arXiv subjects

Diego Díaz

Publications and source records attributed to Diego Díaz.

4 recordsLinked to original sources

arXiv2026

Solid adsorption: the missing mechanism for surfactant contact lines -- a phase-field approach

We develop a thermodynamically consistent phase-field model for soluble surfactants in two-phase flows, incorporating both interfacial and solid surface adsorption. The model is derived via variational principles consistent with the second law of thermodynamics, resulting in modified free energies and boundary conditions that capture surfactant transport, adsorption, and wetting dynamics. A key contribution of this work is the inclusion of surfactant adsorption on solid walls, which leads to qualitative agreement with experimental observations: unlike prior numerical studies that predicted hydrophilic surfaces becoming more hydrophilic and hydrophobic surfaces more hydrophobic, our model shows a shift toward increased hydrophilicity across all contact angles-consistent with experimental trends. Our results establish that solid adsorption provides the missing mechanism required for predictive modelling of surfactant-laden contact line dynamics.

cond-mat.soft↗

arXiv2026

Polyelectrolyte adsorption at the solid-liquid interface favors receding contact line instability

Controlling the motion of non-Newtonian drops on surfaces is crucial for applications ranging from inkjet printing to biomedical devices and food processing. While the macroscopic behavior of viscoelastic drops sliding on tilted hydrophobic surfaces has been characterized, showing reduced velocities and elongation compared to Newtonian fluids, the underlying microscopic mechanisms remain poorly understood. To address this gap, we developed a high-speed, high-resolution reflection microscope that enables direct visualization of the contact line of sliding drops. We used water/soluble polyelectrolyte solutions based on polyacrylamide and let drops sliding on hydrophobic substrates composed of Teflon AF- and PDMS-coated glass slides. The substrate tilting angle was varied between 20° and 45°. We reveal how viscoelasticity influences the dynamics of the receding contact line and drop motion. Our experiments demonstrate that viscoelasticity can destabilize the receding contact line, triggering filament formation. This instability previously observed in the coating of thin viscoelastic films, is reported here for the first time in sliding drops. We further highlight the critical role of polymer charge in this process: while cationic and non-ionic polymers promote filament formation, anionic polymers do not, a difference we attribute to the distinct wetting properties of the solutions. In conclusion, we clarify the interplay between rheology, surface interactions, and drop dynamics.

cond-mat.soft↗

arXiv2025

Stood-up drop to measure receding contact angles

The wetting behavior of drops on natural and industrial surfaces is determined by the advancing and receding contact angles. They are commonly measured by the sessile drop technique, also called goniometry, which doses liquid through a solid needle. Consequently, this method requires substantial drop volumes, long contact times, tends to be user-dependent, and is difficult to automate. Here, we propose the stood-up drop (SUD) technique as an alternative to measure receding contact angles. The method consists of depositing a liquid drop on a surface by a short liquid jet, at which it spreads radially forming a pancake-shaped film. Then the liquid retracts, forming a spherical cap drop shape (stood-up drop). At this quasi-equilibrium state, the contact angle ($θ_\text{SUD}$) closely resembles the receding contact angle measured by goniometry. Our method is suitable for a wide variety of surfaces from hydrophilic to hydrophobic, overcoming typical complications of goniometry such as needle-induced distortion of the drop shape, and it reduces user dependence. We delineate when the receding contact angle can be obtained by the stood-up method using Volume-of-Fluid (VoF) simulations that systematically vary viscosity, contact angle, and deposited drop volume. Finally, we provide simple scaling criteria to predict when the stood-up drop technique works.

physics.flu-dyn↗

arXiv2024

Earthquakes and the wealth of nations: The cases of Chile and New Zealand

The consequences of natural disasters, such as earthquakes, are evident: death, coordination problems, destruction of infrastructure, and displacement of population. However, according to empirical research, the impact of a natural disaster on economic activity is mixed. Natural disasters could have significant economic effects, especially in developing economies. This is particularly important for highly seismic countries such as Chile and New Zealand. This paper contributes to the literature on natural disasters and economic development by analyzing the cases of two affected regions within these countries in the wake of major earthquakes experienced during 2010-2011: Maule (Chile) and Canterbury (New Zealand). We examine the impact of natural disasters on GDP per capita by applying the synthetic control method. Using the synthetic approach, we assess the effects of these two earthquakes by building counterfactuals to compare their recovery trajectories. We find that Chile and New Zealand experienced opposite economic effects. The Canterbury region grew 10% more in three years than its synthetic counterfactual without the earthquake, while the Maule region declined by 5%. We build synthetic controls at a regional and economic-sector level, looking at aggregated and sectoral effects. The difference in institutions, such as property rights and the large amount of government spending given for reconstruction after the New Zealand earthquake relative to Chile's, help to explain the difference in outcomes.

econ.GN↗