Switchable Friction Using Contacts of Stimulus-Responsive and Nonresponding Swollen Polymer Brushes

Stimulus-responsive (SR), solvated polymers can switch between an expanded state and a collapsed state via external stimuli. Using molecular dynamics simulations, I show that such SR polymers can be employed to control the frictional response between two opposing polymer brushes in relative sliding motion. By using a brush composed of SR polymers in contact with a nonresponding solvated polymer brush, the presence of capillaries and the overlap between molecules of the opposing brushes can be switched. When both brushes are solvated, a capillary is formed and polymers of the opposing brushes interdigitate. Interdigitation dominates friction upon shearing flat brush-bearing surfaces, while the breaking and formation of capillaries dominate friction in the low-velocity limit between rough brush-bearing surfaces. Thus, when either rough or flat polymer-bearing surfaces are sheared, friction between two swollen brushes can be high. In contrast, when the SR brush is collapsed, the solvent absorbs only in the brush that does not respond to the external stimulus. The latter circumvents the presence of capillaries and interdigitation of the brushes, which results in a low friction force upon shearing

Probing the properties of confined liquids

In this thesis we describe Atomic Force Microscopy (AFM) measurements and\ud Molecular Dynamics (MD) simulation of the static and dynamic properties of layered\ud liquids confined between two solid surfaces.\ud Liquid molecules in the proximity of a solid surface assemble into layers. When a fluid\ud is confined between two surfaces, the discrete molecular nature of the liquid becomes\ud observable via the oscillatory solvation forces and can be probed with AFM\ud spectroscopy. Upon approach of an in liquid immersed AFM cantilever – driven with a\ud sub-angstrom amplitude – towards a solid graphite surface, we find that both the\ud amplitude and phase response strongly oscillate as the distance is decreased. From the\ud amplitude and phase response we extract the conservative and dissipative interaction\ud forces. We observe that the conservative forces increasingly oscillate for a decreasing\ud tip-surface distance, as expected for oscillatory solvation forces. For the dissipative\ud interaction forces or the damping on the tip we find pronounced maxima positioned at\ud the transition from 3-2, 2-1 and 1-0 layers. From these observations we conclude that\ud the dynamic transport-properties of the confined liquid significantly change in these\ud transition-regions.\ud Nevertheless, in AFM measurements we only measure forces. We can not see what\ud happens with the confined liquid molecules. To study the effect of confinement on the\ud dynamics of the molecules and how that will affect the response on the cantilever, we\ud also performed MD simulations. In our simulations the average force on the tip shows\ud the same exponential decaying oscillations as we found in our experiments. Next to the\ud average force, we also monitored the force-fluctuations on the tip. Using fluctuationdissipation\ud we converted these force-fluctuations in the dissipative force or damping\ud on the tip. The damping on the tip shows pronounced maxima very similar to our\ud experimental results. The maxima are also positioned at the transition regions of 3-2,\ud 2-1 and 1-0 layers. By monitoring the Mean Squared Displacement and the number of nearest neighbors of the molecules confined under the tip, we find that the damping is\ud closely related to the configuration and the dynamics of the molecules. Regarding\ud these observations one might be tempted to conclude that the confined molecules\ud behave either liquid-like or solid-like depending on the distance between the tip and\ud the surface. However, spectral analysis suggests that the elastic and viscous response\ud of the confined liquid is more complex and would be better described as either a gel or\ud a soft glassy material

Electrostatic Fields Stimulate Absorption of Small Neutral Molecules in Gradient Polyelectrolyte Brushes

Molecules can partition from a solution into a polymer coating, leading to a local enrichment. If one can control this enrichment via external stimuli, one can implement such coatings in novel separation technologies. Unfortunately, these coatings are often resource intensive as they require stimuli in the form changes of bulk solvent conditions such as acidity, temperature, or ionic strength. Electrically driven separation technology may provide an appealing alternative, as this will allow local, surface-bound stimuli instead of system-wide bulk stimuli to induce responsiveness. Therefore, we investigate via coarse grained molecular dynamics simulations the possibility of using coatings with charged moieties, specifically gradient polyelectrolyte brushes, to control the enrichment of the neutral target molecules near the surface with applied electric fields. We find that targets which interact more strongly with the brush show both more absorption and a larger modulation by electric fields. For the strongest interactions evaluated in this work, we obtained absorption changes of over 300 % between the collapsed and extended state of the coating.</p

Capillarity-driven dynamics of water–alcohol mixtures in nanofluidic channels

We investigated the spontaneous capillarity-driven filling of nanofluidic channels with a thickness of 6 and 16 nm using mixtures of ethanol and water of variable composition. To improve the visibility of the fluid, we embedded metal mirrors into the top and bottom walls of the channels that act as a Fabry–Pérot interferometer. The motion of propagating liquid–air menisci was monitored for various concentrations in transmission with an optical microscope. In spite of the visible effects of surface roughness and different affinity of water and ethanol to the channel walls, the dynamics followed the classical t 1/2—dependence according to Lucas and Washburn. While the prefactor of this algebraic relation falls short of the expectations based on bulk properties by 10–30%, the relative variation between mixtures of different composition follows the expectations based on the bulk surface tension and viscosity, implying that—despite the small width of the channels and the large surface-to-volume ratio—specific adsorption or chemical selectivity effects are not relevant. We briefly discuss the impact of surface roughness on our experimental results

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

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

PNIPAM Brushes in Colloidal Photonic Crystals Enable Ex Situ Ethanol Vapor Sensing

Structural colors are formed by the periodic repetition of nanostructures in a material. Upon reversibly tuning the size or optical properties of the repetitive unit inside a nanostructured material, responsive materials can be made that change color due to external stimuli. This paper presents a simple method to obtain films of ethanol vapor-responsive structural colors based on stacked poly(N-isopropylacrylamide) (PNIPAM)-grafted silica nanoparticles. Our materials show clear, reversible color transitions in the presence of near-saturated ethanol vapor. Moreover, due to the absorption of ethanol in the PNIPAM brushes, relatively long recovery times are observed (∼30 s). Materials based on bare or poly(methyl methacrylate) (PMMA) brush-grafted silica nanoparticles also change color in the presence of ethanol vapor but possess significantly shorter recovery times (∼1 s). Atomic force microscopy reveals that the delayed recovery originates from the ability of PNIPAM brushes to swell in ethanol vapor. This renders the films highly suitable for ex situ ethanol vapor sensing.</p

Stress anisotropy in polymer brushes and its effects on wetting

Polymer brushes, coatings consisting of densely grafted macromolecules, have been known to experience an intrinsic lateral compressive stress, originating from chain elasticity and excluded volume interactions. This lateral stress complicates a proper definition of the interface and, thereby, of the interfacial tension. Moreover, its effect on wettability has remained unclear. Here, we study the link between grafting-induced compressive lateral stress in polymer brushes, interfacial tension, and brush wettability using coarse-grained molecular dynamics simulations. A central result is that the liquid contact angle is independent of grafting density, which implies that the strength of the compressive stress inside brush has no influence on the wettability. Interestingly, though the interfacial tensions lack a proper definition, 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. It is demonstrated how these results can be explained from the fact that the compressive stress appears "symmetrically" in wet and dry brushes. We discuss our findings in the light of autophobic dewetting and point out the connection to the Shuttleworth effect for wetting on elastomers