Effect of the Chain Length and Temperature on the Adhesive Properties of Alkanethiol Self-Assembled Monolayers

Stable and hydrophobic self-assembled monolayers of alkanethiols are promising materials for use as lubricants in microdevices and nanodevices. We applied high-rate dynamic force spectroscopy measurements to study in detail the influence of the chain length and temperature on the adhesion between methyl-terminated thiol monolayers and a silicon nitride tip. We used the Johnson–Kendall–Roberts model to calculate the number of molecules in adhesive contact and then the Dudko–Hummer–Szabo model to extract the information about the position and the height of the activation barrier per single molecule. Both parameters were determined and analyzed in the temperature range from 25 to 65 °C for three thiols: 1-decanethiol (measured previously), 1-tetradecanethiol, and 1-hexadecanethiol. We associate the increase of the activation barrier parameters versus the chain length with lower stiffness of longer molecules and higher effectiveness of adhesive bond formation. However, we relate the thermal changes of the parameters rather to rearrangements of molecules than to the direct influence of temperature on the adhesive bonds

Squeezing Drops: Force Measurements of the Cassie-to-Wenzel Transition

Superhydrophobic surfaces have long been the center of attention of many researchers due to their unique liquid repellency and self-cleaning properties. However, these unique properties rely on the stability of the so-called Cassie state, which is a metastable state with air-filled microstructures. This state tends to transit to the stable Wenzel state, where the inside of the microstructures eventually wets. For potential industrial applications, it is therefore critical to maintain the Cassie state. We investigate the Cassie-to-Wenzel transition on superhydrophobic micropillar surfaces by squeezing a water drop between the surface and a transparent superhydrophobic force probe. The probe’s transparency allows the use of top-view optics to monitor the area of the drop as it is squeezed against a micropillared surface. The impalement, or Cassie-to-Wenzel transition, is identified as a sharp decrease in force accompanied by an abrupt change in the drop’s contact area. We compare the force measured by the sensor with the capillary pressure force calculated from the observed drop shape and find a good agreement between both quantities. We also study the force and pressure at impalement as a function of the pillar’s slenderness ratio. Finally, we compare the impalement pressure with three literature predictions and find that our experimental values are consistently lower than the theoretical values. We find that a possible cause of this earlier Cassie-to-Wenzel transition may be the coalescence of the squeezed drop with microdroplets that nucleate around the base of the micropillars

Squeezing Drops: Force Measurements of the Cassie-to-Wenzel Transition

Superhydrophobic surfaces have long been the center of attention of many researchers due to their unique liquid repellency and self-cleaning properties. However, these unique properties rely on the stability of the so-called Cassie state, which is a metastable state with air-filled microstructures. This state tends to transit to the stable Wenzel state, where the inside of the microstructures eventually wets. For potential industrial applications, it is therefore critical to maintain the Cassie state. We investigate the Cassie-to-Wenzel transition on superhydrophobic micropillar surfaces by squeezing a water drop between the surface and a transparent superhydrophobic force probe. The probe’s transparency allows the use of top-view optics to monitor the area of the drop as it is squeezed against a micropillared surface. The impalement, or Cassie-to-Wenzel transition, is identified as a sharp decrease in force accompanied by an abrupt change in the drop’s contact area. We compare the force measured by the sensor with the capillary pressure force calculated from the observed drop shape and find a good agreement between both quantities. We also study the force and pressure at impalement as a function of the pillar’s slenderness ratio. Finally, we compare the impalement pressure with three literature predictions and find that our experimental values are consistently lower than the theoretical values. We find that a possible cause of this earlier Cassie-to-Wenzel transition may be the coalescence of the squeezed drop with microdroplets that nucleate around the base of the micropillars

Hydrodynamic Force between a Sphere and a Soft, Elastic Surface

The hydrodynamic drainage force between a spherical silica particle and a soft, elastic polydimethylsiloxane surface was measured using the colloidal probe technique. The experimental force curves were compared to finite element simulations and an analytical model. The hydrodynamic repulsion decreased when the particle approached the soft surface as compared to a hard substrate. In contrast, when the particle was pulled away from the surface again, the attractive hydrodynamic force was increased. The hydrodynamic attraction increased because the effective area of the narrow gap between sphere and the plane on soft surfaces is larger than on rigid ones

Measuring Adhesion Forces in Powder Collectives by Inertial Detachment

One way of measuring adhesion forces in fine powders is to place the particles on a surface, retract the surface with a high acceleration, and observe their detachment due to their inertia. To induce detachment of micrometer-sized particles, an acceleration in the order of 500 000<i>g</i> is required. We developed a device in which such high acceleration is provided by a Hopkinson bar and measured via laser vibrometry. Using a Hopkinson bar, the fundamental limit of mechanically possible accelerations is reached, since higher values cause material failure. Particle detachment is detected by optical video microscopy. With subsequent automated data evaluation a statistical distribution of adhesion forces is obtained. To validate the method, adhesion forces for ensembles of single polystyrene and silica particles on a polystyrene coated steel surface were measured under ambient conditions. We were able to investigate more than 150 individual particles in one experiment and obtained adhesion values of particles in a diameter range of 3–13 μm. Measured adhesion forces of small particles agreed with values from colloidal probe measurements and theoretical predictions. However, we observe a stronger increase of adhesion for particles with a diameter larger than roughly 7–10 μm. We suggest that this discrepancy is caused by surface roughness and heterogeneity. Large particles adjust and find a stable position on the surface due to their inertia while small particles tend to remain at the position of first contact. The new device will be applicable to study a broad variety of different particle–surface combinations on a routine basis, including strongly cohesive powders like pharmaceutical drugs for treatment of lung diseases

Efficient Encapsulation of Self-Healing Agents in Polymer Nanocontainers Functionalized by Orthogonal Reactions

An orthogonal polymerization reaction between two monomers partitioned in the two liquid phases of a miniemulsion yielded nanocapsules with various functional groups (sulfonate, amine, carboxylic acid, and poly­(ethylene glycol) (PEG). The formation of the nanocapsules could be realized in the presence of a self-healing agent in the liquid core. We present here the conditions for the successful preparation of functional polymer nanocapsules by free-radical polymerization as orthogonal reaction for self-healing materials. The stability of the nanocapsules was assessed by AFM measurements in the dried state as well as in water

Contact Forces between TiO₂ Nanoparticles Governed by an Interplay of Adsorbed Water Layers and Roughness

Interparticle forces govern the mechanical behavior of granular matter and direct the hierarchical assembling of nanoparticles into supramolecular structures. Understanding how these forces change under different ambient conditions would directly benefit industrial-scale nanoparticle processing units such as filtering and fluidization. Here we rationalize and quantify the contributions of dispersion, capillary, and solvation forces between hydrophilic TiO₂ nanoparticles with sub-10 nm diameter and show that the humidity dependence of the interparticle forces is governed by a delicate interplay between the structure of adsorbed water layers and the surface roughness. All-atom molecular dynamics modeling supported by force-spectroscopy experiments reveals an unexpected decrease in the contact forces at increasing humidity for nearly spherical particles, while the forces between rough particles are insensitive to strong humidity changes. Our results also frame the limits of applicability of discrete solvation and continuum capillary theories in a regime where interparticle forces are dominated by the molecular nature of surface adsorbates

Mechanical Properties of Poly(dimethylsiloxane)-<i>block</i>-poly(2-methyloxazoline) Polymersomes Probed by Atomic Force Microscopy

Poly­(dimethylsiloxane)-<i>block</i>-poly­(2-methyloxazoline) (PDMS-<i>b</i>-PMOXA) vesicles were characterized by a combination of dynamic light scattering (DLS), cryogenic transmission electron microscopy (cryo-TEM), and atomic force microscopy imaging and force spectroscopy (AFM). From DLS data, a hydrodynamic radius of ∼150 nm was determined, and cryo-TEM micrographs revealed a bilayer thickness of ∼16 nm. In AFM experiments on a silicon wafer substrate, adsorption led to a stable spherical caplike conformation of the polymersomes, whereas on mica, adsorption resulted also in vesicle fusion and formation of bilayer patches or multilayer stacks. This indicates a delicate balance between the mechanical stability of PDMS-<i>b</i>-PMOXA polymersomes on one hand and the driving forces for spreading on the other. A Young’s modulus of 17 ± 11 MPa and a bending modulus of 7 ± 5 × 10^–18 J were derived from AFM force spectroscopy measurements. Therefore, the elastic response of the PDMS-<i>b</i>-PMOXA polymersomes to external stimuli is much closer to that of lipid vesicles compared to other types of polymersomes, such as polystyrene-<i>block</i>-poly­(acrylic acid) (PS-<i>b</i>-PAA)

Highly respirable dry powder inhalable formulation of voriconazole with enhanced pulmonary bioavailability

<p><b>Objective</b>: To develop and characterize a highly respirable dry powder inhalable formulation of voriconazole (VRZ).</p> <p><b>Methods</b>: Powders were prepared by spray drying aqueous/alcohol solutions. Formulations were characterized in terms of particle size, morphology, thermal, moisture responses and aerosolization performance. Optimized powder was deposited onto an air-interface Calu-3 model to assess their uptake across Calu-3 lung epithelia. Optimized formulation was evaluated for stability (drug content and aerosol performance) for 3 months. Additionally, Calu-3 cell viability, lung bioavailability and tissue distribution of optimized formulation were evaluated.</p> <p><b>Results</b>: Particle size and aerosol performance of dry powder containing 80% w/w VRZ and 20% w/w leucine was appropriate for inhalation therapy. Optimized formulation showed irregular morphology, crystalline nature, low moisture sensitivity and was stable for 3 months at room temperature. Leucine did not alter the transport kinetics of VRZ, as evaluated by air-interface Calu-3 model. Formulation was non-cytotoxic to pulmonary epithelial cells. Moreover, lung bioavailability and tissue distribution studies in murine model clearly showed that VRZ dry powder inhalable formulation has potential to enhance therapeutic efficacy at the pulmonary infection site whilst minimizing systemic exposure and related toxicity.</p> <p><b>Conclusion</b>: This study supports the potential of inhaled dry powder VRZ for the treatment of fungal infections.</p

Homogeneous Nucleation of Predominantly Cubic Ice Confined in Nanoporous Alumina

The nucleation mechanism of water can be precisely regulated by confinement within nanoporous alumina. We found a transition from heterogeneous nucleation of hexagonal ice (I_h) to homogeneous nucleation of predominantly cubic ice (I_c) with decreasing pore diameter. These results lead to a phase diagram of water under confinement. It contains a (stable) predominant I_c form, a form known to exist only in the upper atmosphere. Possible applications range from cryopreservation to construction materials like cement