Ref HAL: hal-04105013_v1
Ref Arxiv: 2304.06751
Ref INSPIRE: 2651465
DOI: 10.1007/JHEP08(2023)134
Ref. & Cit.: NASA ADS
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Résumé:

The tadpole conjecture proposes that complex structure moduli stabilisation by fluxes that have low tadpole charge can be realised only at special points in moduli space, leading generically to (large) gauge symmetries. Here we provide an exhaustive survey of the gauge symmetries arising in F-theory flux compactifications on products of attractive $\mbox{K3}$ surfaces, with complex structure moduli fully stabilised. We compute the minimal rank of the left-over non-abelian gauge group for all flux configurations within the tadpole bound, finding that it is always non-zero. It decreases in a roughly linear fashion with the tadpole charge, reaching zero at charge 30. By working out possible gauge algebras for different values of the tadpole, we find that all simple ADE Lie algebras of rank $\le 18$ appear.

Ref HAL: hal-04072206_v1
Ref Arxiv: 2109.14668
DOI: 10.1103/RevModPhys.95.015004
Ref. & Cit.: NASA ADS
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The Helfrich-Hurault (HH) elastic instability is a well-known mechanism behind patterns that form as a result of strain upon liquid crystal systems with periodic ground states. In the HH model, layered structures undulate and buckle in response to local, geometric incompatibilities in order to maintain the preferred layer spacing. Classic HH systems include cholesteric liquid crystals under electromagnetic field distortions and smectic liquid crystals under mechanical strains, where both materials are confined between rigid substrates. However, richer phenomena are observed when undulation instabilities occur in the presence of deformable interfaces and variable boundary conditions. Understanding how the HH instability is affected by deformable surfaces is imperative for applying the instability to a broader range of materials. In this review, the HH mechanism is reexamined and special focus is given to how the boundary conditions influence the response of lamellar systems to geometrical frustration. Lamellar liquid crystals confined within a spherical shell geometry are used as the model system. Made possible by the relatively recent advances in microfluidics within the past 15 years, liquid crystal shells are composed entirely of fluid interfaces and have boundary conditions that can be dynamically controlled at will. Past and recent work that exemplifies how topological constraints, molecular anchoring conditions, and boundary curvature can trigger the HH mechanism in liquid crystals with periodic ground states is examined. The review ends by identifying similar phenomena across a wide variety of materials, both biological and synthetic. The fact that the HH mechanism is a generic and often overlooked response of periodic materials to geometrical frustration is highlighted.

Ref HAL: hal-04072170_v1
Ref Arxiv: 2207.01341
DOI: 10.1016/j.jcis.2022.09.099
Ref. & Cit.: NASA ADS
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HypothesisAlthough the dynamics of colloids in the vicinity of a solid interface has been widely characterized in the past, experimental studies of Brownian diffusion close to an air–water interface are rare and limited to particle-interface gap distances larger than the particle size. At the still unexplored lower distances, the dynamics is expected to be extremely sensitive to boundary conditions at the air–water interface. There, ad hoc experiments would provide a quantitative validation of predictions.ExperimentsUsing a specially designed dual wave interferometric setup, the 3D dynamics of 9 μm diameter particles at a few hundreds of nanometers from an air–water interface is here measured in thermal equilibrium.FindingsIntriguingly, while the measured dynamics parallel to the interface approaches expected predictions for slip boundary conditions, the Brownian motion normal to the interface is very close to the predictions for no-slip boundary conditions. These puzzling results are rationalized considering current models of incompressible interfacial flow and deepened developing an ad hoc model which considers the contribution of tiny concentrations of surface active particles at the interface. We argue that such condition governs the particle dynamics in a large spectrum of systems ranging from biofilm formation to flotation process.

Ref HAL: hal-04072095_v1
PMID 36967649
DOI: 10.1039/D2SM01665F
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In this article, we present the mobilities of prolate ellipsoidal micrometric particles close to an air–water interface measured by dual wave reflection interference microscopy. Particle's position and orientation with respect to the interface are simultaneously measured as a function of time. From the measured mean square displacement, five particle mobilities (3 translational and 2 rotational) and two translational–rotational cross-correlations are extracted. The fluid dynamics governing equations are solved by the finite element method to numerically evaluate the same mobilities, imposing either slip and no-slip boundary conditions to the flow at the air–water interface. The comparison between experiments and simulations reveals an agreement with no-slip boundary conditions prediction for the translation normal to the interface and the out-of-plane rotation, and with slip ones for parallel translations and in-plane rotation. We rationalize these evidences in the framework of surface incompressibility at the interface.

Ref HAL: hal-04071077_v1
DOI: 10.1016/j.conbuildmat.2023.130592
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An experimental program is oriented to the mechanical behaviour of the Interfacial Transition Zone (ITZ) exposed to leaching, for a single mix. Leaching kinetics of the degraded depth, measured through grey level on cross sections, reveals a maximum gap of 10 % between ITZ and bulk cement paste. ITZ, with a main thickness of 15 µm suffers a significant decalcification, which is reflected on the mechanical properties of the bond. Grey level, Calcium/Silicon (Ca/Si) molar ratio and Young’s modulus in the degraded zone present similar evolution. Effect of ITZ causing cracking and then debonding is noticed, through chemical analysis, image observation and mechanical tests.

Résumé:

I will discuss recent results: (i) on the Casimir torque between two metallic one-dimensional gratings rotated by an angle θ with respect to each other [1]; and (ii) on the quantitative effect of the presence of the metallic contact grid at the front side of thermophotovoltaic cells, based on a rigorous approach [2]. This effect can be tremendous both in terms of optical and electrical (resistive) losses. However, it has been theoretically either discarded or studied by means of extremely simplified models (like shadowing methods, consisting in just ignoring the fraction of the cell surface covered by metal). ------- [1] Mauro Antezza, H. B. Chan, Brahim Guizal, V.N. Marachevsky, Riccardo Messina, Mingkang Wang, Phys. Rev. Lett. 124 013903 (2020). [2] K. Austry, Y. Jeyar, M. Luo, B. Guizal, R. Messina, R. Vaillon, and M. Antezza, in preparation (2023)

Résumé:

(1) We will discuss recent results: (i) on the Casimir torque between two metallic one-dimensional gratings rotated by an angle θ with respect to each other [1]; and (ii) on the Casimir force occurring between interpenetrating gratings [2]. These findings pave the way to the design of a contactless quantum vacuum torsional spring, and sensors with possible relevance to micro and nanomechanical devices. (2) We propose a scheme for a quantum thermal machine made by atoms interacting with a single nonequilibrium electromagnetic field. We show that these machines can deliver all thermodynamic tasks (cooling, heating, and population inversion) by establishing quantum coherence with the body on which they act. Remarkably, this system allows these machines to reach efficiencies at maximum power very close to the Carnot limit, which is much more than in existing models [3]. ------- [1] Mauro Antezza, H. B. Chan, Brahim Guizal, V.N. Marachevsky, Riccardo Messina, Mingkang Wang, Phys. Rev. Lett. 124 013903 (2020) [2] Mingkang Wang, L. Tang, C.Y. Ng, Riccardo Messina, Brahim Guizal, J. A. Crosse, Mauro Antezza, C.T. Chan, H.B. Chan, Nature Communication 12 600 (2021) [3] Bruno Leggio, Bruno Bellomo, and Mauro Antezza, Phys. Rev. A 91, 012117 (2015)