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Bandi, M. M., Concha, A., Wood, R., & Mahadevan, L. (2013). A pendulum in a flowing soap film. Phys. Fluids, 25(4), 6 pp.
Abstract: We consider the dynamics of a pendulum made of a rigid ring attached to an elastic filament immersed in a flowing soap film. The system shows an oscillatory instability whose onset is a function of the flow speed, length of the supporting string, the ring mass, and ring radius. We characterize this system and show that there are different regimes where the frequency is dependent or independent of the pendulum length depending on the relative magnitude of the addedmass. Although the system is an infinitedimensional, we can explain many of our results in terms of a one degreeoffreedom system corresponding to a forced pendulum. Indeed, using the vorticity measured via particle imaging velocimetry allows us to make the model quantitative, and a comparison with our experimental results shows we can capture the basic phenomenology of this system. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4800057]

Basu, S., Yawar, A., Concha, A., & Bandi, M. M. (2017). On angled bounceoff impact of a drop impinging on a flowing soap film. Fluid Dyn. Res., 49(6), 19 pp.
Abstract: Small drops impinging obliquely on thin flowing soap films frequently demonstrate the rare emergence of bulk elastic effects working intandem with the more commonplace hydrodynamic interactions. Three collision regimes are observable: (a) drop piercing through the film, (b) it coalescing with the flow, and (c) it bouncing off the film surface. During impact, the drop deforms along with a bulk elastic deformation of the film. For impacts that are closetotangential, the bounceoff regime predominates. We outline a reduced order analytical framework assuming a deformable drop and a deformable threedimensional film, and the idealization invokes a phasebased parametric study. Angular inclination of the film and the ratio of post and preimpact drop sizes entail the phase parameters. We also perform experiments with vertically descending droplets (constituted from deionized water) impacting against an inclined soap film, flowing under constant pressure head. Modelpredicted phase domain for bounceoff compares well to our experimental findings. Additionally, the experiments exhibit momentum transfer to the film in the form of shed vortex dipoles, along with propagation of free surface waves. On consulting prior published work, we note that for locomotion of waterwalking insects using an impulsive action, the momentum distribution to the shed vortices and waves are both significant, taking up respectively 2/3 and 1/3 of the imparted streamwise momentum. Considering the visually similar impulse actions, this theory, despite its assumption of a quiescent liquid bath of infinite depth, is applied to the drop bounceoff experiments, and the resultant shed vortex dipole momenta are compared to the momenta of the coherent vortex structures computed from particle imaging velocimetry data. The magnitudes reveal identical order (10(7) N s), suggesting that notwithstanding the disparities, the bounceoff regime may be tapped as a toy analog for impulsebased interfacial biolocomotion.
Keywords: drop impact; soap film; interfacial flow; bouncing drops

Carrasco, M., Mery, D., Concha, A., Velazquez, R., De Fazio, R., & Visconti, P. (2021). An Efficient PointMatching Method Based on Multiple Geometrical Hypotheses. Electronics, 10(3), 246.
Abstract: Point matching in multiple images is an open problem in computer vision because of the numerous geometric transformations and photometric conditions that a pixel or point might exhibit in the set of images. Over the last two decades, different techniques have been proposed to address this problem. The most relevant are those that explore the analysis of invariant features. Nonetheless, their main limitation is that invariant analysis all alone cannot reduce false alarms. This paper introduces an efficient pointmatching method for two and three views, based on the combined use of two techniques: (1) the correspondence analysis extracted from the similarity of invariant features and (2) the integration of multiple partial solutions obtained from 2D and 3D geometry. The main strength and novelty of this method is the determination of the pointtopoint geometric correspondence through the intersection of multiple geometrical hypotheses weighted by the maximum likelihood estimation sample consensus (MLESAC) algorithm. The proposal not only extends the methods based on invariant descriptors but also generalizes the correspondence problem to a perspective projection model in multiple views. The developed method has been evaluated on three types of image sequences: outdoor, indoor, and industrial. Our developed strategy discards most of the wrong matches and achieves remarkable Fscores of 97%, 87%, and 97% for the outdoor, indoor, and industrial sequences, respectively.

Cisternas, J., & Concha, A. (2024). Searching nontrivial magnetic equilibria using the deflated Newton method. Chaos Solitons Fractals, 179, 114468.
Abstract: Nonlinear systems that model physical experiments often have many equilibrium configurations, and the number of these static solutions grows with the number of degrees of freedom and the presence of symmetries. It is impossible to know a priori how many equilibria exist and which ones are stable or relevant, therefore from the modeler's perspective, an exhaustive search and symmetry classification in the space of solutions are necessary. With this purpose in mind, the method of deflation (introduced by Farrell as a modification of the classic Newton iterative method) offers a systematic way of finding every possible solution of a set of equations. In this contribution we apply deflated Newton and deflated continuation methods to a model of macroscopic magnetic rotors, and find hundreds of new equilibria that can be classified according to their symmetry. We assess the benefits and limitations of the method for finding branches of solutions in the presence of a symmetry group, and explore the high dimensional basins of attraction of the method in selected 2 dimensional sections, illustrating the effect of deflation on the convergence.
Keywords: Magnets; Symmetry; Newton method; Deflation; Basins of attraction

Cisternas, J., Mellado, P., Urbina, F., Portilla, C., Carrasco, M., & Concha, A. (2021). Stable and unstable trajectories in a dipolar chain. Phys. Rev. B, 103(13), 134443.
Abstract: In classical mechanics, solutions can be classified according to their stability. Each of them is part of the possible trajectories of the system. However, the signatures of unstable solutions are hard to observe in an experiment, and most of the times if the experimental realization is adiabatic, they are considered just a nuisance. Here we use a small number of XY magnetic dipoles subject to an external magnetic field for studying the origin of their collective magnetic response. Using bifurcation theory we have found all the possible solutions being stable or unstable, and explored how those solutions are naturally connected by points where the symmetries of the system are lost or restored. Unstable solutions that reveal the symmetries of the system are found to be the culprit that shape hysteresis loops in this system. The complexity of the solutions for the nonlinear dynamics is analyzed using the concept of boundary basin entropy, finding that the damping timescale is critical for the emergence of fractal structures in the basins of attraction. Furthermore, we numerically found domain wall solutions that are the smallest possible realizations of transverse walls and vortex walls in magnetism. We experimentally confirmed their existence and stability showing that our system is a suitable platform to study domain wall dynamics at the macroscale.
Keywords: MAGNETIC MONOPOLES; CRYSTAL STATISTICS; INTERFERENCE; HYSTERESIS; RELAXATION; ENTROPY

Cisternas, J., Navarro, M., Duarte, S., & Concha, A. (2022). Equilibrium and symmetries of altitudinal magnetic rotors on a circle. Chaos, 32(12), 123120.
Abstract: Macroscopic magnets can easily be manipulated and positioned so that interactions between themselves and with external fields induce interesting dynamics and equilibrium configurations. In this work, we use rotating magnets positioned in a line or at the vertices of a regular polygon. The rotation planes of the magnets can be modified at will. The rich structure of stable and unstable configurations is dictated by symmetry and the side of the polygon. We show that both symmetric solutions and their symmetrybreaking bifurcations can be explained with group theory. Our results suggest that the predicted magnetic textures should emerge at any length scale as long as the interaction is polar, and the system is endowed with the same symmetries.
Keywords: ARTIFICIAL SPIN ICE; DYNAMICS

Concha, A., Aguayo, D., & Mellado, P. (2018). Designing Hysteresis with Dipolar Chains. Phys. Rev. Lett., 120(15), 5 pp.
Abstract: Materials that have hysteretic response to an external field are essential in modern information storage and processing technologies. A myriad of magnetization curves of several natural and artificial materials have previously been measured and each has found a particular mechanism that accounts for it. However, a phenomenological model that captures all the hysteresis loops and at the same time provides a simple way to design the magnetic response of a material while remaining minimal is missing. Here, we propose and experimentally demonstrate an elementary method to engineer hysteresis loops in metamaterials built out of dipolar chains. We show that by tuning the interactions of the system and its geometry we can shape the hysteresis loop which allows for the design of the softness of a magnetic material at will. Additionally, this mechanism allows for the control of the number of loops aimed to realize multiplevalued logic technologies. Our findings pave the way for the rational design of hysteretical responses in a variety of physical systems such as dipolar cold atoms, ferroelectrics, or artificial magnetic lattices, among others.

Concha, A., Mellado, P., MoreraBrenes, B., Costa, C. S., Mahadevan, L., & MongeNajera, J. (2015). Oscillation of the velvet worm slime jet by passive hydrodynamic instability. Nat. Commun., 6, 6 pp.
Abstract: The rapid squirt of a proteinaceous slime jet endows velvet worms (Onychophora) with a unique mechanism for defence from predators and for capturing prey by entangling them in a disordered web that immobilizes their target. However, to date, neither qualitative nor quantitative descriptions have been provided for this unique adaptation. Here we investigate the fast oscillatory motion of the oral papillae and the exiting liquid jet that oscillates with frequencies f similar to 3060 Hz. Using anatomical images, highspeed videography, theoretical analysis and a physical simulacrum, we show that this fast oscillatory motion is the result of an elastohydrodynamic instability driven by the interplay between the elasticity of oral papillae and the fast unsteady flow during squirting. Our results demonstrate how passive strategies can be cleverly harnessed by organisms, while suggesting future oscillating microfluidic devices, as well as novel ways for micro and nanofibre production using bioinspired strategies.

Mellado, P., Concha, A., Hofhuis, K., & Tapia, I. (2023). Intrinsic chiral field as vector potential of the magnetic current in the zigzag lattice of magnetic dipoles. Sci. Rep., 13(1), 1245.
Abstract: Chiral magnetic insulators manifest novel phases of matter where the sense of rotation of the magnetization is associated with exotic transport phenomena. Effective control of such phases and their dynamical evolution points to the search and study of chiral fields like the DzyaloshinskiiMoriya interaction. Here we combine experiments, numerics, and theory to study a zigzag dipolar lattice as a model of an interface between magnetic inplane layers with a perpendicular magnetization. The zigzag lattice comprises two parallel sublattices of dipoles with perpendicular easy plane of rotation. The dipolar energy of the system is exactly separable into a sum of symmetric and antisymmetric longrange exchange interactions between dipoles, where the antisymmetric coupling generates a nonlocal DzyaloshinskiiMoriya field which stabilizes winding textures with the form of chiral solitons. The DzyaloshinskiiMoriya interaction acts as a vector potential or gauge field of the magnetic current and gives rise to emergent magnetic and electric fields that allow the manifestation of the magnetoelectric effect in the system.

Mellado, P., Concha, A., & Rica, S. (2020). Magnetoelectric Effect in Dipolar Clusters. Phys. Rev. Lett., 125(23), 237602.
Abstract: We combine the anisotropy of magnetic interactions and the point symmetry of finite solids in the study of dipolar clusters as new basic units for multiferroics metamaterials. The Hamiltonian of magnetic dipoles with an easy axis at the vertices of polygons and polyhedra, maps exactly into a Hamiltonian with symmetric and antisymmetric exchange couplings. The last one gives rise to a DzyaloshinskiiMoriya contribution responsible for the magnetic modes of the systems and their symmetry groups, which coincide with those of a particle in a crystal field with spinorbit interaction. We find that the clusters carry spin current and that they manifest the magnetoelectric effect. We expect our results to pave the way for the rational design of magnetoelectric devices at room temperature

TapiaBelmonte, F., Concha, A., & Poupin, M. J. (2023). The Effects of Uniform and Nonuniform Magnetic Fields in Plant Growth: A MetaAnalysis Approach. Bioelectromagnetics, Early Access.
Abstract: Magnetic field (MF) effects have been reported in plants' growth, seed germination, gene expression, and water consumption. Accordingly, magnetic treatments have been proposed as a sustainable alternative to improve yields. Nevertheless, a comprehensive quantitative assessment is needed to understand whether their effects are general, speciesspecific, or dependent on the experimental setting. We conducted a multilevel metaanalysis of 45 articles that studied 29 different plant species. A positive and neutral effect of a nonuniform MF was found on fresh weight and germination rate, respectively. A significant association was found between a uniform MF and germination. These results suggest that MFs improve plant growth. However, the effects are highly dependent on the experimental setting. This opens exciting questions about the biophysical mechanisms underlying the perception and transduction of this environmental cue and about the possible translation to agricultural practices

Urbina, F., Franco, A. F., & Concha, A. (2022). Frequency dynamics of a chain of magnetized rotors: dumbbell model vs LandauLifshitz equation. J. Phys. Condens. Matter, 34(48), 485801.
Abstract: During the past decades magnetic materials and structures that span several length scales have been of interest mainly due to their application in data storage and processing, flexible electronics, medicine, between others. From a microscopic point of view, these systems are typically studied using the LandauLifshitz equation (LLE), while approaches such as the dumbbell model are used to study macroscopic magnetic structures. In this work we use both the LLE and the dumbbell model to study spin chains of various lengths under the effect of a time dependentmagnetic field, allowing us to compare qualitatively the results obtained by both approaches. This has allowed us to identify and describe in detail several frequency modes that appear, with additional modes arising as the chain length increases. Moreover, we find that high frequency modes tend to be absorbed by lower frequency ones as the amplitude of the field increases. The results obtained in this work are of interest not only to better understand the behavior of the macroscopic spins chains, but also expands the available tools for qualitative studies of both macroscopic and microscopic versions of the studied system, or more complex structures such as junctions or lattices. This would allow to study the qualitative behavior of microscopic systems (e.g. nanoparticles) using macroscopic arrays of magnets, and vice versa.
Keywords: macroscopic system; magnetic rotors; frequency modes
