Europhysics News

Highlights

Droplet explosion by shock waves, relevant to nuclear medicine (Vol. 49, No. 1)

Simulation of a disintegrating droplet.”

An arrow shooting through an apple, makes for a spectacular explosive sight in slow motion. Similarly, energetic ions passing through liquid droplets induce shock waves, which can fragment the droplets. In a study published recently, the authors have proposed a solution to observe the predicted ion-induced shock waves. They believe these can be identified by observing the way incoming ions fragment liquid droplets into multiple smaller droplets. The discovery of such shock waves would change our understanding of the nature of radiation damage with ions to cancerous tumour. This matters for the optimisation of ion-beam cancer therapy, which requires a thorough understanding of the relation between the physical characteristics of the incoming ion beam and its effects on biological tissues. The predicted shock waves significantly contribute to the thermomechanical damage deliberately inflicted on tumour tissue. Specifically, the collective flow intrinsic to the shock waves helps to propagate biologically harmful reactive species, such as free radicals, stemming from the ions. This mechanism increases the volume of tumour cells exposed to reactive species.

E. Surdutovich, A. Verkhovtsev and A. V. Solov’yov, Ion-impact-induced multifragmentation of liquid droplets, Eur. Phys. J. D 71, 285 (2017)
[Abstract]

Droplet production for electrolyte solutions (Vol. 44 No. 1)

The micro-jet length L_j and nanoaerosol plume angle θ_plume are observed to be strong functions of the ionic strength C_∞ and interfacial tension γ_f.

Proteomic mass spectrometry and chemical imaging require the generation of nanometer-sized charged water drops that are emitted in a precise direction. This is done with the remarkable ability of an electric field to sharpen a mm-sized spherical drop into a sharp cone with a universal half angle of 49 degrees, to emit a micro-jet at the cone tip and to breakup the charged micro-jet explosively into a plume of nanodrops, whose plume angle and charge per drop sensitively affect the measurement accuracy of the instrument. This paper captures the underlying nanoscale physics of DC electrospray with a combination of strong electrolyte theory, electrostatics, interfacial phenomena and potential spectral theory for geometric singularities. The electric field of the most singular harmonic near a cone that sharpens the drop is also shown to charge a nanoscale capacitor at the interface of the micro-jet, with a dimension of the Debye length. The interfacial charging increases downstream of the cone tip until the space charge separation is below the Bjerrum length, when the repulsive Coulombic energy between two charges exceeds the thermal energy. The resulting Coulombic fission accounts for the explosive breakup of the micro-jet into the nanodrop plume. The micro-jet length, the charge per nanodrop and the nanodrop plume angle as functions of ion strength are accurately captured by the theory as functions of the ionic strength and surface tension. Strong ionic strength and low surface tension produce the longest micro-jet, the smallest plume and the most precisely directed nanodrops.

Y. Wang, M. K. Tan, D. B. Go and H-C. Chang, ‘Electrospray cone-jet breakup and droplet production for electrolyte solutions’, EPL, 99, 64003 (2012)
[Abstract]

Drops hop from hydrophilic surfaces (Vol. 46 No. 1)

The impact of liquid droplets on solid surfaces is ubiquitous in many natural and industrial settings. It is now well known that drops can bounce on super-hydrophobic surfaces such as the leaf of a lotus plant or a patterned engineered surface, coated to repel water. Furthermore, it is commonly thought that when impacting hydrophilic substrates, for instance a glass window, drops will splash or spread but never bounce. Here, this assumption is shown to be incorrect - drops do in fact bounce on smooth and defect-free hydrophilic substrates such as very clean glass, silicon wafers, or the surface of cleaved mica.

And they do so by never actually contacting the surface.

Direct experimental evidence supports a surprising mechanism for drop rebound. If the velocity of the drop is relatively low, on impact it spreads over a thin film of air, approaching to within 10 nanometres of the surface, but never truly contacting it; then the drop retracts and detaches from the surface. If, however, the impacting drop exceeds a threshold impact velocity, the nanometre-thick air film breaks down and liquid-solid contact initiates.

J. M. Kolinski, L. Mahadevan and S. M. Rubinstein, “Drops can bounce from perfectly hydrophilic surfaces”, EPL 108, 24001 (2014)
[Abstract]

Dynamic Screening acts on radiating GaAs/AlGaAs quantum wells (Vol. 44 No. 5)

Dependencies of the THz amplitude on the excitation fluence at different bias voltages. The solid curves are results of calculations by the developed model with the following parameters: m_e = 0.065m₀, m_h = 0.5m0, ε = 12.9, α₀ = 1.8x104 cm⁻¹ (electron and hole effective masses, dielectric constant and bulk optical absorption coefficient for ħω_ex = 1.56 eV, respectively). The initial electric field F₀ in the QWs was supposed to be equal to the averaged electric field in the structure part containing QWs.

The article reports results of investigations of generation of coherent terahertz (THz) radiation from 29 nm thick GaAs/Al0.37Ga0.63As quantum wells (QWs) with transverse electric bias under interband femtoseconds laser photoexcitation at room temperature. The detected THz radiation is attributed to the excitation of time-varying dipole moment induced by polarization of non-equilibrium electron–hole pairs in QWs. Noticeable sub-linearity in the dependence of THz amplitude on excitation density is observed. A theoretical model, which accounts for the dynamic screening of the electric field in wide GaAs QWs by nonequilibrium carriers, has been developed. The model describes well the properties of the observed THz signal.

A.V. Andrianov, P.S. Alekseev, G.V. Klimko, S.V. Ivanov, V.L. Tscheglov, I.V. Sedova, and A.O. Zakhar’in, ‘Influence of dynamic screening effect on coherent terahertz radiation from biased GaAs/AlGaAs quantum wells’, Semicond. Sci. Technol. 28, 105012 (2013)
[Abstract]

Dynamical origin of complex motor patterns (Vol. 41, No. 6)

The average activity of a large set of forced, globally coupled excitable units fits the subtleties of the air pressure patterns used by domestic canaries during their song. Each excitable unit is the dynamical caricature of excitatory and inhibitory neural subpopulations in the neural song pathway. Upper panels show different experimental gestures. Lower panels display the simulations.

Many motor patterns in biology are surprisingly simple, particularly taking into account that they are generated by complex neural networks. During song, for example, some songbirds have to generate periodic fluctuations in their air sac pressure and syringeal muscle tension in order to achieve sounds with the adequate acoustic properties. For the case of domestic canaries, the different respiratory patterns used during song were found to be sub-harmonic solutions of a simple, low dimensional dynamical system. Yet, these gestures are generated by thousands of neurons operating in a non-synchronous regime. How can the average activity present the precise, non-trivial features of the solutions of a low dimensional nonlinear dynamical system?

Inspired by this example, we address the general issue of the emergence of low dimensional, non-trivial dynamics out of large, complex interacting units.

In the spirit of classical statistical mechanics, the equations obeyed by the order parameter of a population of globally coupled nonlinear units are derived. The analysis allows showing that non-trivial yet low-dimensional dynamics is possible in average, even in a non-synchronic regime, providing a new mechanism for dimensionality reduction in the dynamics of complex systems.

Dynamical origin of complex motor patterns
L.M. Alonso, J.A. Alliende and G.B. Mindlin, Eur. Phys. J. D 60, 361-367 (2010)
[Abstract] | [PDF]

Dynamics of ultrathin soft matter systems (Vol. 45 No. 1)

Scheme (not in scale) of the molding approach used to determine the interfacial free volume ζ (white circles in the left lower panel), seen here as the opposite of the sur-face coverage of PS (blue circles).

Thin films of liquids and polymers are interesting systems for those seeking to test glass transition theories and their prediction of a characteristic transition length scale of a few nanometers. The anomalous phenomena observed in some of these nano-confined films has greatly advanced our understanding of theoretical and experimental soft matter physics.

These films are treated as equilibrium systems where surfaces and interfaces introduce monotonous long-range mobility gradients. Considering finite size and interfacial effects provides an intuitive but oversimplified picture that falls short of explaining many phenomena, such as enhancement of segmental mobility near an absorbing surface or long-lasting metastable states in the liquid.

The authors propose a new picture of the dynamics of these confined soft matter systems, which focuses on non-equilibrium states and on the impact of irreversible chain adsorption on the structural relaxation.

They review the experimental approaches that have been used to study the structural relaxation of films with one, two or no free surfaces by means of dielectric spectroscopy. Then propose methods to determine gradients of mobility in thin films and discuss the as-yet untapped potential of analyses based on the time, temperature and thickness dependence of the orientational polarization.

S. Napolitano, S. Capponi, and B. Vanroy, “Glassy dynamics of soft matter under 1D confinement: How irreversible adsorption affects molecular packing, mobility gradients and orientational polarization in thin films”, Eur. Phys. J. E, 36, 61 (2013)
[Abstract]

Échelon cracks in soft solids (Vol. 45 No.3)

Stepped crack surface developing from a straight notch (dashed line).

While under pure tension loading, crack surfaces are usually planar, whereas under superimposed shear they generally exhibit steps. Explaining the emergence of this ubiquitous instability remains a challenge in fracture mechanics. We study it here for a highly deformable solid (a hydrogel) and show that:
- échelon steps appear beyond a finite shear/tension threshold;
- contrary to linear elastic fracture mechanics predictions, they do not emerge homogeneously along the crack front via a direct bifurcation, but nucleate on local toughness/stiffness fluctuations. As such, the échelon instability continues the cross
-hatching one, observed on soft solids under pure tension, here biased by shear loading.

We argue that this behavior results from the controlling role of elastic non-linearity.

These results point to the importance of studying whether they remain relevant for stiffer materials, in order to assess the validity limit of the linear elastic approximation.

O. Ronsin et al., "Crack front echelon instability in mixed mode fracture of a strongly nonlinear elastic solid", EPL, 105, 34001 (2014)
[Abstract]

Economics made simple with physics models (Vol. 48 No. 2)

Snapshot of the study of economic phenomena using the tools of physics

How would you go about understanding how markets can suddenly be gripped by panic? To physicists, using a model originally developed to explain magnetism might make sense. Yet, economists may find this extremely counter-intuitive. Both physical and economic phenomena may possess universal features that could be uncovered using the tools of physics. The principal difference is that in economic systems — unlike physical ones — current actions may be influenced by the perception of future events. This European Physical Journal Special Topics issue examines the question as to whether econophysics, a physics-based approach to understanding economic phenomena, is more useful and desirable than conventional economics theories. One of the features emerging from the issue is that the much coveted idea of universality may be the exception rather than the rule in the economic and the social world. Also, many of the originally proposed models of econophysics can be argued to be simplistic rather than simple. Most importantly, a clear-cut demonstration of superiority of econophysics models over standard economics models has yet to be delivered.

S. Sinha, A. S. Chakrabarti and M. Mitra, Can economics be a physical science? Eur. Phys. J. ST 225 Issue 17-18 (2016)
[Abstract]

Effect of chaos on relativistic quantum tunnelling (Vol. 43 No. 6)

Tunnelling rates and local density of states for massless Dirac fermion in integrable and chaotic double-well systems where Φ and Χ are two components of Dirac spinor.

What can classical chaos do to a quantum system is a fundamental issue, which is highly relevant to a number of branches in physics. The field, named quantum chaos, has been active for at least three decades, where the focus has been on non-relativistic quantum systems described by the Schrodinger equation. With respect to relativistic quantum systems governed by the Dirac equation, Berry and Mondragon were the first to investigate the energy-level statistics of a chaotic neutrino billiard.

The present work presents an astonishing case of how chaos may affect relativistic quantum tunnelling dynamics. By developing an efficient method to solve the Dirac equation in the setting where relativistic quantum particles can tunnel between two symmetric cavities through a potential barrier, it appears that chaotic cavities can mostly suppress the spread in the tunnelling rate. Specifically, when the classical dynamics is integrable, the tunnelling rate for any given energy can assume values in a range that increases with energy (fig. upper panel). However, when the cavities allow fully chaotic dynamics, spread in the tunnelling rate is strongly reduced (lower panel). This suppression can be explained by the emergence of certain class of pointer states (fig.). A remarkable feature, which does not arise in non-relativistic quantum tunnelling systems, is that substantial tunnelling exists even when the particle energy nears zero. This is a consequence of the relativistic quantum phenomenon of Klein tunnelling. The authors found similar results in tunnelling devices made of graphene, implying that the field of relativistic quantum chaos can be highly relevant to the development of such devices.

Xuan Ni, Liang Huang, Ying-Cheng Lai and L. M. Pecora, ‘Effect of chaos on relativistic quantum tunnelling’, EPL, 98, 50007 (2012)
[Abstract]

Effective long-range interactions in confined curved dimensions (Vol. 42, No. 6)

Phase space picture of the effective long-range interactions indicating many local equilibriums. (Inset: Number of minima as a function of the pitch of the helix).

Fundamental forces of nature such as the gravitational or Coulomb force mediated by the mass and charge of particles, respectively, decay according to universal laws with increasing distance between the particles. Still, these simple laws lead to the enormous beauty and amazing complexity of matter surrounding us.

In ultra-cold atomic physics the three-dimensional motion of neutral atoms or charged particles can be confined to a lower dimensional typically one- or two-dimensional trap by employing electromagnetic forces. If these traps are of curved character, which is possible e.g. in evanescent fields surrounding optical nano-fibres, they confine the motion to a curved low-dimensional manifold.

This opens the perspective of designing novel effective finite-range and possibly also long-range interactions since the dynamics is constrained to the curved geometry but the interaction takes places via the dynamically forbidden dimensions. The corresponding forces can now become oscillating with increasing distance between the particles and are widely tunable via the parameters of the confining curved manifold. Exploring as a prototype example the one-dimensional helix, it can be immediately shown that a plethora of local equilibrium configurations and consequently bound states emerge already for two particles even if the particles were repelling each other in free space. The number and depths of the local minima and wells can be tuned by modifying the pitch or curvature of the helix thereby establishing bound state configurations of different symmetries.

With an increasing number of interacting particles an ever-increasing wealth of symmetry-adapted and symmetry-distorted configurations create a very complex energy landscape exhibiting a dense spectrum of local equilibriums. It can be anticipated that the thermodynamical properties and quantum physics of the many-body interacting helical chain show novel structural properties such as enriched phase diagrams as well as an intriguing dynamical behaviour.

Effective long-range interactions in confined curved dimensions
P. Schmelcher, EPL, 95, 50005 (2011)
[Abstract]

Effects of electric field-induced versus conventional heating (Vol. 43 No. 1)

Study of mobile phone electric field shown no extraordinary heating

The effect of microwave heating and cell phone radiation on sample material is no different than a temperature increase, according to the present work.

Richert and coworkers attempted for the first time to systematically quantify the difference between microwave-induced heating and conventional heating using a hotplate or an oil-bath, with thin liquid glycerol samples. The authors measured molecular mobility and reactivity changes induced by electric fields in these samples, which can be gauged by what is known as configurational temperature. They realised that thin samples exposed to low-frequency electric field heating can have a considerably higher mobility and reactivity than samples exposed to standard heating, even if they are exactly at the same temperature. They also found that at frequencies exceeding several megahertz and for samples thicker than one millimetre, the type of heating does not have a significant impact on the level of molecular mobility and reactivity, which is mainly dependent on the sample temperature. Actually, the configurational temperatures are only marginally higher than the real measurable one.

Previous studies were mostly fundamental in nature and did not establish a connection between microwaves and mobile phone heating effects. These findings imply that for heating with microwave or cell phone radiation operating in the gigahertz frequency range, no other effect than a temperature increase should be expected.

Since the results are based on averaged temperatures, future work will be required to quantify local overheating, which can, for example, occur in biological tissue subjected to a microwave field, and better assess the risks linked to using both microwaves and mobile phones.

Heating liquid dielectrics by time dependent fields
A. Khalife, U. Pathak and R. Richert, Eur. Phys. J. B 83, 429-435 (2011)
[Abstract]

Effects of time defects in modulated systems (Vol. 51, No. 5)

The spatial periodicity in crystals induces energy band-gaps. Similarly, time modulated systems possess momentum band-gaps. Is there a temporal analogue to the localised edge modes induced by topological defects in spatial crystals?

We show that in a vertically vibrated liquid with a pi-shift in the excitation as a time defect, waves grow exponentially before the defect and decay exponentially after. Because of causality and non-energy conservation, this apparent time localisation must, in fact, be interpreted as a permutation of band-gap modes. However, as such, time defects provide an original way to explore these gaps.

G. d’Hardemare, A. Eddi and E. Fort, Probing Floquet modes in a time periodic system with time defects using Faraday instability, EPL 131, 24007 (2020)
[Abstract]

Efficient Lattice-Boltzmann simulation techniques for nonlinear thermoacoustic engines (Vol. 46 No. 2)

Thermoacoustics is the physics of the interaction of thermal and acoustic fields. The nonlinear acoustic effect and low Mach number compressible flow in thermoacoustic engines make the theoretical analysis of such systems extremely complicated. A new study investigates the nonlinear self-excited thermoacoustic onset in a Rijke tube via the lattice Boltzmann method (LBM), which simulates the fluid flow by tracking the evolution of particles and obtains flow stream and heat transfer patterns from the kinetic level. The adopted LBM model, which was developed by the authors, convincingly simulates the Navier-Stokes-Fourier equations, treating accurately the nonlinear process of wave excitation of coupled fields and providing reliable estimates for pressure, density, velocity and temperature in such a finite geometry.

A nonlinear self-excited standing wave in the Rijke tube is observed from simulations. Agreement is obtained with theoretical predictions when they exist. Instantaneous velocity fields and temperature fields are discussed. The maximal Mach number in the Rijke tube is about 0.035, indicating that the air flow under the limit cycle is the low Mach number compressible flow.

Y. Wang, D.-K. Sun, Y.-L. He, and W.-Q. Tao, “Lattice Boltzmann study on thermoacoustic onset in a Rijke tube”, Eur. Phys. J. Plus, 130, 9 (2015)
[Abstract]

Efficiently manipulating magnetism with a sputtered topological insulator (Vol. 50, No. 3)

The field of spintronics aims to efficiently control the magnetic state of magnetic material using electric currents for better magnetic memories. In this regard, the spin-orbit coupling (SOC) effect, which couples the orbital and spin degrees of the freedom of electrons, has received enormous research attention. The SOC provides a pathway to convert an electric current in a non-magnetic material to a spin current which can subsequently be utilized for fast and efficient control of a magnetic material.

Recently, the material topological insulator (TI) has gained research interest due to its exotic properties including high SOC effect. Although earlier works have utilized the SOC effect of TI to efficiently control the magnetization, the TI material was mostly grown using molecular beam epitaxy technique which is not common in the memory industry. In this work, the authors utilize the more common growth technique of sputtering to grow the TI material (Bismuth Selenide), which allows both the TI and the magnet to be grown in situ. Subsequently, the authors demonstrate a highly efficient switching of the magnetic information storage layer using the high SOC from the sputtered Bismuth Selenide.

R. Ramaswamy, T. Dutta, S. Liang, G. Yang, M. S. M. Saifullah, and H. Yang, J. Phys. D: Appl. Phys. 52, 224001 (2019)

Eigenvalue problem in 2D for an irregular boundary (Vol. 42, No. 5)

Comparison of the eigenvalues obtained numerically and analytically for an elliptical boundary (in units of 1/R_o²) with Neumann condition for the first 7 states.

The Helmholtz equation arises in a number of physical contexts as one reduces the wave equation by considering single frequency propagation. One such application appears in studying wave behaviour in waveguides. While waveguides in technology are carefully engineered to have, for instance, constant and simple cross-sectional geometries, natural waveguides relax such engineering constraints. (Natural waveguides range across systems from atmospheric and oceanic ducts, to biological systems such as seal and polar bear hair fibres.) In particular the cross section of natural waveguides tends to have a complex boundary shape that is far from the ideal circular or rectangular form of engineered waveguides.

The present team proposes a new approach to the irregular boundary 2-d Helmholtz equation with Neumann boundary conditions (specified normal derivatives of the field at the boundary). This boundary condition has significant physical importance: it is the natural one for electromagnetic propagation in the TM mode in waveguides.

To date the most successful efforts to solve the irregular boundary Helmholtz equation have been computational, but even this general method has its drawbacks. The present analytic perturbative approach solves the irregular boundary problem via a perturbative series. The authors explicitly work out several nontrivial examples. The benefits of this approach include, most importantly, an analytical understanding of the behaviour of the solution as the amplitude of the boundary distortion is increased.

Another important feature of Panda et al.’s expression is its analytic precision in the terms computed and its analytic error estimates for the terms truncated from the series. Together these give the analytic methods a much larger dynamic range than available computationally.

Eigenvalue problem in two dimensions for an irregular boundary: Neumann condition
S. Panda, S. Chakraborty, and S.P. Khastgir, Eur. Phys. J. Plus, 126, 62 (2011)
[Abstract]

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