Deep-UV photoinduced chemical patterning at the micro- and nanoscale for directed self-assembly.

Deep-UV (DUV) laser patterning has been widely used in recent years for micro- and nanopatterning, taking advantage of the specific properties of irradiation with high-energy photons. In this paper, we show the usefulness of DUV laser patterning for preparing surfaces with controlled chemical properties at the micro- and nanoscale. Our motivation was to develop a simple and versatile method for chemical patterning at multiscales (from mm to nm) over relatively wide areas (mm2 to cm2). The chemical properties were provided by self-assembled monolayers (SAMs), prepared on glass or silicon wafers. We first investigated their modification under our irradiation conditions (ArF laser) using AFM, XPS and contact angle measurements. Photopatterning was then demonstrated with minimum feature sizes as small as 75 nm, and we showed the possibility to regraft a second SAM on the irradiated regions. Finally, we used these chemically patterned surfaces for directed self-assembly of several types of objects, such as block copolymers, sol-gel materials and liquids by vapor condensation.

Tensiometric Characterization of Superhydrophobic Surfaces As Compared to the Sessile and Bouncing Drop Methods.

We have considered in this work the Wilhelmy plate tensiometer to characterize the wetting properties of two model surface textures: (i) a series of three superhydrophobic micropillared surfaces and (ii) a series of two highly water-repellent surfaces microtextured with a femtosecond laser. The wetting forces obtained on these surfaces with the Wilhelmy plate technique were compared to the contact angles of water droplets measured with the sessile drop technique and to the bouncing behavior of water droplets recorded at a high frame rate. We showed that it is possible with this technique to directly measure triple-line anchoring forces that are not accessible with the commonly used sessile drop technique. In addition, we have demonstrated on the basis of the bouncing drop experiments wetting transitions induced by the specific test conditions associated with the Wilhelmy plate tensiometer for the two series of textured surfaces. Finally, the tensiometer technique is proposed as an alternative test for characterizing the wetting properties of highly liquid-repellent surface, especially under immersion conditions.

Role of Cellulose Nanocrystals on the Microstructure of Maleic Anhydride Plasma Polymer Thin Films.

Recently, it was shown that the microstructure of a maleic anhydride plasma polymer (MAPP) could be tailored ab initio by adjusting the plasma process parameters. In this work, we aim to investigate the ability of cellulose nanocrystals (CNCs) to induce topographical structuration. Thus, a new approach was designed based on the deposition of MAPP on CNCs model surfaces. The nanocellulosic surfaces were produced by spin-coating the CNC suspension on a silicon wafer substrate and on a hydrophobic silicon wafer substrate patterned with circular hydrophilic microsized domains (diameter of 86.9 ± 4.9 µm), resulting in different degrees of CNC aggregation. By depositing the MAPP over these surfaces, it was possible to observe that the surface fraction of nanostructures increased from 20% to 35%. This observation suggests that CNCs can act as nucleation points resulting in more structures, although a critical density of the CNCs is required.

Impact of Chemical Heterogeneities of Surfaces on Colonization by Bacteria.

An essential yet never addressed parameter for the control of bacteria on functionalized biomaterial is surely the accessibility and heterogeneity of the functional groups immobilized on the surface. In this context, we investigated the colonization (Escherichia coli K12, Staphylococcus epidermidis RP62A) of precisely engineered surfaces revealing various densities of NH2 and CH3 functional groups. We demonstrated for the first time nonlinear relationships between the NH2/CH3 surface fraction and the quantity of adhered, adhering or detaching bacteria. Plateaus and transition zones were related to the range of NH2/CH3 surface fraction offering stability or sharp variation in bacterium/surface interactions. The nonlinear behavior was attributed to the discrete distribution of positive charges revealed by the bacterial membrane in the continuum of negative charges resulting from the phospholipids, which may correlate with one single specific distribution of positive NH3+ charges on the material surface, because of electrostatic, repulsive interactions occurring at the local, molecular scale.

Experimental characterization of the nanoparticle size effect on the mechanical stability of nanoparticle-based coatings.

We present an experimental investigation of the mechanical stability of silica nanoparticle-based coatings as a function of the size of the nanoparticles. The coatings are built following a layer-by-layer procedure, alternating positive and negative surface charges. The mechanical stability of the multilayers is studied in water, on the basis of an ultrasonic cavitation test. The resistance of the coating to cavitation is found to remarkably increase with decreasing the size of the nanoparticles, indicating an increase of the cohesive energy density. The relative contribution of van der Waals and electrical double-layer interactions to the stability of the multilayer is discussed toward their size dependence.

Wetting-mediated collective tubulation and pearling in confined vesicular drops of DDAB solutions.

Whether driven by external mechanical stresses (shear flow) or induced by membrane-active peptides and/or proteins, the collective growth of tubules in membranous fluids has seldom been reported. The pearling destabilization of these membranous tubules which requires an activation of the shape distortion, often induced by optical tweezers, membrane-active biomolecules or an electrical field, has also rarely been observed under mild experimental conditions. Here we report such events of collective tubulation and pearling destabilization in sessile drops of a didodecyl-dimethylammonium bromide (DDAB) vesicular solution that are confined by a surrounding oil medium. Based on the wetting dynamics and the features of the tubulation process, we show that the growth of the tubules here relies on a mechanism of "pinning-induced pulling" from the retracting drop, rather than the classical hydrodynamic fingering instability. We show that the whole tubulation process is driven by a strong coupling between the bulk properties of the ternary (DAAB/water/oil) system and the dynamics of wetting. Finally, we discuss the pearling destabilization of these tubules under vanishing static interface tension and quite mild tensile force arising from their pulling. We show that under those mild conditions, shape disturbances readily grow, either as pearling waves moving toward the drop-reservoir or as Rayleigh-type peristaltic modulations. Besides revealing singular non-Rayleigh pearling modes, this work also brings new insights into the flow dynamics in membranous tubules anchored to an infinite reservoir.

Model experimental study of scale invariant wetting behaviors in Cassie-Baxter and Wenzel regimes.

In this work, we discuss quantitatively two basic relations describing the wetting behavior of microtopographically patterned substrates. Each of them contains scale invariant topographical parameters that can be easily expressed onto substrates decorated with specifically designed micropillars. The first relation discussed in this paper describes the contact angle hysteresis of water droplets in the Cassie-Baxter regime. It is shown that the energy at the origin of the hysteresis, that has to be overcome for moving the triple line, can be invariantly expressed for hexagonal pillars by varying the pillars width and interpillar distance. Identical contact angle hystereses are thus measured on substrates expressing this scale invariance for pillar widths and interpillar distances ranging from 4 to 128 µm. The second relation we discuss concerns the faceting of droplets spreading on microtopographically patterned substrates. It is shown in this case that the condition for pinning of the triple line can be fulfilled by simultaneously varying the height of the pillars and the interpillar distance, leading to faceted droplets of similar morphologies. The invariance of these two wetting phenomena resulting from the simultaneous and homothetic variation of topographical parameters is demonstrated for a wide range of pattern dimensions. Our results show that either of those two wetting behaviors can be simply achieved by the proper choice of a dimensionless ratio of topographical length scales.

From contact angle titration to chemical force microscopy: a new route to assess the pH-dependent character of the stratum corneum.

Despite of its complex multicomponent organization and its compact architecture, the Stratum corneum (SC) is not completely impermeable to substances directly applied on the skin surface. A huge number of works have been dedicated to the understanding of the mechanisms involved in substance permeation by exploring deeper layers than the SC itself. Surprisingly, there is a poor interest in studies relating to interactions which may occur in the near-surface region (i.e. approximately 1 nm depth) of the SC. In this work, equilibrium proton-transfer reactions have been used as probes to define in a fundamental point of view the nature of the SC interactions with its environment. Such titration curves are investigated on 'in vitro' SC (isolated SC from abdominal skin tissue) and on 'in vivo' volar forearm (a sebum poor area). The results are discussed in term of work of adhesion and surface pKa values. Because SC can 'reconstruct' under heating, influence of the temperature on titration curves is investigated and the role of the different components is discussed. Different sigmoidal transitions were observed. Two common pKa values (pKa(1) = 4 and pKa(2) = 11.5) were clearly identified in both cases and associated to an acid-base character. By playing with the temperature of 'in vitro' SC, the 'accessibility' of polar functions was increased, thus refining the results by revealing an amphoteric character with an acid-to-base transition at pH 3.5 and two acid transitions at pH = 6.5 and pH = 11.5. Adhesion forces between an Atomic Force Microscopy (AFM) tip and a single isolated corneocyte through buffered liquid media were also investigated to better understand the role of the individual corneocytes.

Manipulation of gold colloidal nanoparticles with atomic force microscopy in dynamic mode: influence of particle-substrate chemistry and morphology, and of operating conditions.

One key component in the assembly of nanoparticles is their precise positioning to enable the creation of new complex nano-objects. Controlling the nanoscale interactions is crucial for the prediction and understanding of the behaviour of nanoparticles (NPs) during their assembly. In the present work, we have manipulated bare and functionalized gold nanoparticles on flat and patterned silicon and silicon coated substrates with dynamic atomic force microscopy (AFM). Under ambient conditions, the particles adhere to silicon until a critical drive amplitude is reached by oscillations of the probing tip. Beyond that threshold, the particles start to follow different directions, depending on their geometry, size and adhesion to the substrate. Higher and respectively, lower mobility was observed when the gold particles were coated with methyl (-CH(3)) and hydroxyl (-OH) terminated thiol groups. This major result suggests that the adhesion of the particles to the substrate is strongly reduced by the presence of hydrophobic interfaces. The influence of critical parameters on the manipulation was investigated and discussed viz. the shape, size and grafting of the NPs, as well as the surface chemistry and the patterning of the substrate, and finally the operating conditions (temperature, humidity and scan velocity). Whereas the operating conditions and substrate structure are shown to have a strong effect on the mobility of the particles, we did not find any differences when manipulating ordered vs random distributed particles.

Complex aggregation patterns in drying nanocolloidal suspensions: size matters when it comes to the thermomechanical stability of nanoparticle-based structures.

We report the results of a model study on the interrelation among the occurrence of complex aggregation patterns in drying nanofluids, the size of the constitutive nanoparticles (NPs), and the drying temperature, which is a critical issue in the genesis of complex drying patterns that was never systematically reported before. We show that one can achieve fine control over the occurrence and topological features of these drying-mediated complex structures through the combination of the particle size, the drying temperature, and the substrate surface energy. Most importantly, we show that a transition in the occurrence of the patterns appears with the temperature and the particle size, which accounts for the size dependence of the thermomechanical stability of the aggregates in the nanoscale range. Using simple phenomenological and scaling considerations, we showed that the thermomechanical stability of the aggregates was underpinned by physical quantities that scale with the size of the NPs (R) either as R(-2) or R(-3). These insights into the size-dependent dissipation mechanisms in nanoclusters should help in designing NPs-based structures with tailored thermomechanical and environmental stability and hence with an optimized morphological stability that guarantees their long-term functional properties.

Plasma polymer tailoring of the topography and chemistry of surfaces at the nanoscale.

We demonstrate in this paper that plasma polymer can be advantageously used to provide surfaces with topography and chemical control at the nanoscale. Moreover, this technique was also proved to be of high interest to functionalize atomic force microscopy tips that were used to probe the patterned surfaces in pulsed force mode. This approach allowed demonstration by a direct observation of the possibility of generating alternating hydrophilic/hydrophobic surfaces at the nanoscale prepared by DUV laser irradiation. Such a versatile and simple route opens new possibilities in the field of smart surfaces engineering.

Spontaneous growth of self-relief wrinkles in freely floating lipid-based nanomembranes, formed on a reactive bath of polyoxometalate aqueous solution.

Wetting and capillarity have appeared over the last decadesas potential tools for the guided actuation, self-assembly and nanostructuration, and most recently as powerful "tool-free" techniques of micro-, and nano-fabrication, in the field of nanotechnology. The present work deals with such a use of wetting, achieving for the first time the spontaneous growth of composite (lipid-capped polyoxometalate) nanomembranes, using the reactive spreading film formed by the droplets of a lipid solution, on an aqueous foundation hosting the polyoxometalate ions. Moreover, we show that the internal stress produced by the reactive wetting, and the resulting self-movements of the drop could be accumulated within the drying spreading film, driving the spontaneous occurrence of wrinkles, in the freely floating nanomembranes that are thus formed. Finally, we showed that gravity-driven scaling relations for buckling instabilities apply to these spontaneously occurring wrinkles, allowing the in situ characterization of the physical properties of the nanomembranes. This approach that allowed producing floating nanomembranes of "lipid-capped Keggin ions" that were collectable as freestanding nanosheets may constitute a potential route for the fabrication of a wide range of functional (copolymers/metal nanoparticles) nanocomposite membranes.

Liquid ring formation from contacting, nonmiscible sessile drops.

When two pure and nonmiscible liquid drops at rest on a rigid substrate come into close contact with a quasi-zero spreading velocity, one of them may be sucked around the second into a liquid ring, leading in some cases to the complete engulfment of the latter. We here show that the conditions for this amazing and unusual capillary effect to develop are defined by two sets of criteria: the "reciprocal" spreading of one drop with respect to the other and a "geometrical-wetting" criterion related to the opening of the groovelike channels along the base of the attracting drop. Despite the exceeding simplicity and roughness of liquid drops as compared to living cells, the phenomenon strangely recalls, at least in its mechanistic aspect, the fundamental biological process of phagocytosis. Besides these fundamental aspects, this effect may also have interesting implications for microstructuring techniques.