Roman Pogreb

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.

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.

Composite non-stick droplets and their actuation with electric field

Composite non-stick droplets comprised of di-iodomethane and water, coated by a common shell built from hydrophobic particles, are reported. Activation of the composite marbles by an electric field was studied. The water drop climbed onto the di-iodomethane drop when the composite marble was exposed to the electric field. A dimensionless constant describing sensitivity of dielectric droplets to an electric field is introduced. An explanation of the observed phenomena is proposed.

Spatial light modulator based on a deformed-helix ferroelectric liquid crystal and a thin a-Si:H amorphous photoconductor

We have developed an optically addressed spatial light modulator (OASLM) based on a deformed-helix ferroelectric liquid crystal (DHFLC) and a thin (0.5-mum) amorphous a-Si:H photoconductor. The thin photoconductor permits operation in the transmission mode of a read beam. The OASLM, operated in the integrated mode, exhibits gray levels, a high contrast (>1:40), and a high spatial resolution (exceeding 57 lines/mm). This is a significant improvement over previously reported OASLMs based on DHFLCs.

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.

Superoleophobic Surfaces Obtained via Hierarchical Metallic Meshes

Hierarchical metallic surfaces demonstrating pronounced water and oil repellence are reported. The surfaces were manufactured with stainless-steel microporous meshes, which were etched with perfluorononanoic acid. As a result, a hierarchical relief was created, characterized by roughness at micro- and sub-microscales. Pronounced superoleophobicity was registered with regard to canola, castor, sesame, flax, crude (petroleum), and engine oils. Relatively high sliding angles were recorded for 5 μL turpentine, olive, and silicone oil droplets. The stability of the Cassie-like air trapping wetting state, established with water/ethanol solutions, is reported. The omniphobicity of the surfaces is due to the interplay of their hierarchical relief and surface fluorination.

The Reversible Giant Change in the Contact Angle on the Polysulfone and Polyethersulfone Films Exposed to UV Irradiation

Water contact angles on polysulfone and polyethersulfone films exposed to UV irradiation have been found to decrease dramatically. We relate this phenomenon to the formation and release of disulfonic acid from the irradiated films, a well-known surfactant. The phenomenon appears to be reversible, namely, cleansed surfaces retained their initial contact angle. The revealed phenomenon may provide a means of controlling the spreading of liquids on polysulfone and polyethersulfone films and seems promising for use in microfluidics applications.