Stimulus-responsive polymers and other functional polymer surfaces as components in glass microfluidic channels.

The integration of smart stimulus-responsive polymers as functional elements within microfluidic devices has greatly improved the performance capabilities of controlled fluid delivery. For their use as actuators in microfluidic systems, reversible expansion and shrinking are unique mechanisms which can be utilized as both passive and active fluid control elements to establish gate and valve functions (passive) and pumping elements (active). Various constituents in microfluidic glass channels based on stimulus-responsive elements have been reported based on pH-responsive, thermoresponsive and photoresponsive coatings. Fluid control and robust performance have been demonstrated in microfluidic devices in a number of studies. Here we give a brief overview of selected examples from the literature reporting on the use of stimulus response polymers as active or passive elements for fluid control in microfluidic devices, with specific emphasis on glass-based devices. The remaining challenges include improving switching times and achieving local addressability of the responsive constituent. We envisage tackling these challenges by utilizing redox-responsive polymers which offer fast and reversible switching and local addressability in combination with nanofabricated electrodes.

Solvent-induced immiscibility of polymer brushes eliminates dissipation channels.

Polymer brushes lead to small friction and wear and thus hold great potential for industrial applications. However, interdigitation of opposing brushes makes them prone to damage. Here we report molecular dynamics simulations revealing that immiscible brush systems can form slick interfaces, in which interdigitation is eliminated and dissipation strongly reduced. We test our findings with friction force microscopy experiments on hydrophilic and hydrophobic brush systems in both symmetric and asymmetric setups. In the symmetric setup both brushes are chemically alike, while the asymmetric system consists of two different brushes that each prefer their own solvent. The trends observed in the experimentally measured force traces and the friction reduction are similar to the simulations and extend to fully immersed contacts. These results reveal that two immiscible brush systems in mechanical contact slide at a fluid-fluid interface while having load-bearing ability. This makes them ideal candidates for tribological applications.

Collapse from the top: brushes of poly(N-isopropylacrylamide) in co-nonsolvent mixtures.

Using a combination of ellipsometry and friction force microscopy, we study the reversible swelling, collapse and variation in friction properties of covalently bound poly(N-isopropylacrylamide) (PNIPAM) layers on silicon with different grafting densities in response to exposure to good solvents and co-nonsolvent mixtures. Changes in the thickness and segment density distribution of grafted films are investigated by in situ ellipsometry. Based on quantitative modelling of the ellipsometry spectra, we postulate a structural model, which assumes that collapse takes place in the contacting layer between the brush and the co-nonsolvent and the top-collapsed brushes remain hydrated in the film interior. Using the structural model derived from ellipsometry spectra, we analyse the AFM based friction force microscopy data, which were obtained by silica colloidal probes. Results show a large increase of the friction coefficient of PNIPAM grafts when the grafts swollen by water are brought in contact with co-nonsolvents. For instance, the value of the friction coefficient for a medium density brush in water is four times lower than the value observed in a water-methanol (50% v/v) mixture. This increase of friction is accompanied by an increase in adherence between the PNIPAM chains and the silica colloidal probes, and is a result of chain collapse in the graft when contacted by a co-nonsolvent mixture in agreement with the model postulated on the basis of ellipsometric characterisation. The kinetic behaviour of the collapse is assessed by measuring the temporal variation of friction in situ as a function of elapsed time following contact with the co-nonsolvent as a function of graft density. In conclusion, the effect of co-nonsolvency influenced both the thickness of the PNIPAM brushes and the tribological behavior of the brush surfaces.

Sieving of hot gases by hyper-cross-linked nanoscale-hybrid membranes.

Macromolecular networks consisting of homogeneously distributed covalently bonded inorganic and organic precursors are anticipated to show remarkable characteristics, distinct from those of the individual constituents. A novel hyper-cross-linked ultrathin membrane is presented, consisting of a giant molecular network of alternating polyhedral oligomeric silsesquioxanes and aromatic imide bridges. The hybrid characteristics of the membrane are manifested in excellent gas separation performance at elevated temperatures, providing a new and key enabling technology for many important industrial scale applications.

The effect of PeakForce tapping mode AFM imaging on the apparent shape of surface nanobubbles.

Until now, TM AFM (tapping mode or intermittent contact mode atomic force microscopy) has been the most often applied direct imaging technique to analyze surface nanobubbles at the solid-aqueous interface. While the presence and number density of nanobubbles can be unequivocally detected and estimated, it remains unclear how much the a priori invasive nature of AFM affects the apparent shapes and dimensions of the nanobubbles. To be able to successfully address the unsolved questions in this field, the accurate knowledge of the nanobubbles' dimensions, radii of curvature etc is necessary. In this contribution we present a comparative study of surface nanobubbles on HOPG (highly oriented pyrolytic graphite) in water acquired with (i) TM AFM and (ii) the recently introduced PFT (PeakForce tapping) mode, in which the force exerted on the nanobubbles rather than the amplitude of the resonating cantilever is used as the AFM feedback parameter during imaging. In particular, we analyzed how the apparent size and shape of nanobubbles depend on the maximum applied force in PFT AFM. Even for forces as small as 73 pN, the nanobubbles appeared smaller than their true size, which was estimated from an extrapolation of the bubble height to zero applied force. In addition, the size underestimation was found to be more pronounced for larger bubbles. The extrapolated true nanoscopic contact angles for nanobubbles on HOPG, measured in PFT AFM, ranged from 145° to 175° and were only slightly underestimated by scanning with non-zero forces. This result was comparable to the nanoscopic contact angles of 160°-175° measured using TM AFM in the same set of experiments. Both values disagree, in accordance with the literature, with the macroscopic contact angle of water on HOPG, measured here to be 63° ± 2°.

Molecular dynamics and energy landscape of decanethiolates in self-assembled monolayers on Au(111) studied by scanning tunneling microscopy.

The energetics and dynamics of the various phases of decanethiolate self-assembled monolayers on Au(111) surfaces were studied with scanning tunneling microscopy. We have observed five different phases of the decanethiolate monolayer on Au(111): four ordered phases (ß, δ, χ*, and φ) and one disordered phase (ε). We have determined the boundary free energies between the disordered and order phases by analyzing the thermally induced meandering of the domain boundaries. On the basis of these results, we are able to accurately predict the two-dimensional phase diagram of the decanethiolate/Au(111) system. The order-disorder phase transition of the χ* phase occurs at 295 K, followed by the order-disorder phase transition of the ß phase at 325 K. Above temperatures of 325 K, only the densely packed φ and disordered ε phases remain. Our findings are in good agreement with the phase diagram of the decanethiolate/Au(111) system that was put forward by Poirier et al. [Langmuir 2001, 17 (4), 1176-1183].

Dynamics of decanethiol self-assembled monolayers on Au(111) studied by time-resolved scanning tunneling microscopy.

We investigated the dynamics of decanethiol self-assembled monolayers on Au(111) surfaces using time-resolved scanning tunneling microscopy at room temperature. The expected ordered phases (ß, δ, χ*, and φ) and a disordered phase (ε) were observed. Current-time traces with the feedback loop disabled were recorded at different locations on the surface. The sulfur end group of the decanethiolate molecule exhibits a stochastic two-level switching process when the molecule is adsorbed in a (local) ß phase registry. This two-level process is attributed to the diffusion of the Au-thiolate complex between two adjacent adsorption sites. The irregular current jumps in the current-time traces recorded on the tails of decanethiolate molecules in the ordered ß, δ, and χ* phases are ascribed to wagging of the alkyl tails. Finally, the disordered phase is characterized by even larger current jumps, which indicates that the tail of the decanethiolate flips up occasionally and makes contact with the tip. Our experiments reveal that the massive dynamics of the self-assembled monolayer is due to diffusion of decanethiol-Au complexes, rather than the diffusion of decanethiolate molecules.

Austenitic and duplex stainless steels in simulated physiological solution characterized by electrochemical and X-ray photoelectron spectroscopy studies.

A study of oxide layers grown on 2205 duplex stainless steel (DSS) and AISI 316L austenitic stainless steel in simulated physiological solution is presented here in order to establish the possibility of replacement of AISI 316 L with 2205 DSS in biomedical applications. The results of the potentiodynamic measurements show that the extent of the passive range significantly increased for DSS 2205 compared to AISI 316L stainless steel. Cyclic voltammetry was used to investigate electrochemical processes taking place on the steel surfaces. Oxide layers formed by electrochemical oxidation at different oxidation potentials were studied by X-ray photoelectron spectroscopy, and their compositions were analyzed as a function of depth. The main constituents on both the investigated materials were Cr- and Fe-oxides. Atomic force microscopy topography studies revealed the higher corrosion resistance of the DSS 2205 compared to the AISI 316L under the chosen experimental conditions.