Table of contents

We show that the first order magneto-structural phase transitions observed in various classes of magnetic solids are often accompanied by useful multi-functional properties, namely giant magneto-resistance, magneto-caloric effect and magneto-striction. We highlight various characteristic features associated with a disorder influenced first order phase transition namely supercooling, superheating, phase-coexistence and metastability, in several magnetic materials and discuss how a proper understanding of the transition process can help in fine tuning of the accompanied functional properties. Magneto-elastic coupling is a key element in this first order phase transition, and methods need to be explored for maximizing the contributions from both the lattice and the magnetic degree of freedom while simultaneously minimizing the thermomagnetic hysteresis loss. An analogy is also drawn with the first order phase transition observed in dielectric materials and vortex matter of type-II superconductors.

When looking at a wetted surface with a technique that can probe the nanoscale, a high surface coverage of gas bubbles is often revealed. So what? Well, if we believe in classical diffusion, these bubbles should dissolve in microseconds, but in reality they are found to remain stable for as long as anyone has observed (five days thus far, which is 10–11 orders of magnitude longer than would be expected). As well as the obvious question of why the lifetime is so long, and also the question of how the bubbles nucleate in the first place, we rapidly find ourselves asking can we use the bubbles to our benefit? A clear example would be in controlling slip in micro/nanofluidics: effectively, replacing a solid wall with a 'gassy' wall replaces the no-slip boundary condition with one of slip. Several other potential applications have also been suggested and nanobubbles have, in fact, already proven useful in the antifouling world.

Returning to fundamentals, another near-wall gas domain has also come to light through our investigations into nanobubbles. The micropancake is thought to be a quasi-2D dense adsorbate of gas molecules (i.e. N₂ or O₂) which grows epitaxially on the surface. New questions now include: why are micropancakes stable, how do they form, and what is their relationship with nanobubbles?

Progress is being made in this field and, as with all new topics, the community is rapidly converging toward a standard set of 'minimum' requirements for scientific reporting. For example, taking single-shot atomic force microscopy data is almost definitely no longer sufficient to be additive to the field (there are far too many unrepeatable single-shot measurements in the literature which are too often used as 'evidence', even though there are a seemingly equal number of single-shot measurements that may disagree). Just quoting a 'set-point' is now also insufficient (both set-point and free (or interaction) amplitude are required to know the applied force of an AFM)—hard core statistics with real numbers are now what matters. Also, carrying out molecular dynamics simulations with either zero-Kelvin walls or Lennard–Jones (LJ) potentials is almost definitely insufficient (the first necessarily forces the production of micropancakes, whilst for the second the community knows that using 'real' water models most often gives different results to their simplistic LJ counterparts). This cautionary note also extends to the new wave of experiments using optical visualization: yes we can now access fast dynamics, but (and I am writing from personal experience) we have an even bigger and necessary job to prove that the objects are not droplet contamination from, e.g. oil (AFM allows us to 'poke' bubbles and coalesce them; ATR allows us to 'see' that the bubbles contain gas; no such probe exists for optical measurements).

I think in the future we will see many strong contributions to the field where a combination of several techniques is used to tell a coherent story—the level of complexity and rigor must advance in order for the field to follow. It is great to see some of the contributions to this special section taking a closer look at the interpretative difficulties that are involved in the field, but which are, we hope, close to being understood.

I would like to thank the invited authors whom have contributed to this special section. I am also grateful to the editorial staff of Journal of Physics: Condensed Matter for their help in producing this special section.

Surface nanobubbles and micropancakes

Surface nanobubbles and micropancakesJames R T Seddon

Nanobubble assisted nanopatterning utilized for ex situ identification of surface nanobubblesH Tarábková and P Janda

Transient nanobubbles in short-time electrolysisVitaly B Svetovoy, Remco G P Sanders and Miko C Elwenspoek

Nanobubbles and their role in slip and dragAbdelhamid Maali and Bharat Bhushan

Modeling of bubble nucleation of an air--water mixture near hydrophobic wallsDi Zhou, Ziaul Haque Ansari and Jianguo Mi

The effect of PeakForce tapping mode AFM imaging on the apparent shape of surface nanobubblesWiktoria Walczyk, Peter M Schön and Holger Schönherr

Coarse-grained modelling of surface nanobubblesPatrick Grosfils

Understanding the stability of surface nanobubblesShuo Wang, Minghuan Liu and Yaming Dong

Hydrogen nanobubble at normal hydrogen electrodeS Nakabayashi, R Shinozaki, Y Senda and H Y Yoshikawa

Particle tracking around surface nanobubblesErik Dietrich, Harold J W Zandvliet, Detlef Lohse and James R T Seddon

Imaging surface nanobubbles at graphite--water interfaces with different atomic force microscopy modesChih-Wen Yang, Yi-Hsien Lu and Ing-Shouh Hwang

Nanobubble assisted nanopatterning of polystyrene (PS) film allows visualization of nanobubble positions and identification of their appearance on the surface ex post by atomic force microscopy (AFM) imaging ex situ. Due to the PS nanograin size, ∼10¹ nm, nanobubbles of diameter less than 50 nm can be resolved, as well as microbubbles and micropancakes. The time scale of the nanopattern formation was found to be in the seconds range. This relatively short exposure time thus provides the possibility of also resolving some aspects of microbubble and nanobubble dynamics. In this work we demonstrate that ex post, ex situ AFM imaging of a PS surface after its exposure to deionized water allows us to examine nanobubble and microbubble formation, distribution and arrangement without any influence of the AFM scanning tip and under experimental conditions where in situ AFM imaging is not feasible, e.g. in liquid flow.

Water electrolysis in a microsystem is observed and analyzed on a short-time scale of ∼10 μs. The very unusual properties of the process are stressed. An extremely high current density is observed because the process is not limited by the diffusion of electroactive species. The high current is accompanied by a high relative supersaturation, S > 1000, that results in homogeneous nucleation of bubbles. On the short-time scale only nanobubbles can be formed. These nanobubbles densely cover the electrodes and aggregate at a later time to microbubbles. The effect is significantly intensified with a small increase of temperature. Application of alternating polarity voltage pulses produces bubbles containing a mixture of hydrogen and oxygen. Spontaneous reaction between gases is observed for stoichiometric bubbles with sizes smaller than ∼150 nm. Such bubbles disintegrate violently affecting the surfaces of the electrodes.

Atomic force microscope images of flat solid surfaces in water reveal that very soft objects can be formed on the surfaces. These objects are nanobubbles of gas with sizes ranging from 10 nm to several micrometers. The bubbles are stable to dissolution, lasting for several hours. In this paper we review some of the methods that allow their generation and observation using the atomic force microscope. Next, we describe the influence of the bubbles on liquid slip close to a hydrophobic surface. The influence of liquid–gas menisci, formed as a result of nanobubbles being present on the surface, on drag reduction is also discussed. Finally, data of liquid flow probed on bubbles entrapped on microstructured surfaces are presented.

In this work, a density functional approach is applied to calculate the interfacial thermodynamic properties of an air–water mixture in the presence of a hydrophobic wall. Without any mixing parameter, the theoretical model can correctly reproduce the measured interfacial tensions of a nitrogen–water binary mixture. The density profiles of the dissolved air and water near the hydrophobic walls are predicted using the validated model. It is shown clearly that a hydrophobic wall leads to air enrichment and water depletion. According to the extent of air enrichment, the free energy barriers and critical radii of bubble nucleation at different hydrophobic interfaces are calculated to evaluate the possibility of spontaneous bubble nucleation.

Until now, TM AFM (tapping mode or intermittent contact mode atomic force microscopy) has been the most often applied direct imaging technique to analyze surface nanobubbles at the solid–aqueous interface. While the presence and number density of nanobubbles can be unequivocally detected and estimated, it remains unclear how much the a priori invasive nature of AFM affects the apparent shapes and dimensions of the nanobubbles. To be able to successfully address the unsolved questions in this field, the accurate knowledge of the nanobubbles' dimensions, radii of curvature etc is necessary. In this contribution we present a comparative study of surface nanobubbles on HOPG (highly oriented pyrolytic graphite) in water acquired with (i) TM AFM and (ii) the recently introduced PFT (PeakForce tapping) mode, in which the force exerted on the nanobubbles rather than the amplitude of the resonating cantilever is used as the AFM feedback parameter during imaging. In particular, we analyzed how the apparent size and shape of nanobubbles depend on the maximum applied force in PFT AFM. Even for forces as small as 73 pN, the nanobubbles appeared smaller than their true size, which was estimated from an extrapolation of the bubble height to zero applied force. In addition, the size underestimation was found to be more pronounced for larger bubbles. The extrapolated true nanoscopic contact angles for nanobubbles on HOPG, measured in PFT AFM, ranged from 145° to 175° and were only slightly underestimated by scanning with non-zero forces. This result was comparable to the nanoscopic contact angles of 160°–175° measured using TM AFM in the same set of experiments. Both values disagree, in accordance with the literature, with the macroscopic contact angle of water on HOPG, measured here to be 63° ± 2°.

Surface nanobubbles are nanoscale gaseous objects that form on hydrophobic surfaces in contact with water. Understanding nanobubble formation and stability remains challenging due to the lack of appropriate theoretical framework and adequate modelling. Here we present a non-equilibrium coarse-grained model for nanobubbles at hydrophobic surfaces. The model is based on a lattice-gas model that has been proposed to understand the hydrophobic effect to which dynamical properties are added. The results presented demonstrate the ability of the model to reproduce the basic features of stable surface nanobubbles, which, thereby, supports the dynamical origin of these objects.

Surface nanobubbles emerging at solid–liquid interfaces show extreme stability. In this paper, the stability of surface nanobubbles in degassed water is discussed and investigated by AFM. The result demonstrates that surface nanobubbles are kinetically stable and the liquid/gas interface is gas impermeable. The force modulation experiment further proves that there is a layer coating on nanobubbles. These critical properties suggest that surface nanobubbles may be stabilized by a layer which has a great diffusive resistance.

Electrochemically formed hydrogen nanobubbles at a platinum rotating disk electrode (RDE) were detected by re-oxidation charge. The dissolution time course of the hydrogen nanobubbles was measured by AFM tapping topography under open-circuit conditions at stationary platinum and gold single-crystal electrodes. The bubble dissolution at platinum was much faster than that at gold because two types of diffusion, bulk and surface diffusion, proceeded at the platinum surface, whereas surface diffusion was prohibited at the gold electrode. These findings indicated that the electrochemical reaction of normal hydrogen electrode partly proceeded heterogeneously on the three-phase boundary around the hydrogen nanobubble.

The exceptionally long lifetime of surface nanobubbles remains one of the biggest questions in the field. One of the proposed mechanisms for producing the stability is the dynamic equilibrium model, which describes a constant flux of gas in and out of the bubble. Here, we describe results from particle tracking experiments carried out to measure this flow. The results are analysed by measuring the Voronoï cell size distribution, the diffusion, and the speed of the tracer particles. We show that there is no detectable difference in the movement of particles above nanobubble-laden surfaces as compared to ones above nanobubble-free surfaces.

We have imaged nanobubbles on highly ordered pyrolytic graphite (HOPG) surfaces in pure water with different atomic force microscopy (AFM) modes, including the frequency-modulation, the tapping, and the PeakForce techniques. We have compared the performance of these modes in obtaining the surface profiles of nanobubbles. The frequency-modulation mode yields a larger height value than the other two modes and can provide more accurate measurement of the surface profiles of nanobubbles. Imaging with PeakForce mode shows that a nanobubble appears smaller and shorter with increasing peak force and disappears above a certain peak force, but the size returns to the original value when the peak force is reduced. This indicates that imaging with high peak forces does not cause gas removal from the nanobubbles. Based on the presented findings and previous AFM observations, the existing models for nanobubbles are reviewed and discussed. The model of gas aggregate inside nanobubbles provides a better explanation for the puzzles of the high stability and the contact angle of surface nanobubbles.

We study the charge transport and heat transfer through a nano-junction composed of a small metallic grain weakly coupled to two metallic leads. We focus on the cotunneling regime out-of-equilibrium, where the bias voltage and the temperature gradient between the leads strongly drive electron and phonon degrees of freedom in the grain, which in turn have a strong feedback on the transport through the grain. We derive and solve the heat balance equation for electron and phonon degrees of freedom in the grain and self-consistently find the current–voltage characteristics. We demonstrate that the transport in the nano-junction is very sensitive to the spectrum of the bosonic modes in the grain.

Powder samples of Fe_1−xCu_xCr₂S₄ with x = 0,0.2,0.5,0.8 were studied, between 5 and 300 K. The results reveal that for x < 1, the magnetic order in the series is more varied than the simple collinear ferrimagnetic structure traditionally assumed to exist everywhere from the Curie point to T → 0. In FeCr₂S₄ several ordered magnetic phases are present, with the ground state likely to have an incommensurate cone-like helical structure. Fe_0.8Cu_0.2Cr₂S₄ is the compound for which simple collinear ferrimagnetism is best developed. In Fe_0.5Cu_0.5Cr₂S₄ the ferrimagnetic spin structure is not stable, causing spin reorientation around 90 K. In Fe_0.2Cu_0.8Cr₂S₄ the ferrimagnetic structure is at low temperatures considerably distorted locally, but with rising temperature this disorder shows a rapid reduction, coupled to increased spin fluctuation rates. In summary, the present data show that the changes induced by the replacement of Fe by Cu have more profound influences on the magnetic properties of the Fe_1−xCu_xCr₂S₄ compounds than merely a shift of Curie temperature, saturation magnetization and internal field magnitude.

We use the model of covalent magnetism and its application to magnetic insulators applied to the case of insulating carbon doped BaTiO₃. Since the usual Stoner mechanism is not applicable we study the possibility of the formation of magnetic order based on a mechanism favoring singly occupied orbitals. On the basis of our model parameters we formulate a criterion similar to the Stoner criterion but also valid for insulators. We describe the model of covalent magnetism using a molecular orbital picture and determine the occupation numbers for spin-up and spin-down states. Our model allows a simulation of the results of our ab initio calculations for E() which are found to be in very good agreement.

We report on a comprehensive investigation of the magnetic properties of [NdCo(bpdo)(H₂O)₄(CN)₆]⋅3H₂O (bpdo=4, 4'-bipyridine-N,N'-dioxide) by use of electron paramagnetic resonance, magnetization, specific heat and susceptibility measurements. The studied material was identified as a magnet with an effective spin S = 1/2 and a weak exchange interaction J/k_B = 25 mK. The ac susceptibility studies conducted at audio frequencies and at temperatures from 1.8 to 9 K revealed that the application of a static magnetic field induces a slow spin relaxation. It is suggested that the relaxation in the magnetic field appears due to an Orbach-like process between the two lowest doublet energy states of the magnetic Nd³⁺ ion. The appearance of the slow relaxation in a magnetic field cannot be associated with a resonant phonon trapping. The obtained results suggest that the relaxation is influenced by nuclear spin driven quantum tunnelling which is suppressed by external magnetic field.

Polycrystalline Nd₂Ru₂O₇ samples have been prepared and examined using a combination of structural, magnetic, and electrical and thermal transport studies. Analysis of synchrotron x-ray and neutron diffraction patterns suggests some site disorder on the A-site in the pyrochlore sublattice: Ru substitutes on the Nd-site up to 7.0(3)%, regardless of the different preparative conditions explored. Intrinsic magnetic and electrical transport properties have been measured. Ru 4d spins order antiferromagnetically at 143 K, as seen both in the susceptibility and in the specific heat, and there is a corresponding change in the electrical resistivity. The onset of a second antiferromagnetic ordering transition seen below 5 K is attributed to ordering of Nd 4f spins. Nd₂Ru₂O₇ is an electrical insulator, and this behaviour is believed to be independent of the Ru-antisite disorder on the Nd-site. The electrical properties of Nd₂Ru₂O₇ are presented in the light of data published on all A₂Ru₂O₇ pyrochlores, and we emphasize the special structural role that Bi³⁺ ions on the A-site play in driving metallic behaviour. High-temperature thermoelectric properties have also been measured. When considered in the context of known thermoelectric materials with useful figures-of-merit, it is clear that Nd₂Ru₂O₇ has excessively high electrical resistivity which prevents it from being an effective thermoelectric. A method for screening candidate thermoelectrics is suggested.