文献

Methods of shaping and manipulating liquids on small scales are important for micro-patterning, microfluidics and biosensing, and may provide fundamental insights into phenomena in confined small spaces. Here, we report a simple technique for shaping liquids into micrometre-scale filaments. Microgrooves on microwrinkles generated by thin film buckling on elastic substrates can function as open channel capillaries for liquids with appropriate wettabilities. Tuning the groove depth of the microwrinkles by modulating strain, we explore the capillary action of various liquids in microgrooves, which form liquid filaments emanating from a large reservoir. This behaviour is explained in terms of sinusoidal-geometry-dependent surface energy. Based on this concept, a regular array of liquid filaments can be formed over a large area by a simple coating method, and these filaments can be further reshaped by exploiting strain-induced nonlinear changes in microwrinkle topography. This technology shows promise for applications in micro-patterning, nano- and micro-reactors, and microfluidics.

When a very thin, hard layer supported by a soft substrate is laterally compressed beyond a critical strain, then buckling of the hard layer occurs, leading to the formation of small sinusoidal surface undulations (microwrinkles). The orientation of the wrinkle grooves can be reversibly altered by simply adding a strain externally. Using this nonlinear microtopological change, we demonstrate that the morphology of an array of liquid filaments formed on microwrinkle grooves is dramatically and reversibly transformed into liquid filaments with a different orientation or a regular array of microdroplets, “dots”, depending on the predefined contact angle. The novel liquid transformation at nanomicrometer scales will find unique applications, such as switchable light diffraction grating, laboratories-on-a-chip systems as well as a simple liquid micropatterning technique.

In this work, we successfully developed bacterial templates on the surface of an overoxidized polypyrrole film using both gram-negative and gram-positive bacteria in which bacterial surface chemical structures are precisely transferred at a molecular level. The sensor film identified target bacteria within minutes through a unique combination with dielectrophoresis. The bacterial cavities had high selectivity for distinguishing specific target bacteria in bacterial mixtures containing gram-negative (Escherichia coli and Pseudomonas aeruginosa) and gram-positive (Bacillus subtilis and Staphylococcus aureus) bacteria. This rapid and specific recognition system will enable not only bacterial sensing but also analysis of various biological species.

Primary aliphatic amines were introduced into most of (>95%) carboxyl groups densely present on the TEMPO-oxidized cellulose nanofibril (TOCN) surfaces via carboxyl/amine salt formation in a water-isopropyl alcohol mixture. The carbon number of alkyl chain lengths introduced into TOCN varied using n-decyl-, n-dodecyl, n-tetradecyl-, n-hexadecyl- and n-octadecyl-amines. When n-dodecyl-, n-tetradecyl- and n-hexadecyl-amines were used for neutralization of carboxyl groups, the TOCN-COOH/amine salts were dispersed at the individual nanofibril level in pure isopropyl alcohol (IPA) by ultrasonication treatment, and gave stable and transparent IPA dispersions with birefringence when observed between cross polarizers. Flexible, highly transparent and surface-hydrophobic self-standing films of n-dodecylamine-treated TOCN-COOH (TOCN-COOH/C12-amine salt) were prepared by casting and drying of the dispersion. However, the introduction of abundant long-alkyl chains on the TOCN surfaces via the amine salt formation resulted in clearly lower mechanical and oxygen barrier properties of the films than those of TOCN-COONa films.

Friction on soft materials is strongly correlated with the associated deformation, which may be controlled by the surface topography. We investigate the wearless sliding friction between a rigid hemispherical indenter and a deformable textured surface, which is shape-tunable wrinkles. The size of the indenter is comparable to the wavelength of the wrinkles. We evaluate the effects on the friction of the aspect ratio of the wrinkles, the applied normal load, and the alignment direction of the wrinkles relative to the sliding direction. The frictional oscillations are observed during sliding in the direction perpendicular to the alignment using optical images and friction profiles. The correlation of friction force oscillation with deformation of the wrinkles is elucidated using Hertz contact theory. Within a cycle of frictional oscillation, the friction force increases as the front part of the indenter elastically plows the crests. When the normal load is high and/or the aspect ratio of the wrinkles is low, the indenter continues to squash the wrinkles and remains in contact with them during sliding. Consequently, the amplitude of friction force oscillation relative to the averaged friction force decreases.