Tamir Stein

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.

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.

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.

Electrically Deformable Liquid Marbles

Liquid marbles, which are droplets coated with a hydrophobic powder, were exposed to a uniform electric field. It was established that a threshold value of the electric field, 15 cgse, should be surmounted for deformation of liquid marbles. The shape of the marbles was described as a prolate spheroid. A semi-quantitative theory describing deformation of liquid marbles in a uniform electric field is presented. The scaling law relating the radius of the contact area of the marble to the applied electric field shows a satisfactory agreement with the experimental data.

Electrostatically driven droplets deposited on superhydrophobic surfaces

The behavior of dielectric droplets deposited on a superhydrophobic surface and exposed to a constant and uniform electric field is investigated. The change in the droplet shape is described by a linear dependence of its eccentricity on the applied electric field. The proposed control of the droplet shape with the electric field may serve as an alternative to electrowetting technologies.