arXiv subjects

Nicolas Giovambattista

Publications and source records attributed to Nicolas Giovambattista.

At least 19 recordsLinked to original sources

arXiv2026

A Local Structural Basis to Resolve Amorphous Ices

Phases with distinct thermodynamic properties must differ in their underlying microscopic configurations. While ordered phases are readily distinguished by unit cells and space groups, the local structural basis differentiating amorphous phases is less apparent. Here, using a new probabilistic data-driven framework applied to molecular simulations of water, we identify local collective variables that discriminate low-density and high-density amorphous (LDA and HDA) ices and characterize pressure-induced transitions between them. As expected, descriptors related to local density effectively distinguish LDA and HDA; however, phase identity is surprisingly encoded within the first coordination shell. Furthermore, the pressure-induced LDA--HDA transformation proceeds through redistribution between LDA- and HDA-like local environments with no evidence for intermediate structures, consistent with a first-order-like phase transition. This contrasts with the gradual structural evolution observed in other amorphous systems, such as metallic glasses. Critically, local hydrogen density reveals pronounced structural hysteresis between compression and decompression pathways, which is not apparent in orientational order parameters, demonstrating that the microscopic interpretation of amorphous transformations depends fundamentally on descriptor choice. These findings are robust across force fields and provide a general strategy for characterizing disordered phases lacking obvious distinguishing features.

cond-mat.stat-mech↗

arXiv2025

Configurational Entropy and Adam-Gibbs Relation for Quantum Liquids

As a liquid approaches the glass state, its dynamics slows down rapidly, by a few orders of magnitude in a very small temperature range. In the case of light elements and small molecules containing hydrogen (e.g., water), such a process can be affected by nuclear quantum effects (due to quantum fluctuations/atoms delocalization). In this work, we apply the potential energy landscape (PEL) formalism and path-integral computer simulations to study the low-temperature behavior of a Lennard-Jones binary mixture (LJBM) that obeys quantum mechanics. We show that, as for the case of classical liquids, (i) a configurational entropy $S_{IS}$ can be defined, and (ii) the Adam-Gibbs equation, which relates the diffusion coefficient of a liquid and its $S_{IS}$, holds for the studied quantum LJBM. Overall, our work shows that one theoretical approach, the PEL formalism, can be used to describe low-temperature liquids close to their glass transition, independently of whether the system obeys classical or quantum mechanics.

cond-mat.soft↗

arXiv2025

Probing interfacial water via color-center-enabled spin magnetometry

Understanding the behavior of confined water at liquid-solid interfaces is central to numerous physical, chemical, and biological processes, yet remains experimentally challenging. Here, we utilize shallow nitrogen-vacancy (NV) centers in diamond to investigate the nanoscale dynamics of interfacial water confined between the diamond surface and an overlying fluorinated oil droplet. Using NV-based nuclear magnetic resonance protocols selectively sensitive to 1H and 19F, we independently track water and oil near the interface under ambient conditions. Comparing opposite sides of a doubly-implanted diamond membrane - one exposed to oil, the other not - we uncover a slow, multi-day process in which the interfacial water layer is gradually depleted. This desorption appears to be driven by sustained interactions with the fluorinated oil and is supported by molecular dynamics simulations and surface-sensitive X-ray spectroscopies. Our findings provide molecular-level insight into long-timescale hydration dynamics and underscore the power of NV-NMR for probing liquid-solid heterointerfaces with chemical specificity.

physics.chem-ph↗

arXiv2024

Slow water in engineered nano-channels revealed by color-center-enabled sensing

Nanoscale confinement of molecules in a fluid can result in enhanced viscosity, local fluidic order, or collective motion. Confinement also affects ion transport and/or the rate and equilibrium concentration in a chemical reaction, all of which makes it the subject of broad interest. Studying these effects, however, is notoriously difficult, mainly due to the lack of experimental methods with the required sensitivity and spatial or time resolution. Here we leverage shallow nitrogen-vacancy (NV) centers in diamond to probe the dynamics of room-temperature water molecules entrapped within ~6-nm-tall channels formed between the diamond crystal and a suspended hexagonal boron nitride (hBN) flake. NV-enabled nuclear magnetic resonance measurements of confined water protons reveal a much reduced H2O self-diffusivity, orders of magnitude lower than in bulk water. We posit the slow dynamics stem from the accumulation of photogenerated carriers at the interface and trapped fluid, a notion we support with the help of molecular dynamics modeling. Our results provide feedback for theories describing interfacial water, and lay out a route for investigating other fluids under confinement.

cond-mat.mes-hall↗

arXiv2024

Supercritical density fluctuations and structural heterogeneity in supercooled water-glycerol microdroplets

Recent experiments and theoretical studies strongly indicate that water exhibits a liquid-liquid phase transition (LLPT) in the supercooled domain. An open question is how the LLPT of water can affect the properties of aqueous solutions. Here, we study the structural and thermodynamic properties of supercooled glycerol-water microdroplets at dilute conditions ($χ_g=3.2~\%$ glycerol mole fraction). The combination of rapid evaporative cooling with ultrafast X-ray scattering allows us to outrun crystallization and gain access to the deeply supercooled regime down to $T=229.3$ K. We find that the density fluctuations of the glycerol-water solution or, equivalently, its isothermal compressibility, $κ_T$, increases upon cooling. This is confirmed by molecular dynamics simulations, which indicate that the presence of glycerol shifts the temperature of maximum $κ_T$ from $T=230$ K in pure water down to $T=223$ K in the solution. Our findings elucidate the interplay between the complex behavior of water, including its LLPT, and the properties of aqueous solutions at low temperatures, which can have practical consequences in cryogenic biological applications and cryopreservation techniques.

cond-mat.soft↗

arXiv2024

Potential Energy Landscape of a Flexible Water Model: Equation-of-State, Configurational Entropy, and Adam-Gibbs Relationship

The potential energy landscape (PEL) formalism is a tool within statistical mechanics that has been used in the past to calculate the equation of states (EOS) of classical rigid model liquids at low temperatures, where computer simulations may be challenging. In this work, we use classical molecular dynamics (MD) simulations and the PEL formalism to calculate the EOS of the flexible q-TIP4P/F water model. This model exhibits a liquid-liquid critical point (LLCP) in the supercooled regime, at ($P_c = 150$ MPa, $T_c = 190$ K, $ρ_c = 1.04$ g/cm$^3$) [using the reaction field technique]. The PEL-EOS of q-TIP4P/F water, and the corresponding location of the LLCP, are in very good agreement with the MD simulations. We show that the PEL of q-TIP4P/F water is Gaussian which allows us to calculate the configurational entropy of the system, $S_{conf}$. The $S_{conf}$ of q-TIP4P/F water is surprisingly similar to that reported previously for rigid water models, suggesting that intramolecular flexibility does not necessarily add roughness to the PEL. We also show that the Adam-Gibbs relation, which relates the diffusion coefficient $D$ with $S_{conf}$, holds for the flexible q-TIP4P/F water model. Overall, our results indicate that the PEL formalism can be used to study molecular systems that include molecular flexibility, the common case in standard force fields. This is not trivial since the introduction of large bending/stretching mode frequencies is problematic in classical statistical mechanics. For example, as shown previously, we find that such high-frequencies lead to an unphysical (negative) entropy for q-TIP4P/F water (yet the PEL formalism can be applied successfully).

cond-mat.soft↗

arXiv2021

Liquid-liquid phase transition in simulations of ultrafast heating and decompression of amorphous ice

A recent experiment [K. H. Kim, et al., Science 370, 978 (2020)] showed that it may be possible to detect a liquid-liquid phase transition (LLPT) in supercooled water by subjecting high density amorphous ice (HDA) to ultrafast heating, after which the sample reportedly undergoes spontaneous decompression from a high density liquid (HDL) to a low density liquid (LDL) via a first-order phase transition. Here we conduct computer simulations of the ST2 water model, in which a LLPT is known to occur. We subject various HDA samples of this model to a heating and decompression protocol that follows a thermodynamic pathway similar to that of the recent experiments. Our results show that a signature of the underlying equilibrium LLPT can be observed in a strongly out-of-equilibrium process that follows this pathway despite the very high heating and decompression rates employed here. Our results are also consistent with the phase diagram of glassy ST2 water reported in previous studies.

cond-mat.stat-mech↗

arXiv2019

Glass Polymorphism in TIP4P/2005 Water: A Description Based on the Potential Energy Landscape Formalism

The potential energy landscape (PEL) formalism is a statistical mechanical approach to describe supercooled liquids and glasses. Here we use the PEL formalism to study the pressure-induced transformations between low-density amorphous ice (LDA) and high-density amorphous ice (HDA) using computer simulations of the TIP4P/2005 molecular model of water. We find that the properties of the PEL sampled by the system during the LDA-HDA transformation exhibit anomalous behavior. In particular, at conditions where the change in density during the LDA-HDA transformation is approximately discontinuous, reminiscent of a first-order phase transition, we find that (i) the inherent structure (IS) energy, $e_\text{IS}(V)$, is a concave function of the volume, and (ii) the IS pressure, $P_\text{IS}(V)$, exhibits a van der Waals-like loop. In addition, the curvature of the PEL at the IS is anomalous, a non-monotonic function of $V$. In agreement with previous studies, our work suggests that conditions (i) and (ii) are necessary (but not sufficient) signatures of the PEL for the LDA-HDA transformation to be reminiscent of a first-order phase transition. We also find that one can identify two different regions of the PEL, one associated to LDA and another to HDA. Our computer simulations are performed using a wide range of compression/decompression and cooling rates. In particular, our slowest cooling rate (0.01 K/ns) is within the experimental rates employed in hyperquenching experiments to produce LDA. Interestingly, the LDA-HDA transformation pressure that we obtain at $T=80$ K and at different rates extrapolates remarkably well to the corresponding experimental pressure.

cond-mat.stat-mech↗

arXiv2018

Searching for crystal-ice domains in amorphous ices

We employ classical molecular dynamics simulations to investigate the molecular-level structure of water during the isothermal compression of hexagonal ice (I$h$) and low-density amorphous (LDA) ice at low temperatures. In both cases, the system transforms to high-density amorphous ice (HDA) via a first-order-like phase transition. We employ a sensitive local order metric (LOM) [Martelli et. al., Phys. Rev. B, 97, 064105 (2018)], that can discriminate among different crystalline and non crystalline ice structures and is based on the positions of the oxygen atoms in the first and/or second hydration shell. Our results confirm that LDA and HDA are indeed amorphous, i.e., they lack of polydispersed ice domains. Interestingly, HDA contains a small number of domains that are reminiscent of the unit cell of ice IV, although the hydrogen-bond network (HBN) of these domains differ from the HBN of ice IV. The presence of ice IV-like domains provides some support to the hypothesis that HDA could be the result of a detour on the HBN rearrangement along the I$h$-to-ice IV pressure induced transformation. Both nonequilibrium LDA-to-HDA and I$h$-to-HDA transformations are two-steps processes where a small distortion of the HBN first occurs at low pressures and then, a sudden, extensive re-arrangement of hydrogen bonds at the corresponding transformation pressure follows. Interestingly, the I$h$-to-HDA and LDA-to-HDA transformations occur when LDA and I$h$ have similar local order, as quantified by the site-averaged LOMs. Since I$h$ has a perfect tetrahedral HBN, while LDA does not, it follows that higher pressures are needed to transform I$h$ into HDA than that for the conversion of LDA to HDA. In correspondence with both first-order-like phase transitions, the samples are composed of a large HDA cluster that percolates within the I$h$/LDA samples.

physics.chem-ph↗

arXiv2017

Influence of sample preparation on the transformation of low-density to high-density amorphous ice: An explanation based on the Potential Energy Landscape

Experiments and computer simulations of the transformations of amorphous ices display different behavior depending on sample preparation methods, and on the rates of change of temperature and pressure to which samples are subjected. In addition to these factors, simulation results also depend strongly on the chosen water model. Using computer simulations of the ST2 water model, we study how the sharpness of the compression-induced transition from low-density amorphous ice (LDA) to high-density amorphous ice (HDA) is influenced by the preparation of LDA. By studying LDA samples prepared using widely different procedures, we find that the sharpness of the LDA-to- HDA transformation is correlated with the depth of the initial LDA sample in the potential energy landscape (PEL), as characterized by the inherent structure energy. Our results show that the complex phenomenology of the amorphous ices reported in experiments and computer simulations can be understood and predicted in a unified way from a knowledge of the PEL of the system.

cond-mat.stat-mech↗

arXiv2017

Large-scale structure and hyperuniformity of amorphous ices

We investigate the large-scale structure of amorphous ices and transitions between their different forms by quantifying their large-scale density fluctuations. Specifically, we simulate the isothermal compression of low-density amorphous ice (LDA) and hexagonal ice (Ih) to produce high-density amorphous ice (HDA). Remarkably, both HDA and LDA are nearly hyperuniform, meaning that they are characterized by an anomalous suppression of large-scale density fluctuations. By contrast, in correspondence with both non-equilibrium phase transitions to HDA, the presence of structural heterogeneities strongly suppresses the hyperuniformity and, remarkably, the system becomes hyposurficial (devoid of "surface-area" fluctuations). Our investigation challenges the largely accepted "frozen-liquid" picture, which views glasses as structurally arrested liquids. Beyond implications for water, our findings enrich our understanding of the structural transformations that occur in glasses with varying pressures.

cond-mat.stat-mech↗

arXiv2016

Potential Energy Landscape of the Apparent First-Order Phase Transition between Low-Density and High-Density Amorphous Ice

The potential energy landscape (PEL) formalism is a valuable approach within statistical mechanics for describing supercooled liquids and glasses. Here we use the PEL formalism and computer simulations to study the pressure-induced transformations between low-density amorphous ice (LDA) and high-density amorphous ice (HDA) at different temperatures. We employ the ST2 water model for which the LDA-HDA transformations are remarkably sharp, similar to what is observed in experiments, and reminiscent of a first-order phase transition. Our results are consistent with the view that LDA and HDA configurations are associated with two distinct regions (megabasins) of the PEL that are separated by a potential energy barrier. At higher temperature, we find that low-density liquid (LDL) configurations are located in the same megabasin as LDA, and that high-density liquid (HDL) configurations are located in the same megabasin as HDA. We show that the pressure-induced LDL-HDL and LDA-HDA transformations occur along paths that interconnect these two megabasins, but that the path followed by the liquid is different than the path followed by the amorphous solid. At higher pressure, we also study the liquid-to-ice-VII first-order phase transition, and find that the behavior of the PEL properties across this transition are qualitatively similar to the changes found during the LDA-HDA transformation. This similarity supports the interpretation that the LDA-HDA transformation is a first-order-like phase transition between out-of-equilibrium states.

cond-mat.stat-mech↗

arXiv2007

Correspondence Between the Phase Diagrams of TIP5P Water and a Spherically Symmetric Repulsive Ramp Potential

We perform molecular dynamics simulations of a well-known water model (the TIP5P pair potential) and a simple liquid model (a two-scale repulsive ramp potential) to compare the regions of anomalous behavior in their phase diagrams. We select the parameters of the ramp potential by mapping it to an effective pair potential derived from the TIP5P model. We find that the regions of anomalous behavior in the phase diagrams of both systems can be mapped onto each other if (i) pressure $P$ and temperature $T$ are replaced by $T-T_{C}$ and $P-P_{C}$, respectively, where $(T_{C},P_{C})$ are the coordinates of the liquid-liquid critical point of the corresponding system; and (ii) a single ramp particle corresponds to two TIP5P molecules. We present heuristic arguments supporting point (ii). We also argue that the water-like anomalies in the ramp potential are due to the ability of the particles to reproduce, upon compression or heating, the migration of water molecules from the second shell to its first shell.

cond-mat.soft↗

arXiv2007

Dynamical heterogeneities and the breakdown of the Stokes-Einstein and Stokes-Einstein-Debye relations in simulated water

We study the Stokes-Einstein (SE) and the Stokes-Einstein-Debye (SED) relations using molecular dynamics simulations of the extended simple point charge model of water. We find that both the SE and SED relations break down at low temperature. To explore the relationship between these breakdowns and dynamical heterogeneities (DH), we also calculate the SE and SED relations for subsets of the 7% ``fastest'' and 7% ``slowest'' molecules. We find that the SE and SED relations break down in both subsets, and that the breakdowns occur on all scales of mobility. Thus these breakdowns appear to be generalized phenomena, in contrast with the view where only the most mobile molecules are the origin of the breakdown of the SE and SED relations, embedded in an inactive background where these relations hold. At low temperature, the SE and SED relations in both subsets of molecules are replaced with ``fractional'' SE and SED relations, $D_t\sim(τ/T)^{-ξ_t}$ and $D_r\sim(τ/T)^{-ξ_r}$ where $ξ_t\approx0.84<1$ and $ξ_r\approx0.75<1$. We also find that there is a decoupling between rotational and translational motion, and that this decoupling occurs in both fastest and slowest subsets of molecules. We also find that when the decoupling increases, upon cooling, the probability of a molecule being classified as both translationally and rotationally fastest also increases. To study the effect of time scale for SE and SED breakdown and decoupling, we introduce a time-dependent version of the SE and SED relations, and a time-dependent function that measures the extent of decoupling. Our results suggest that both the decoupling and SE and SED breakdowns are originated at the time scale corresponding to the end of the cage regime, when diffusion starts. This is also the time scale when the DH are more relevant.

cond-mat.stat-mech↗

arXiv2007

Structure of the First and Second Neighbor Shells of Water: Quantitative Relation with Translational and Orientational Order

We perform molecular dynamics simulation of water using the TIP5P model to quantify structural order in both the first shell (defined by four nearest neighbors)and second shell (defined by twelve next-nearest neighbors) of a central water molecule. We find the anomalous decrease of orientational order upon compression occurs in both shells, but the anomalous decrease of translational order upon compression occurs {\it mainly in the second shell}. The decreases of translational and orientational orders upon compression ("structural anomaly") are thus correlated only in the second shell. Our findings quantitatively confirm the qualitative idea that the thermodynamic, dynamic and structural anomalies of water are related to changes in the second shell upon compression.

cond-mat.soft↗

arXiv2006

Effect of pressure on the phase behavior and structure of water confined between nanoscale hydrophobic and hydrophilic plates

We perform systematic molecular dynamics simulations of water confined between two nanoscale plates at T=300 K. We investigate the effect of pressure (-0.15 GPa <= P <= 0.2 GPa) and plate separation (0.4 nm <= d <= 1.6 nm) on the phase behavior of water when the plates are either hydrophobic or hydrophilic. When water is confined between hydrophobic plates, capillary evaporation occurs between the plates at low enough P. The threshold value of d at which this transition occurs decreases with P (e.g., 1.6 nm at P ~ -0.05 GPa; 0.5 nm at P \~ 0.1 GPa), until, at high P, no capillary evaporation occurs. For d ~ 0.6 nm and P >= 0.1 GPa, the system crystallizes into a bilayer ice. A P-d phase diagram showing the vapor, liquid, and bilayer ice phases is proposed. When water is confined by hydrophilic (hydroxylated silica) plates, it remains in the liquid phase at all P and d studied. Interestingly, we observe, for this case, that even at the P at which bulk water cavitates, the confined water remains in the liquid state. We also study systematically the state of hydration at different P for both kinds of plates. For the range of conditions studied here, we find that in the presence of hydrophobic plates the effect of P is to enhance water structure and to push water molecules toward the plates. The average orientation of water molecules next to the hydrophobic plates does not change upon pressurization. In contrast, in the presence of hydrophilic plates, water structure is insensitive to P. Hence, our results suggest that upon pressurization, hydrophobic plates behave as `soft' surfaces (in the sense of accommodating pressure-dependent changes in water structure) while hydrophilic walls behave as `hard' surfaces.

physics.chem-ph↗

arXiv2006

A Family of Tunable Spherically-Symmetric Potentials that Span the Range from Hard Spheres to Water-like Behavior

We investigate the equation of state, diffusion coefficient, and structural order of a family of spherically-symmetric potentials consisting of a hard core and a linear repulsive ramp. This generic potential has two characteristic length scales: the hard and soft core diameters. The family of potentials is generated by varying their ratio, $λ$. We find negative thermal expansion (thermodynamic anomaly) and an increase of the diffusion coefficient upon isothermal compression (dynamic anomaly) for $0\leqλ<6/7$. As in water, the regions where these anomalies occur are nested domes in the ($T, ρ$) or ($T, P$) planes, with the thermodynamic anomaly dome contained entirely within the dynamic anomaly dome. We calculate translational and orientational order parameters ($t$ and $Q_6$), and project equilibrium state points onto the ($t, Q_6$) plane, or order map. The order map evolves from water-like behavior to hard-sphere-like behavior upon varying $λ$ between 4/7 and 6/7. Thus, we traverse the range of liquid behavior encompassed by hard spheres ($λ=1$) and water-like ($λ\sim4/7$) with a family of tunable spherically-symmetric potentials by simply varying the ratio of hard to soft-core diameters. Although dynamic and thermodynamic anomalies occur almost across the entire range $0\leqλ\leq1$, water-like structural anomalies (i.e., decrease in both $t$ and $Q_6$ upon compression and strictly correlated $t$ and $Q_6$ in the anomalous region) occur only around $λ=4/7$. Water-like anomalies in structure, dynamics and thermodynamics arise solely due to the existence of two length scales, orientation-dependent interactions being absent by design.

cond-mat.soft↗

arXiv2006

Structural Order for One-Scale and Two-Scale Potentials

We perform molecular dynamics simulations to investigate the relationship between structural order and water-like dynamic and thermodynamic anomalies in spherically-symmetric potentials having either one or two characteristic length scales. %The first potential has only one length scale which is the diameter of the ramp %without the hard core, and the second potential has two length scales: one is the %diameter of a ramp(softcore) and another one is the diameter of a %hard core with a ratio of 1.76. Structural order is characterized by translational and orientational order parameters. %analogous to those used in previous cases for water and %silica.Only the two-scale ramp potential exhibits properties %remarkably similar to those found for water and silica regarding the %relationship between structural order, dynamic anomalies, and thermodynamic %anomalies. We find that (i) dynamic and thermodynamic anomalies exist for both one-scale and two-scale ramp potentials, and (ii) water-like structural order anomalies exist only for the two-scale ramp potential. Our findings suggest that the water-like relationship between structural order and anomalies is related to the presence of two different length scales in the potential.

cond-mat.soft↗