arXiv subjects

Robin H. A. Ras

Publications and source records attributed to Robin H. A. Ras.

8 recordsLinked to original sources

arXiv2025

Capillary Condensation in Nanogaps: Nucleation or Film Coalescence?

Nucleation and film coalescence represent two fundamentally different pathways for capillary condensation. Yet, both have so far been proposed as the processes driving the condensation in nanometric confinements, leading to a long-standing and overlooked ambiguity. Here, we delineate the dichotomy between these mechanisms and test their validity using an experimental method capable of absolute distance measurement during capillary condensation. We show that the molecular content of the capillary meniscus given by the first nucleation theorem is far smaller than what the confinement geometry and the Kelvin equation require. In contrast, the analysis based on film coalescence reproduces the experimental observations and describes the final meniscus formation as a barrierless process, while allowing for an intermediate, first-order-like film-thickening transition prior to the meniscus formation.

cond-mat.soft↗

arXiv2025

Unlocking the full potential of jumping condensation on microstructured surfaces

Water condensation on superhydrophobic surfaces can generate spontaneous droplet jumping, enabling rapid condensate removal and improved thermal and mass transfer. Although this effect has been extensively demonstrated on densely packed nanostructures, the capability of microscale textures to support jumping condensation remains poorly understood. Here, we show that engineered microscale conical arrays can achieve efficient microdroplet jumping and reveal a previously unreported spacing-dependent critical transition between jumping and non-jumping regimes. In the jumping regime, by varying only the cone pitch, we identify a geometric threshold below which sub-10 micron droplets are rapidly removed, and above which jumping is suppressed, resulting in slower dynamics and larger departing droplets. In situ optical and environmental scanning electron microscopies reveal the mechanistic origin of this transition: dense arrays favour full Cassie droplets, which depart cleanly, while wider spacing favours partial Cassie droplets that retain a localized wet region initiating new nucleation. From these results, we construct a geometry-wetting design map linking microstructure spacing, droplet morphology, and nucleation density. These findings establish design principles for scalable, mechanically robust microstructured surfaces capable of high-performance condensation management for anti-fogging, water harvesting, and heat-transfer applications.

cond-mat.soft↗

arXiv2024

Super-droplet-repellent carbon-based printable perovskite solar cells

Despite attractive cost-effectiveness, scalability, and superior stability, carbon-based printable perovskite solar cells (CPSCs) still face moisture-induced degradation that limits their lifespan and commercial potential. Here, we investigate the moisture-preventing mechanisms of thin nanostructured super-repellent coating (advancing contact angle $>$167$^{\circ}$ and contact angle hysteresis 7$^{\circ}$ integrated into CPSCs for different moisture forms (falling water droplets vs water vapor vs condensed water droplets). We show that unencapsulated super-repellent CPSCs have superior performance under continuous droplet impact for 12h (rain simulation experiments) compared to unencapsulated pristine (uncoated) CPSCs that degrade within seconds. Contrary to falling water droplets, where super-repellent coating serves as a shield, we found water vapor to physisorb through porous super-repellent coating (room temperature and relative humidity, RH 65\% and 85\%) that increased the CPSCs performance for 21\% during ~43 days similarly to pristine CPSCs. We further showed that, water condensation forms within or below the super-repellent coating (40$^{\circ}$ C and RH 85\%), followed by chemisorption and degradation of CPSCs. Because different forms of water have distinct effect on CPSC, we suggest that future standard tests for repellent CPSCs should include rain simulation and condensation tests. Our findings will thus inspire the development of super-repellent coatings for moisture prevention.

physics.app-ph↗

arXiv2023

Flattened and wrinkled encapsulated droplets: Shape-morphing induced by gravity and evaporation

We report surprising morphological changes of suspension droplets (containing class II hydrophobin protein HFBI from Trichoderma reesei and water) as they evaporate with a contact line pinned on a rigid solid substrate. Both pendant and sessile droplets display the formation of an encapsulating elastic film as the bulk concentration of solute reaches a critical value during evaporation, but the morphology of the droplet varies significantly: for sessile droplets, the elastic film ultimately crumples in a nearly flattened area close to the apex while in pendant droplets, circumferential wrinkling occurs close to the contact line. These different morphologies are understood through a gravito-elasto-capillary model that predicts the droplet morphology and the onset of shape changes, as well as showing that the influence of the direction of gravity remains crucial even for very small droplets (where the effect of gravity can normally be neglected). The results pave the way to control droplet shape in several engineering and biomedical applications.

cond-mat.soft↗

arXiv2021

Ferrofluidic Manipulator: Automatic Manipulation of Non-magnetic Microparticles at Air-Ferrofluid Interface

Manipulation of small-scale matter is a fundamental topic in micro- and nanorobotics. Numerous magnetic robotic systems have been developed for the manipulation of microparticles in an ambient environment, liquid as well as on the air-liquid interface. These systems move intrinsically magnetic or magnetically tagged objects by inducing a magnetic torque or force. However, most of the materials found in nature are non-magnetic. Here, we report a ferrofluidic manipulator for automatic two-dimensional manipulation of non-magnetic objects floating on top of a ferrofluid. The manipulation system employs eight centimeter-scale solenoids, which can move non-magnetic particles by deforming the air-ferrofluid interface. Using linear programming, we can control the motion of the non-magnetic particles with a predefined trajectory of a line, square, and circle with a precision of 25.1+/-19.5 um, 34.4+/-28.4 um and 33.4+/-26.6 um, respectively. The ferrofluidic manipulator is versatile with the materials and the shapes of the objects under manipulation. We have successfully manipulated particles made of polyethylene, polystyrene, a silicon chip, and poppy and sesame seeds. This work shows a promising venue for the manipulation of living and non-living matter at the air-liquid interface.

cs.RO↗

arXiv2018

Antifouling membranes for oily wastewater treatment: interplay between wetting and membrane fouling

Oily wastewater is an extensive source of pollution to soil and water, and its harmless treatment is of great importance for the protection of our aquatic ecosystems. Membrane filtration is highly desirable for removing oil from oily water because it has the advantages of energy efficiency, easy processing and low maintenance cost. However, membrane fouling during filtration leads to severe flux decline and impedes long-term operation of membranes in practical wastewater treatment. Membrane fouling includes reversible fouling and irreversible fouling. The fouling mechanisms have been explored based on classical fouling models, and on oil droplet behaviors (such as droplet deposition, accumulation, coalescence and wetting) on the membranes. Membrane fouling is dominated by droplet-membrane interaction, which is influenced by the properties of the membrane (e.g., surface chemistry, structure and charge) and the wastewater (e.g., compositions and concentrations) as well as the operation conditions. Typical membrane antifouling strategies, such as surface hydrophilization, zwitterionic polymer coating, photocatalytic decomposition and electrically enhanced antifouling are reviewed, and their cons and pros for practical applications are discussed.

cond-mat.soft↗

arXiv2018

Wetting of ferrofluids: phenomena and control

Ferrofluids are liquids exhibiting remarkably strong response to magnetic fields, which leads to fascinating properties useful in various applications. Understanding the wetting properties and spreading of ferrofluids is important for their use in microfluidics and magnetic actuation. However, this is challenging as magnetically induced deformation of the ferrofluid surface can affect contact angles, which are commonly used to characterize wetting properties in other systems. In addition, interaction of the magnetic nanoparticles and solid surface at nanoscale can have surprising effects on ferrofluid spreading. In this review we discuss these issues with focus on interpretation of ferrofluid contact angles. We review recent literature examining ferrofluid wetting phenomena and outline novel wetting related ferrofluid applications. To better understand wetting of ferrofluids, more careful experimental work is needed.

cond-mat.soft↗

arXiv2014

Ferromagnetic resonance in $ε$-Co magnetic composites

We investigate the electromagnetic properties of assemblies of nanoscale $ε$-cobalt crystals with size range between 5 nm to 35 nm, embedded in a polystyrene (PS) matrix, at microwave (1-12 GHz) frequencies. We investigate the samples by transmission electron microscopy (TEM) imaging, demonstrating that the particles aggregate and form chains and clusters. By using a broadband coaxial-line method, we extract the magnetic permeability in the frequency range from 1 to 12 GHz, and we study the shift of the ferromagnetic resonance with respect to an externally applied magnetic field. We find that the zero-magnetic field ferromagnetic resonant peak shifts towards higher frequencies at finite magnetic fields, and the magnitude of complex permeability is reduced. At fields larger than 2.5 kOe the resonant frequency changes linearly with the applied magnetic field, demonstrating the transition to a state in which the nanoparticles become dynamically decoupled. In this regime, the particles inside clusters can be treated as non-interacting, and the peak position can be predicted from Kittel's ferromagnetic resonance theory for non-interacting uniaxial spherical particles combined with the Landau-Lifshitz-Gilbert (LLG) equation. In contrast, at low magnetic fields this magnetic order breaks down and the resonant frequency in zero magnetic field reaches a saturation value reflecting the interparticle interactions as resulting from aggregation. Our results show that the electromagnetic properties of these composite materials can be tuned by external magnetic fields and by changes in the aggregation structure.

cond-mat.mtrl-sci↗