound on the weighted average of the null energy along a finite portion of a null geodesic. We then prove a semiclassical singularity theorem using SNEC as an assumption. This theorem extends the Penrose theorem to semiclassical gravity. We also apply our bound to evaporating black holes and the traversable wormhole of Maldacena-Milekhin-Popov, and comment on the relationship of our results to other proposed semiclassical singularity theorems.

gr-qc↗

arXiv2021

A singularity theorem for evaporating black holes

The classical singularity theorems of General Relativity rely on energy conditions that are easily violated by quantum fields. Here, we provide motivation for an energy condition obeyed in semiclassical gravity: the smeared null energy condition (SNEC), a proposed bound on the weighted average of the null energy along a finite portion of a null geodesic. Using SNEC as an assumption we proceed to prove a singularity theorem. This theorem extends the Penrose singularity theorem to semiclassical gravity and has interesting applications to evaporating black holes.

gr-qc↗

arXiv2021

The Smeared Null Energy Condition

We propose a new bound on the average null energy along a finite portion of a null geodesic. We believe our bound is valid on scales small compared to the radius of curvature in any quantum field theory that is consistently coupled to gravity. If correct, our bound implies that regions of negative energy density are never strongly gravitating, and that isolated regions of negative energy are forbidden.

hep-th↗

arXiv2018

Violations of the Null Energy Condition in QFT and their Implications

We study violations of the Null Energy Condition (NEC) in Quantum Field Theory (QFT) and their implications. For the first part of the project, we examine these violations for classes of already known and novel (first discussed here) QFT states. Next, we discuss the implications of these violations focusing on the example of Wormhole Traversability. After reviewing the current literature on the existing restrictions on these violations, we conjecture that NEC violating states are incompatible with the Semi-Classical Gravity approximation. We argue that this conjecture provides the only way out of the problems introduced by the violations of NEC in this regime. Building on this, we propose a bound that should hold for all QFT states. Finally, we show that both our conjecture and bound hold for some relevant classes of QFT states.

hep-th↗