arXiv subjects

Sissi de Beer

Publications and source records attributed to Sissi de Beer.

7 recordsLinked to original sources

arXiv2026

The role of polyelectrolyte brushes in tunable synaptic devices

With the ever-increasing digitization of society, the development of materials with low-power memory storage -similar to synapses- is becoming more relevant. The field of iontronic artificial synapses has gained traction, in particular with polymers as the memory-active material which allows for additional bio-compatibility, flexibility and tunability. Polyelectrolyte brushes are an example of stimulus-responsive materials that can be used in iontronic devices. However, the complexity of current neuromorphic devices does not allow us to isolate and understand the role of polyelectrolyte brushes in their synaptic response. In this paper, we show that polyelectrolyte brushes are capable of synaptic behavior in the most simple of electrochemical cell designs. Furthermore, by combining theory and experimental work, we shed light on the role of brushes in this synaptic behavior and their dynamic stimuli-responsiveness to polarity changes for different salt concentrations. The obtained trends and interpretations of the nonlinear potential-current response, paired-pulse experiments, and accumulative learning lay the foundation for designing and developing polymer brush-based neuromorphic devices.

cond-mat.soft↗

arXiv2024

The reentrant condensation of polyelectrolytes induced by diluted multivalent salts: A mean-field level revisiting

We study the reentrant condensation of polyelectrolytes in dilute solutions of small multivalent salts, whose phase-transition mechanism remains poorly understood. Motivated by recent full atomic simulation results reported by the Caltech group on phase behaviors of polyelectrolytes in presence of multivalent salts (DOIs: 10.1021/acs.macromol.3c02437 and 10.1021/acs.langmuir.3c03640), in this work we construct a simple but effective mean-field model which can rationalize the essential features of the reentrant condensation including the phase diagram of polyelectrolyte. The model unveils that the strong adsorption between the ionic monomers and multivalent ions can be at the origin of the peculiar phenomenon that rather low concentrations of multivalent salts trigger both collapse and re-entry transitions. For the first time, the analytical solution of the model indicates that a minimum of coupling energy due to sharing multivalent salt ions between ionic monomers is essential for a phase transition to occur, which can explain the enigmatic observation that polyelectrolytes can only show phase transition in a dilute solution of salts with selective multivalency. Our analytical calculations also show that the incompatibility of the uncharged moieties of the polyelectrolytes with water is critical to regulate phase behaviors of polyelectrolytes in aqueous solutions. This is in agreement with recent experimental investigations on solution properties of amphiphilic proteins. The obtained results will contribute to the understanding of liquid-liquid phase separation in biological systems where multivalent ions bound to bio-polyelectrolytes play an essential role.

cond-mat.soft↗

arXiv2024

Millisecond-resolved infrared spectroscopy study of polymer brush swelling dynamics

We present the study of millisecond-resolved polymer brush swelling dynamics using infrared spectroscopy with a custom-built quantum cascade laser-based infrared spectrometer at a 1 kHz sampling rate after averaging. By cycling the humidity of the environment of the polymer brush, we are able to measure the swelling dynamics sequentially at different wavenumbers. The high sampling rate provides us with information on the reconformation of the brush at a higher temporal resolution than previously reported. Using spectroscopic ellipsometry, we study the brush swelling dynamics as a reference experiment and to correct artefacts of the infrared measurement approach. This technique informs on the changes in the brush thickness and refractive index. Our results indicate that the swelling dynamics of the polymer brush are poorly described by Fickian diffusion and the Berens-Hopfenberg formalism, pointing toward more complicated underlying transport.

physics.ins-det↗

arXiv2023

Pressure anisotropy in polymer brushes and its effects on wetting

Polymer brushes, coatings consisting of densely grafted macromolecules, experience an intrinsic lateral compressive pressure, originating from chain elasticity and excluded volume interactions. This lateral pressure complicates a proper definition of the interface and, thereby, the determination and interpretation of the interfacial tension and its relation to the wetting behavior of brushes. Here, we study the link between grafting-induced compressive lateral pressure in polymer brushes, interfacial tension, and brush wettability using coarse-grained molecular dynamics simulations. We focus on grafting densities and polymer-liquid affinities such that the polymer and liquid do not tend to mix. For these systems, a central result is that the liquid contact angle is independent of the grafting density, which implies that the grafting-induced lateral compressive pressure in the brush does not influence its wettability. Although the definition of brush interfacial tensions is complicated by the grafting-induced pressure, the difference in interfacial tension between wet and dry brushes is perfectly well-defined. We confirm explicitly from Young's law that this difference offers an accurate description of the brush wettability. We then explore a method to isolate the grafting-induced contribution to the lateral pressure, assuming the interfacial tension to be independent of grafting density. This scenario indeed allows to disentangle interfacial and grafting effects for a broad range of parameters, except close to the mixing point. We separately discuss the latter case in the light of autophobic dewetting.

cond-mat.soft↗

arXiv2023

Non-equilibrium configurations of swelling polymer brush layers induced by spreading drops of weakly volatile oil

Polymer brush layers are responsive materials that swell in contact with good solvents and their vapors. We deposit drops of an almost completely wetting volatile oil onto an oleophilic polymer brush layer and follow the response of the system upon simultaneous exposure to both liquid and vapor. Interferometric imaging shows that a halo of partly swollen polymer brush layer forms ahead of the moving contact line. The swelling dynamics of this halo is controlled by a subtle balance of direct imbibition from the drop into the brush layer and vapor phase transport and can lead to very long-lived transient swelling profiles as well as non-equilibrium configurations involving thickness gradients in a stationary state. A gradient dynamics model based on a free energy functional with three coupled fields is developed and numerically solved. It describes experimental observations and reveals how local evaporation and condensation conspire to stabilize the inhomogeneous non-equilibrium stationary swelling profiles. A quantitative comparison of experiments and calculations provides access to the solvent diffusion coefficient within the brush layer. Overall, the results highlight the - presumably generally applicable - crucial role of vapor phase transport in dynamic wetting phenomena involving volatile liquids on swelling functional surfaces.

cond-mat.soft↗

arXiv2022

OpenHumidistat: Humidity-controlled experiments for everyone

Humidity control is a crucial element for a wide variety of experiments. Yet, often naive methods are used that do not yield stable regulation of the humidity, are slow, or are inflexible. PID-based electropneumatic humidistats solve these problems, but commercial devices are not widespread, typically proprietary and/or prohibitively expensive. Here we describe OpenHumidistat: a free and open-source humidistat for laboratory-scale humidity control that is affordable (<500 EUR) and easy to build. The design is based around mixing a humid and dry air flow in varying proportions, using proportional solenoid valves and flow sensors to control flow rates. The mixed flow is led into a measurement chamber, which contains a humidity sensor to provide feedback to the controller, to achieve closed-loop humidity control.

physics.ins-det↗

arXiv2017

Dramatic effect of fluid chemistry on cornstarch suspensions: linking particle interactions to macroscopic rheology

Suspensions of cornstarch in water exhibit strong dynamic shear-thickening. We show that partly replacing water by ethanol strongly alters the suspension rheology. We perform steady and non-steady rheology measurements combined with atomic force microscopy to investigate the role of fluid chemistry on the macroscopic rheology of the suspensions and its link with the interactions between cornstarch grains. Upon increasing the ethanol content, the suspension goes through a yield-stress fluid state and ultimately becomes a shear-thinning fluid. On the cornstarch grain scale, atomic force microscopy measurements reveal the presence of polymers on the cornstarch surface, which exhibit a co-solvency effect. At intermediate ethanol content, a maximum of polymer solubility induces high microscopic adhesion which we relate to the macroscopic yield stress.

cond-mat.soft↗