Mindaugas Rackaitis

Gelling nature of aluminum soaps in oils

Aluminum soaps are notable for their ability to form soap-hydrocarbon gels of high viscosity. For more than half a century, it has been believed that the gelling mechanism is due to a formation of polymeric chains of aluminum molecules with the aluminum atoms linking along the axis and with the fatty acid chain extended sideways. Here we report results from an investigation using high-resolution electron microscopy and rheology measurements that clearly resolve the ambiguity. Our results reveal that the gelling mechanism stems from the formation of spherical nano-sized micelles from aluminum soap molecules, and those colloidal micelle particles then aggregate into networks of highly fractal and jammed structures. The earlier proposed polymer chain-like structure is definitely incorrect. The discovery of aluminum soap particles could expand application of these materials to new technologies.

Microtribological properties of diamond-like and hydrogenated carbon coatings grown by different methods

Abstract Microadhesion and wear behaviour of diamond-like coatings and hydrogenated carbon coatings produced by several different methods was investigated. An atomic force microscope (AFM) was used for microadhesion tests. Adhesion force between AFM cantilever and coating surface was measured. Adhesion force values were obtained from AFM force-displacement curves. Wear of coatings was evaluated using diamond “tip-on-plate” and steel “ball-on-plate” tribotesters. Investigated coatings were grown by graphite electron beam evaporation and ion beam bombardment assisted electron beam evaporation in hydrogenous ionic atmosphere. Quartz, silicon and GaAs wafers were used as substrates for deposition. Adhesion forces evaluated for samples of different microstructure varied in the range 1–100 nN. Different shapes of the force-displacement curve were observed for coatings grown using different methods. Our investigations point to the ion assisted deposition as the preferred fabrication method for hard protective coatings.

Synthesis and characterisation of porphyrin nanotubes obtained by ionic self-assembly

In this work, the porphyrin nanotubes were built by ionic self-assembly in acidic aqueous solution of two oppositely charged non-metal porphyrins, namely, the anionic meso-tetra (4-sulfonatophenyl) porphine dihydrochloride (TPPS4) and cationic meso-tetra (4-pyridyl) porphine (T4MPyP). The electrostatic forces between these porphyrin blocks contribute to the formation of porphyrin aggregates in the form of nanosheets and nanotubes, enhance the structural stability of these nanostructures. The nanosheets thickness approximately varies in the range of 3-15 nm and the sheets diameter up to 1 micron. Using the TEM and SEM, it was revealed that the mechanism of nanotubes formation is based on the wrapping of nanosheets in multiwall cigar-like structures. The porphyrin nanotubes obtained are hollow structures with a length from 200 nm up to 1,000 nm and have a diameter in the range of 50-140 nm with 20-40 nm thick walls. TEM images confirm a hollow tubular structure of the aggregates.

Topological peculiarities in liquid phase of styrene butadiene rubber thin films induced by electrostatic nanolithography

Nanostructures of 50–200nm in width were formed in styrene butadiene rubber using atomic force microscopy electrostatic nanolithography combined with vertical manipulation of the tip. The formation of circular nanostructures is attributed to factors such as the strength of the electric field (109–1010Vm−1), a contribution to the radial component of the pressure gradient, and a film thickness of less than 10nm. Stability of the features (48h) suggests cross-linking between macromolecules at the nanoscale.