Mikhail S. Kondratenko

Omniphobic coatings effectively repelling water, oils, and other liquids are of great interest and have a broad number of applications including self-cleaning, anti-icing surfaces, biofouling protection, selective filtration, etc. To create such coatings, one should minimize the pinning force that resists droplet motion and causes contact angle hysteresis. The minimization of the free surface energy by means of the chemical modification of the solid surface is not enough to obtain a nonsticky slippery omniphobic surface. One should minimize the contact between the solid and the droplet. Besides coating the surface with flat polymer films, among the major approaches to create omniphobic coatings, one can reveal "lotus effect" textured coatings, slippery liquid-infused porous surfaces (SLIPS), and slippery omniphobic covalently attached liquid (SOCAL) coatings. It is possible to turn one surface type into other by texturizing, impregnating with liquids, or grafting flexible liquid-like polymer chains. There are a number of models describing the pinning force on surfaces, but the transitions between states with different wetting regimes remain poorly understood. At the same time, such studies can significantly broaden existing ideas about the physics of wetting, help to design coatings, and also contribute to the development of generalized models of the pinning force. Here we review the existing pinning force (contact angle hysteresis) models on various omniphobic substrates. Also, we discuss the current studies of the pinning force in the transitions between different wetting regimes.

In modern life, people face a wide number of sticky problems when adhesion is highly undesirable: water and dirt stick to clothes, useful materials stick to the walls of their containers and cannot be fully used, water sticking and freezing on airplane wings affects handling and can be dangerous, biological liquids can stick and form clots inside medical devices threatening patients' lives, etc. Slippery liquid-infused porous surfaces (SLIPSs) with pressure stable omniphobicity could help to solve these issues. Lubricant depletion from porous surface and subsequent degradation of omniphobic properties is the major problem for SLIPS. It could be resolved by attaching flexible, liquid-like sidechains to the polymer matrix. Understanding the relationship between the structure of such polymer films and wetting effects is therefore of great importance. The present work is devoted to the study of droplet pinning on crosslinked polydimethylsiloxane (PDMS) polymer films with varied amounts of attached flexible PDMS sidechains and clarification of the relationship between slippery and viscoelastic properties of the films. An one-stage approach to the synthesis of such slippery coatings on smooth and porous substrates in "eco-friendly" pressurized CO₂ solutions is proposed. Pinning force and Young's modulus (E) of the films on silicon substrates with variation of the grafted sidechains amount (x) are measured. The non-monotonic dependence of the pinning force on the amount of sidechains is obtained: the pinning force decreases at small x values (region I) and starts to increase at higher x (region II). The effects of the grafted sidechains amount, as well as matrix softening, are discussed for each case. It is demonstrated that the proposed method of film synthesis allows one to obtain thin, uniform coatings on fabrics without gluing the fibers. Such coatings with an optimal amount of PDMS sidechains demonstrate decreased sliding angles for droplets of water and aqueous alcohol solutions, as compared to PDMS coatings without grafted sidechains. The proposed technique may be of interest for deposition of coatings on porous surfaces having a complex morphology, such as textiles, aerogels, porous electrodes, etc.

The change in the pinning force during the transition from dry to oil-impregnated thin polymer films is studied for droplets of water and hexadecane. A careful variation of the oil amount in the films is performed by means of supercritical impregnation. The film thickness dependence on the oil content is measured using ellipsometry and compared to gel swelling theory estimates. Depending on the oil content, two cases of pinning force behavior have been identified. For each case, the factors that determine the pinning force are discussed. The pinning force in the transition from dry to equilibrium swollen gel films is well approximated by the Joanny and de Gennes hysteresis model of dilute defects.

The study addresses the relationship between the omniphobic properties of coatings based on polymer matrices doped with liquid lubricants (Slippery Liquid-Infused Porous Surfaces, SLIPS) and the degree of swelling of the matrices in the lubricant. A method for fabrication of matrices based on poly(perfluoroalkyl methacrylates) impregnated with fluorinated oils with different lubricant contents by one-stage synthesis in supercritical (SC) CO₂ is proposed. The dependence of the contact angle hysteresis and the sliding angle on the degree of swelling was studied for water and saturated hydrocarbon droplets. The lubricant content in the matrices was determined by thermogravimetric analysis and ellipsometry.

The possibility of electrochemical intercalation of tetrafluoroborate anion into graphite from a supercritical electrolyte containing carbon dioxide, acetonitrile as a co-solvent, and tetrabutylammonium tetrafluoroborate has been demonstrated for the first time.

For the first time, the electrodeposition of platinum from a carbon dioxide-based supercritical electrolyte with the addition of acetonitrile as a co-solvent and tetrabutylammonium tetrafluoroborate salt was studied. Dimethyl (1,5-cyclooctadiene) platinum is used as a precursor. It has been established that as a result of potentiostatic electrodeposition, not a continuous film is formed, but agglomerates of densely packed platinum nanoparticles.

The electrodeposition of platinum from a carbon dioxide-based supercritical electrolyte with the addition of acetonitrile as a cosolvent and tetrabutylammonium tetrafluoroborate has been studied for the first time. Dimethyl(1,5-cyclooctadiene)platinum has been used as a precursor. It has been demonstrated that potentiostatic electrodeposition leads to the formation of agglomerates of densely packed platinum nanoparticles rather than to a continuous film.