arXiv subjects

Hans Riegler

Publications and source records attributed to Hans Riegler.

8 recordsLinked to original sources

arXiv2025

Interfacially arrested melting in thin films: capillarity-driven suspension of phase transitions

Melting is typically viewed as a bulk first-order phase transition that proceeds once nucleation barriers are overcome. Here we demonstrate an interfacially arrested melting regime in molecularly thin crystalline films, where large liquid droplets remain stably trapped well above the bulk melting temperature. Using long-chain alkane films as a model system, we show that melting is suspended by the competition between bulk melting enthalpy and interfacial energy costs associated with capillary confinement. The arrested state is governed by a single control parameter, the product of temperature offset and film thickness, and is independent of droplet size. As a consequence, small temperature variations produce pronounced and reversible changes in droplet morphology, enabling intrinsic thermodynamic amplification of thermal signals. These results reveal a general mechanism by which interfacial constraints can arrest first-order phase transitions in thin films.

cond-mat.soft↗

arXiv2022

Hole-initiated melting process of thin films

We perform numerical and experimental studies on the melting process of thin films initiated by a small hole. The presence of a non-trivial capillary surface, namely the liquid/air interface, leads to a few counter-intuitive results: (1) The melting point is elevated if the film surface is partially wettable, even with a small contact angle. (2) For a film that is finite in size, melting may prefer to start from the outer boundary, rather than a hole inside. (3) More complex melting scenario may arise, including morphology transitions, and the "de facto" melting point being a range instead of a single value. These are verified by experiments on melting alkane films between silica and air. This work continues a series of investigations on the capillary aspects on melting. Both our model and analysis approach can be easily generalized to other systems.

physics.chem-ph↗

arXiv2016

Morphological Transitions during Melting of Small Cylindrical Aggregates

Most studies on melting under confinement focus only on the solid and liquid melt phases. Despite of its ubiquity, contributions from the capillary interface (liquid / vapor interface) are often neglected. In this study the melting behavior of small cylindrical aggregates in vapor attached to planar surfaces is analyzed. For the assumed boundary conditions (cylindrical solid with a non wetting top plane and a wettable side wall) solid and the liquid phases can coexist within a certain temperature range. Due to capillary instability, the liquid phase can form either an axisymmetric rouloid morphology or, above a certain threshold liquid volume fraction, a bulge coexisting with a rouloid-like section. The corresponding melting points are different. The analysis explicitly describes the behavior of a real system of small aggregates of long chain alkanes on planar substrates. It also gives qualitative insights into the melting behavior of small aggregates with anisotropic wetting behaviors in general. It reveals in particular how melting points and melting pathways depend on the energetic respectively morphological pathways leading to complete melting.

cond-mat.soft↗

arXiv2014

Sharp Transition between Coalescence and Noncoalescence of Sessile Drops

Unexpectedly, under certain conditions, sessile drops from different but completely miscible liquids do not always coalesce instantaneously upon contact: the drop bodies remain separated in a temporary state of noncoalescence, connected through a thin liquid bridge. Here we investigate the transition between the states of instantaneous coalescence and temporary noncoalescence. Experiments reveal that it is barely influenced by viscosities and absolute surface tensions. The main system control parameters for the transition are the arithmetic means of the three-phase angles, $\overlineΘ_a$ and the surface tension differences $Δγ$ between both liquids. These relevant parameters can be combined into a single system parameter, a speciffic Marangoni number $\widetilde{M} = 3Δγ/ (2\overlineγ\overlineΘ_a^2)$. This $\widetilde{M}$ universally characterizes the coalescence respectively transition behavior as a function of both, the physicochemical liquid properties and the shape of the liquid body in the contact region. The transition occurs at a certain threshold value $\widetilde{M}_t$ and is sharp within the experimental resolution. The experimentally observed threshold value of $\widetilde{M}_t \approx 2$ agrees quantitatively with values obtained by simulations assuming authentic real space data. The simulations indicate that the absolute value of $\widetilde{M}_t$ very weakly depends on the molecular diffusivity.

physics.flu-dyn↗

arXiv2014

Physics of Spin Casting Dilute Solutions

We analyze the evolution of the vertical composition profile during hydrodynamic-evaporative film thinning as it typically occurs during spin casting mixtures of non-volatile solutes and volatile solvents. We assume that the solvent dominates the hydrodynamic-evaporative film thinning. The internal spatio-temporal evolution of the composition is analyzed with a diffusive-advective approach. The analysis provides transparent physical insights into the influence of the experimental conditions on the evolution of the internal composition. We present power laws that link the process control parameters to the composition evolution, process duration, and final solute coverage. The analysis reveals a characteristic Sherwood Number as fundamental process parameter. It identifies for which stages of the process our analysis is quantitatively relevant and discloses the dominance of either diffusion or evaporation. The analysis is valid for dilute solutions e.g., for the deposition of solute (sub)monolayers. But it is also relevant for the deposition of thicker (polymer) films.

physics.flu-dyn↗

arXiv2012

Non-coalescence of sessile drops from different but miscible liquids: Hydrodynamic analysis of the twin drop contour as self stabilizing, traveling wave

Capillarity always favors drop fusion. Nevertheless sessile drops from different but completely miscible liquids often do not fuse instantaneously upon contact. Rather, intermediate non-coalescence is observed. Two separate drop bodies, connected by a thin liquid neck move over the substrate. Supported by new experimental data a thin film hydrodynamic analysis of this state is presented. Presumably advective and diffusive volume fluxes in the neck region establish a localized and temporarily stable surface tension gradient. This induces a local surface (Marangoni) flow that stabilizes a traveling wave i.e., the observed moving twin drop configuration. The theoretical predictions are in excellent agreement with the experimental findings.

physics.flu-dyn↗

arXiv2012

Surface nanobubbles: Seeing is believing

The existence of surface nanobubbles has been previously suggested using various experimental techniques, including attenuated total reflection spectroscopy, quartz crystal microbalance, neutron reflectometry, and x-ray reflectivity, but all of these techniques provide a sole number to quantify the existence of gas over (usually) hundreds of square microns. Thus `nanobubbles' are indistinguishable from a `uniform gassy layer' between surface and liquid. Atomic force microscopy, on the other hand, does show the existence of surface nanobubbles, but the highly intrusive nature of the technique means that a uniform gassy layer could break down into nanobubbles \textit{due to} the motion of the microscope's probe. Here we demonstrate \textit{optical} visualisation of surface nanobubbles, thus validating their individual existence non-intrusively.

cond-mat.soft↗

arXiv2012

Non-isomorphic nucleation pathways arising from morphological transitions of liquid channels

Motivated by unexpected morphologies of the emerging liquid phase (channels, bulges, droplets) at the edge of thin, melting alkane terraces, we propose a new heterogeneous nucleation pathway. The competition between bulk and interfacial energies and the boundary conditions determine the growth and shape of the liquid phase at the edge of the solid alkane terraces. Calculations and experiments reveal a "pre-critical" shape transition (channel-to-bulges) of the liquid before reaching its critical volume along a putative shape-conserving path. Bulk liquid emerges from the new shape, and depending on the degree of supersaturation, the new pathway may have two, one, or zero energy barriers. The findings are broadly relevant for many heterogeneous nucleation processes because the novel pathway is induced by common, widespread surface topologies (scratches, steps, etc.).

physics.chem-ph↗