Yelena Bormashenko

New Investigations on Ferrofluidics: Ferrofluidic Marbles and Magnetic-Field-Driven Drops on Superhydrophobic Surfaces

The motion of ferrofluidic marbles on flat polymer substrates is reported. Nanopowders of polyvinylidene fluoride and gammaFe2O3 were used for the preparation of ferrofluidic marbles. The marbles are activated easily with an external magnetic field. A microfluidic device based on ferrofluidic marbles (the ferrofluidic bearing) is described. Velocities of marbles as high as 25+/-3 cm/s were registered. The sliding of ferrofluidic drops on superhydrophobic surfaces was studied. It was demonstrated that the threshold magnetic force necessary for the drop displacement depends linearly on the drop radius, thus the motion of the drop is defined by the processes occurring in the vicinity of the triple line only.

Vibration-induced Cassie-Wenzel wetting transition on rough surfaces

The wetting transition is revealed for a water drop placed on a honeycomb polymer pattern under the action of vibration. Water penetration into the pattern cavities is accompanied by a change in the apparent contact angle. The Cassie-Wenzel wetting transition is confirmed by the model calculation. The threshold pressure in the drop for this transition is determined.

Cold Radiofrequency Plasma Treatment Modifies Wettability and Germination Speed of Plant Seeds

We report the possibility to modify the wetting properties of the surfaces of a diversity of seeds including: lentils (Lens culinaris), beans (Phaseolus vulgaris) and wheat (Triticum, species C9) by cold radiofrequency air plasma treatment. Air plasma treatment leads to the dramatic decrease in the apparent contact angle. Moreover, the speed of germination and yield (germination rate) of seeds can be modified by preliminary plasma treatment. The change in the wetting properties of seeds is at least partially due to oxidation of their surface under plasma treatment. Significant growth of the peaks corresponding to the nitrogen containing groups in the mass spectra of air plasma treated seeds was registered by TOF-SIMS spectroscopy.

Janus droplets: liquid marbles coated with dielectric/semiconductor particles.

The manufacturing of water droplets wrapped with two different powders, carbon black (semiconductor) and polytetrafluoroethylene (dielectric), is presented. Droplets composed of two hemispheres (Janus droplets) characterized by various physical and chemical properties are reported first. Watermelon-like striped liquid marbles are reported. Janus droplets remained stable on solid and liquid supports and could be activated with an electric field.

Contact angle hysteresis on polymer substrates established with various experimental techniques, its interpretation, and quantitative characterization.

The effect of contact angle hysteresis (CAH) was studied on various polymer substrates with traditional and new experimental techniques. The new experimental technique presented in the article is based on the slow deformation of the droplet, thus CAH is studied under the constant volume of the drop in contrast to existing techniques when the volume of the drop is changed under the measurement. The energy of hysteresis was calculated in the framework of the improved Extrand approach. The advancing contact angle established with a new technique is in a good agreement with that measured with the needle-syringe method. The receding angles measured with three experimental techniques demonstrated a very significant discrepancy. The force pinning the triple line responsible for hysteresis was calculated.

Water rolling and floating upon water: Marbles supported by a water/marble interface

Floating of liquid marbles on a water/air interface was studied. The critical density allowing floating marbles containing NaCl solution was established experimentally and compared with its calculated value. A satisfactory agreement between experimental and theoretical values of the critical density is reported.

Interaction of cold radiofrequency plasma with seeds of beans (Phaseolus vulgaris)

Highlight The impact of cold plasma on the wetting, water absorption, and germination of beans (Phaseolus vulgaris) is reported. Plasma treatment accelerated the water absorption and germination of seeds.

Non-stick droplet surgery with a superhydrophobic scalpel.

Cutting of nonstick droplets with a double-faced superhydrophobic blade is reported first. The process of manufacturing the superhydrophobic scalpel is presented. Cutting of water marbles and droplets deposited on a superhydrophobic surface is possible when the velocity of the blade is higher than a certain critical value. An estimation of the critical blade velocity coinciding with experimental findings is presented. Cutting of Janus and glycerol-based marbles is discussed. Coalescence of glycerol nonstick drops and marbles under cutting is treated.

Superhydrophobic Metallic Surfaces and Their Wetting Properties

Two-scaled polymer patterns possessing high roughness are produced using the hot-embossing technique and coating with chromium. After covering the rough polymer surface with this metal, the resulting surface demonstrates pronounced hydrophobic properties, with water drops having apparent contact angles as high as 150°. As revealed by SEM, the surface is patterned on two scales which is known to be an important condition for superhydrophobicity of inherently wettable surfaces. The modified Cassie–Baxter approach has been used for the present relief. The calculated values of the apparent contact angles agree with the observed ones. It is important to note that the apparent contact angles almost do not depend on the local contact angles.

Low voltage reversible electrowetting exploiting lubricated polymer honeycomb substrates

Low-voltage electrowetting-on-dielectric scheme realized with lubricated honeycomb polymer surfaces is reported. Polycarbonate honeycomb reliefs manufactured with the breath-figures self-assembly were impregnated with silicone and castor oils. The onset of the reversible electrowetting for silicone oil impregnated substrates occurred at 35 V, whereas for castor oil impregnated ones it took place at 80 V. The semi-quantitative analysis of electrowetting of impregnated surfaces is proposed.