arXiv subjects

Sergio Santos

Publications and source records attributed to Sergio Santos.

11 recordsLinked to original sources

arXiv2026

Hidden Information in Force Curves: Transient- and Brownian-Driven Dynamics

Force distance curves (FCs) are direct probes of tip sample interactions in atomic force microscopy, but their dynamic content is often suppressed by filtering, averaging and quasistatic analysis. Indeed, in the forty years since the invention of the AFM, this hidden dynamic signal has been largely ignored, with exceptions restricted to the large oscillations associated with relatively large adhesive separations (snap from contact) or to time averaged approaches that carry their own challenges. Using high bandwidth interferometric detection, we directly measure fast FCs, including short lived cantilever oscillations during snap in, contact and pull off. Two excitation mechanisms coexist: thermal fluctuations and transients generated by rapid changes in the tip sample boundary condition. Cycle resolved frequency and amplitude estimators provide time local observables of the evolving cantilever and sample system within single FCs, creating a pixel by pixel mechanical contrast channel that requires no external broadband excitation, resonant tracking or multi pass imaging. A state dependent harmonic oscillator model incorporating van der Waals attraction, hysteretic capillary bridge formation and rupture, hydration mediated damping and repulsive contact stiffness partitions each FC into five interaction regimes and reproduces the observed responses. In repulsive contact, frequency shifts yield stiffness maps consistent with contact-mechanics estimates, enabling high throughput stiffness mapping and localization of transient tip sample events from individual pixels. More broadly, this hitherto hidden signal may pave the way for new ways of studying and exploiting Brownian motion in a variety of systems, providing the capability to follow time resolved (non) Markovian and (non) ergodic dynamics mediated by the tip sample interaction, including single event dynamics.

cond-mat.mes-hall↗

arXiv2025

Drops on architected elastic substrates: A repertoire of regimes at the turn of a knob

Drops on a vibrating substrate can experience a variety of motion regimes, including directional motion and climbing. The key ingredient to elicit these regimes is simultaneously activating the in-plane and out-of-plane degrees of freedom of the substrate with the proper phase difference. This is typically achieved by using a rigid substrate and two independent actuators. However, this framework is unable to establish different motion conditions in different regions of the substrate, achieving spatial variability and selectivity, since this would violate the rigid-body assumption and require a proliferation of actuation channels. Challenging this paradigm, we leverage the inherent elasticity of the substrate to provide the modal and spatial diversity required to establish the desired regimes. To this end, we design deformable substrates exhibiting a rich landscape of deformation modes, and we exploit their multi-modal response to switch between drop motion regimes and select desired spatial patterns, using the excitation frequency as our tuning parameter.

physics.flu-dyn↗

arXiv2021

Uncovering contributing factors to interruptions in the power grid: An Arctic case

Electric failures are a problem for customers and grid operators. Identifying causes and localizing the source of failures in the grid is critical. Here, we focus on a specific power grid in the Arctic region of North Norway. First, we collect data pertaining to the grid topology, the topography of the area, the historical meteorological data, and the historical energy consumption/production data. Then, we exploit statistical and machine-learning techniques to predict the occurrence of failures. We interpret the variables that mostly explain the classification results to be the main driving factors of power interruption. We are able to predict 57% (F1-score 0.53) of all failures reported over a period of 1 year with a weighted support-vector machine model. Wind speed and local industry activity are found to be the main controlling parameters where the location of exposed power lines is a likely trigger. In summary, we discuss causing factors for failures in the power grid and enable the distribution system operators to implement strategies to prevent and mitigate incoming failures.

physics.soc-ph↗

arXiv2020

Advances in dynamic AFM: from nanoscale energy dissipation to material properties in the nanoscale

Since the inception of the atomic force microscope AFM, dynamic methods have been very fruitful by establishing methods to quantify dissipative and conservative forces in the nanoscale and by providing a means to apply gentle forces to the samples with high resolution. Here we review developments that cover over a decade of our work on energy dissipation, phase contrast and the extraction of relevant material properties from observables. We describe the attempts to recover material properties via one dimensional amplitude and phase curves from force models and explore the evolution of these methods in terms of force reconstruction, fits of experimental measurements, and the more recent advances in multifrequency AFM.

cond-mat.mes-hall↗

arXiv2018

The clustering of typical Ly$α$ emitters from $z \sim 2.5 - 6$: host halo masses depend on Ly$α$ and UV luminosities

We investigate the clustering and halo properties of $\sim 5000$ Ly$α$-selected emission line galaxies (LAEs) from the Slicing COSMOS 4K (SC4K) and from archival NB497 imaging of SA22 split in 15 discrete redshift slices between $z \sim 2.5 - 6$. We measure clustering lengths of $r_0 \sim 3 - 6\ h^{-1}$ Mpc and typical halo masses of $\sim 10^{11}$ M$_\odot$ for our narrowband-selected LAEs with typical $L_{\rm{Ly}α} \sim 10^{42 - 43}$ erg s$^{-1}$. The intermediate band-selected LAEs are observed to have $r_0 \sim 3.5 - 15\ h^{-1}$ Mpc with typical halo masses of $\sim 10^{11 - 12}$ M$_\odot$ and typical $L_{\rm{Ly}α} \sim 10^{43 - 43.6}$ erg s$^{-1}$. We find a strong, redshift-independent correlation between halo mass and Ly$α$ luminosity normalized by the characteristic Ly$α$ luminosity, $L^\star(z)$. The faintest LAEs ($L \sim 0.1\ L^\star(z)$) typically identified by deep narrowband surveys are found in $10^{10}$ M$_\odot$ halos and the brightest LAEs ($L \sim 7\ L^\star(z)$) are found in $\sim 5 \times 10^{12}$ M$_\odot$ halos. A dependency on the rest-frame 1500 Å~UV luminosity, M$_\rm{UV}$, is also observed where the halo masses increase from $10^{11}$ to $10^{13}$ M$_\odot$ for M$_\rm{UV} \sim -19$ to $-23.5$ mag. Halo mass is also observed to increase from $10^{9.8}$ to $10^{12.3}$ M$_\odot$ for dust-corrected UV star formation rates from $\sim 0.6$ to $10$ M$_\odot$ yr$^{-1}$ and continues to increase up to $10^{13.5}$ M$_\odot$ in halo mass, where the majority of those sources are AGN. All the trends we observe are found to be redshift-independent. Our results reveal that LAEs are the likely progenitors of a wide range of galaxies depending on their luminosity, from dwarf-like, to Milky Way-type, to bright cluster galaxies. LAEs therefore provide unique insight into the early formation and evolution of the galaxies we observe in the local Universe.

astro-ph.GA↗

arXiv2018

Direct Measurement of the Magnitude of van der Waals interaction of Single and Multilayer Graphene

Vertical stacking of monolayers via van der Waals assembly is an emerging field that opens promising routes toward engineering physical properties of two-dimensional (2D) materials. Industrial exploitation of these engineering heterostructures as robust functional materials still requires bounding their measured properties so to enhance theoretical tractability and assist in experimental designs. Specifically, the short-range attractive van der Waals forces are responsible for the adhesion of chemically inert components and are recognized to play a dominant role in the functionality of these structures. Here we reliably quantify the the strength of van der Waals forces in terms of an effective Hamaker parameter for CVD-grown graphene and show how it scales by a factor of two or three from single to multiple layers on standard supporting surfaces such as copper or silicon oxide. Furthermore, direct measurements on freestanding graphene provide the means to discern the interplay between the van der Waals potential of graphene and its supporting substrate. Our results demonstrated that the underlying substrates could enhance or reduce the van der Waals force of graphene surfaces, and its consequences are explained in terms of a Lifshitz theory-based analytical model.

cond-mat.mes-hall↗

arXiv2016

A large H$α$ survey of star formation in relaxed and merging galaxy cluster environments at $z\sim0.15-0.3$

We present the first results from the largest H$α$ survey of star formation and AGN activity in galaxy clusters. Using 9 different narrow band filters, we select $>3000$ H$α$ emitters within $19$ clusters and their larger scale environment over a total volume of $1.3\times10^5$ Mpc$^3$. The sample includes both relaxed and merging clusters, covering the $0.15-0.31$ redshift range and spanning from $5\times10^{14}$ $M_{\odot}$ to $30\times10^{14}$ $M_{\odot}$. We find that the H$α$ luminosity function (LF) for merging clusters has a higher characteristic density $ϕ^*$ compared to relaxed clusters. $ϕ^*$ drops from cluster core to cluster outskirts for both merging and relaxed clusters, with the merging cluster values $\sim0.3$ dex higher at each projected radius. The characteristic luminosity $L^*$ drops over the $0.5-2.0$ Mpc distance from the cluster centre for merging clusters and increases for relaxed objects. Among disturbed objects, clusters hosting large-scale shock waves (traced by radio relics) are overdense in H$α$ emitters compared to those with turbulence in their intra-cluster medium (traced by radio haloes). We speculate that the increase in star formation activity in disturbed, young, massive galaxy clusters can be triggered by interactions between gas-rich galaxies, shocks and/or the intra-cluster medium, as well as accretion of filaments and galaxy groups. Our results indicate that disturbed clusters represent vastly different environments for galaxy evolution compared to relaxed clusters or average field environments.

astro-ph.GA↗

arXiv2015

Wearing a single DNA molecule with an AFM tip

While the fundamental limit on the resolution achieved in an atomic force microscope (AFM) is clearly related to the tip radius, the fact that the tip can creep and/or wear during an experiment is often ignored. This is mainly due to the difficulty in characterizing the tip, and in particular a lack of reliable methods that can achieve this in situ. Here, we provide an in situ method to characterize the tip radius and monitor tip creep and/or wear and biomolecular sample wear in ambient dynamic AFM. This is achieved by monitoring the dynamics of the cantilever and the critical free amplitude to observe a switch from the attractive to the repulsive regime. The method is exemplified on the mechanically heterogeneous sample of single DNA molecules bound to mica mineral surfaces. Simultaneous monitoring of apparent height and width of single DNA molecules while detecting variations in the tip radius R as small as one nanometer are demonstrated. The yield stress can be readily exceeded for sharp tips (R<10 nm) at typical operating amplitudes (A>10nm). The ability to know the AFM tip radius in situ and in real-time opens up the future for quantitative nanoscale materials properties determination at the highest possible spatial resolution.

cond-mat.mes-hall↗

arXiv2014

Anharmonicity in multifrequency atomic force microscopy

In multifrequency atomic force microscopy higher eigenmodes are externally excited to enhance resolution and contrast while simultaneously increasing the number of experimental observables with the use of gentle forces. Here, the implications of externally exciting multiple frequencies are discussed in terms of cantilever anharmonicity, fundamental period and the onset of subharmonic and superharmonic components. Cantilever anharmonicity is shown to affect and control both the observables, that is, the monitored amplitudes and phases, and the main expressions quantified via these observables, that is, the virial and energy transfer expressions which form the basis of the theory.

cond-mat.mes-hall↗

arXiv2014

Theory of small amplitude bimodal atomic force microscopy in ambient conditions

Small oscillation amplitudes in dynamic atomic force microscopy can lead to invasive and high resolution imaging. Here we discuss small oscillation amplitude imaging in the context of ambient conditions and simultaneously excite the second flexural mode to access contrast channels sensitive to variations in sample's properties. Two physically distinct regimes of operation are discussed, one where the tip oscillates above the hydration layer and another where the tip oscillates in perpetual contact with it. It is shown that the user can control the region to be probed via standard operational parameters. The fundamental theory controlling the sensitivity of the second mode phase shift to compositional variations is then developed. The second mode phase shift is controlled by an interplay between conservative tip-sample interactions, energy transfer between modes and irreversible loss of energy in the tip sample junction.

cond-mat.mes-hall↗

arXiv2014

Deconstructing the governing dissipative phenomena in the nanoscale

An expression describing the controlling parameters involved in short range nanoscale dissipation is proposed and supported by simulations and experimental findings. The expression is deconstructed into the geometrical, dynamic, chemical and mechanical properties of the system. In atomic force microscopy these are translated into 1) tip radius and tip-sample deformation, 2) resonant frequency and oscillation amplitude and 3) hysteretic and viscous dissipation. The latter are characteristic parameters defining the chemical and mechanical properties of the tip-sample system. Long range processes are also discussed and footprints are identified in experiments conducted on mica and silicon samples. The present methodology can be exploited to validate or invalidate nanoscale dissipative models by comparing predictions with experimental observables.

cond-mat.mes-hall↗