Surface tension between immiscible fluids is a well-understood and well-characterized phenomenon. By contrast, much less is known about the effective, off-equilibrium surface tension that arises when a (transient) interface is created between miscible fluids. This effective surface tension plays a key role in many driven phenomena, from jetting and drop formation and coalescence to precipitation and deposition, as it was already recognized more than 100 years ago by Korteweg.

I present a thorough experimental investigation of the Saffman-Taylor (SF) instability arising when a simple fluid (the solvent of a colloidal suspension) is pushed through a miscible, more viscous one (the colloidal suspension itself). I show that the resulting interface pattern can be accounted for by the non-linear rheological properties of the suspension and an effective surface tension. By measuring the effective surface tension for suspensions at a variety of particle volume fractions, I successfully test Korteweg’s theory.

I rationalize such findings by showing that, for microgel spheres, composed of cross-linked polymers, the surface tension is dominated by the entropic contribution associated with the internal degrees of freedom of the particles. Furthermore, by studying SF-instabilities employing hard silica particles suspensions, I show that, at odd with the bulk behavior, microgels and hard particle suspensions exhibit vastly different interfacial stress properties.

We present some four-dimensional purely fermionic gauge theories that
give a UV completion of composite
Higgs models. We start at the group theoretical level, addressing the
necessary (but not sufficient) conditions
for the viability of these models, such as the existence of top partners
and custodial symmetry. We then study
the dynamics of one of the most promising models, based on a SU(4)
hyper-color group in more detail.
Work based on JHEP 1403 (2014) 077 [arXiv:1312.5330] and JHEP 1406
(2014) 142 [arXiv:1404.7137].

The non-geometric fluxes are specific gaugings that appear in four-dimensional (4D) gauged supergravities. They generate particular terms in the potential, that are of phenomenological interest (moduli stabilisation, de Sitter vacua). However, obtaining them from a compactification of a standard ten-dimensional (10D) supergravity is a priori not possible, as they are thought to descend from 10D backgrounds exhibiting a so-called non-geometry. After reviewing these ideas, I will present a reformulation of the standard (NSNS sector of) 10D supergravity into a new 10D theory named beta-supergravity. The latter contains 10D non-geometric fluxes, and can reformulate a non-geometric background into a geometric one. This theory thus provides an uplift to the aforementioned 4D supergravities. In addition, this framework allows to write 10D Bianchi identities for non-geometric fluxes. Those get corrected by a source term and boil down to Poisson equations in presence of NS-branes, as for an NS5-brane or a KK-monopole; in particular the non-geometric Q-flux is sourced by the new exotic 5_2^2- or Q-brane. I will finally comment on supersymmetry for this theory.

The dipolar hard-sphere (DHS) model, a dipole embedded in the centre of a hard sphere, is a paradigm for the self
assembly of anisotropic particles. Notwithstanding its simplicity, the phase diagram of the DHS model is still
partially unknown. In this talk I will focus on the structure and thermodynamics of the low-density DHS fluid, in the region
of the phase diagram where a so-called topological phase transition was predicted to occur[1,2]. I will first
show that the system does not phase separate and that the formation of self-assembled, stable rings suppresses
phase separation[3].
In order to confirm this picture, a much simpler patchy toy model, incorporating the main features of the DHS model, will
be used to acquire more insight on the phase behaviour of dipolar hard spheres. Indeed, the model can be tuned to favour
or disfavour the formation of rings. As ring formation is enhanced, the region of thermodynamic instability shrinks
and, eventually, disappears[4]. Given the similarities between the two systems, ring formation can thus be linked with
the lack of a critical point in the DHS fluid.
In addition to the suppression of the gas-liquid-like phase separation, I will also show that the tendency of the DHS
fluid to form magnetically inert structures such as rings provides a natural explanation for the experimentally
observed non-monotonicity of the initial magnetic susceptibility[5,6].

[1] T. Tlusty and S. A. Safran, Science 290, (2000) p. 1328
[2] G. Ganzenmüller, G. N. Patey, and P. J. Camp, Mol. Phys. 107, (2009) p. 403
[3] L. Rovigatti, J. Russo and F. Sciortino, Phys. Rev. Lett., 107, (2011) p. 237801
[4] L. Rovigatti, J. M. Tavares and F. Sciortino, Phys. Rev. Lett., 111, (2013) p. 168302
[5] A. Lebedev, Colloid J. 72, (2010) p. 815
[6] S. Kantorovich, A. O. Ivanov, L. Rovigatti, J. M. Tavares and F. Sciortino, Phys. Rev. Lett., 110, (2013) p. 148306

Using unitarity, analyticity and crossing symmetry, I present universal sum rules for scattering amplitudes in theories invariant under an arbitrary symmetry group. The sum rules relate the coefficients of the energy expansion of the scattering amplitudes in the IR to total cross sections integrated all the way up to the UV. For effective field theories the amplitudes in the IR are calculable and hence the sum rules set constraints on the parameters of the effective Lagrangian. I discuss explicit examples that are relevant for WW-scattering, composite Higgs models, and chiral perturbation theory. Certain sum rules based on custodial symmetry and its extensions provide constraints on the Higgs boson coupling to the electroweak gauge bosons.

Water and alcohol are well known to form mixtures with properties that are not just a linear
combination of the properties of the two components. Remarkably, such anomaly in the
solvent properties is also reflected in the solvation behaviour of poly‐N‐isopropyl acrylamide
(PNiPAM) in water/alcohol mixtures. At room‐temperature PNiPAM is perfectly soluble in
pure water and pure alcohol, but not in certain mixtures of both.
In this contribution we show that this 'co‐nonsolvency' phenomenon has to be understood
as that the phase behaviour of PNiPAM is governed by two distinct contributions. While the
phase behaviour is governed by classical mixing contributions in the alcohol‐rich regime, in
the water‐rich regime the phase behaviour of PNiPAM is determined by the solvent state.
This denotes that solvation of PNiPAM in water is primarily governed by hydrophobic
hydration, while the PNiPAM‐alcohol interactions are largely unspecific.

I. Bischofberger, D. C. E. Calzolari, P. De Los Rios, I. Jelezarov and V. Trappe, Hydrophobic
hydration of poly‐N‐isopropyl acrylamide: a matter of the mean energetic state of water,
Scientific Report 4 (2014) 4377

I. Bischofberger, D. C. E. Calzolari and V. Trappe, Co‐nonsolvency of PNiPAM at the transition between solvation mechanisms, Soft Matter (2014) accepted DOI: 10.1039/c4sm01345j

Gravity theories with negative cosmological constant in three dimensions (such as the ones on AdS3) play an important role in the understanding of black hole physics, and provided an early example of holography. Their dual 2-dimensional conformal field theories (CFT2) are quite special, since they enjoy (suitable super-symmetric extensions of) Virasoro symmetry. This duality naturally emerges in string theory too, for instance as the near horizon limit of a system of D1/F1-strings and D5/NS5-branes and was much studied in the early days of the Maldacena correspondence.
Recently, the interest in Ad3/CFT2 was revived when Babichenko, Stefanski and Zarembo showed that the maximally super-symmetric AdS3 backgrounds yield classically integrable string non-linear sigma models. It is natural to ask whether the S-matrix integrability approach, which works beautifully for the planar limit of AdS5/CFT4, can be applied here as well. The answer did not appear to be straightforward, due to several new features and some conceptual complications of AdS3/CFT2, and indeed eluded us for four years.
In my talk I will provide substantial evidence for an affirmative answer. To do this, I will discuss in detail the simplest case of superstrings on AdS3xS3xT4, argue how the same approach can be extended to more general cases, and describe the exciting future directions for this integrability program.

Optical and acoustical metamaterials take advantage of resonances. Here, we create mechanical metamaterials by exploiting elastic instabilities. Namely, we exploit buckling of the micro-structure to create materials with transnatural mechanical properties. We use Finite Element simulations, 3D printing, mechanical tests and high resolution imaging to optimize, make and study their mechanical response. I will show several examples of metamaterials in 1D (metabeams), 2D (holey sheets) and 3D (buckli-cube), that have an incredibly rich phenomenology: auxetic behavior, negative stiffness, elastic memory, collective effects. I will show that the symmetries of the micro-structure and the presence of elastic nonlinearities are crucial to understand, to model and to tune the mechanics of such systems. This paves the way towards the rational design of a new generation of materials, with extreme mechanics and enhanced functionality.

"Thé au cube" est la pause café scientifique de l'axe PTh. L'orateur y expose de manière interactive et accessible au théoriciens non-spécialistes, ses intérêts scientifiques du moment. N'oubliez pas de ramener vos tasses et autres mugs préférés.