Plasma treatment switches the regime of wetting and floating of pepper seeds.

Cold radiofrequency plasma treatment modified wetting and floating regimes of pepper seeds. The wetting regime of plasma-treated seeds was switched from the Wenzel-like partial wetting to the complete wetting. No hydrophobic recovery following the plasma treatment was registered. Environmental scanning electron microscopy of the fine structure of the (three-phase) triple line observed with virgin and plasma-treated seeds is reported. Plasma treatment promoted rapid sinking of pepper seeds placed on the water/air interface. Plasma treatment did not influence the surface topography of pepper seeds, while charged them electrically. Electrostatic repulsion of floating plasma-treated seeds was observed. The surface charge density was estimated from the data extracted from floating of charged seeds and independently with the electrostatic pendulum as σ≈1-2µC/m2.

Under-Liquid Self-Assembly of Submerged Buoyant Polymer Particles.

The self-assembly of submerged cold-plasma-treated polyethylene beads (PBs) is reported. The plasma-treated immersed millimetrically sized PBs formed well-ordered 2D quasicrystalline structures. The submerged floating of "light" (buoyant) PBs is possible because of the energy gain achieved by the wetting of the high-energy plasma-treated polymer surface prevailing over the energy loss due to the upward climb of the liquid over the beads. The capillary "immersion" attraction force is responsible for the observed self-assembly. The observed 2D quasicrystalline structures demonstrate "dislocations" and "point defects". The mechanical vibration of self-assembled rafts built of PBs leads to the healing of point defects. The immersion capillary lateral force governs the self-assembly, whereas the elastic force is responsible for the repulsion of polymer beads.

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.

Interpretation of elasticity of liquid marbles.

Liquid marbles are non-stick droplets covered with micro-scaled particles. Liquid marbles demonstrate quasi-elastic properties when pressed. The interpretation of the phenomenon of elasticity of liquid marbles is proposed. The model considering the growth in the marble surface in the course of deformation under the conservation of marble's volume explains semi-quantitatively the elastic properties of marbles in satisfactory agreement with the reported experimental data. The estimation of the effective Young modulus of marbles and its dependence on the marble volume are reported.

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

The impact of cold radiofrequency air plasma on the wetting properties and water imbibition of beans (Phaseolus vulgaris) was studied. The influence of plasma on wetting of a cotyledon and seed coat (testa) was elucidated. It was established that cold plasma treatment leads to hydrophilization of the cotyledon and tissues constituting the testa when they are separately exposed to plasma. By contrast, when the entire bean is exposed to plasma treatment, only the external surface of the bean is hydrophilized by the cold plasma. Water imbibition by plasma-treated beans was studied. Plasma treatment markedly accelerates the water absorption. The crucial role of a micropyle in the process of water imbibition was established. It was established that the final percentage of germination was almost the same in the cases of plasma-treated, untreated, and vacuum-pumped samples. However, the speed of germination was markedly higher for the plasma-treated samples. The influence of the vacuum pumping involved in the cold plasma treatment on the germination was also clarified.

Physical mechanisms of interaction of cold plasma with polymer surfaces.

Physical mechanisms of the interaction of cold plasmas with organic surfaces are discussed. Trapping of plasma ions by the CH2 groups of polymer surfaces resulting in their electrical charging is treated. Polyethylene surfaces were exposed to the cold radiofrequency air plasma for different intervals of time. The change in the wettability of these surfaces was registered. The experimentally established characteristic time scales of the interaction of cold plasma with polymer surfaces are inversely proportional to the concentration of ions. The phenomenological kinetic model of the electrical charging of polymer surfaces by plasmas is introduced and analyzed.

On universality of scaling law describing roughness of triple line.

The fine structure of the three-phase (triple) line was studied for different liquids, various topographies of micro-rough substrates and various wetting regimes. Wetting of porous and pillar-based micro-scaled polymer surfaces was investigated. The triple line was visualized with the environmental scanning electron microscope and scanning electron microscope for the "frozen" triple lines. The value of the roughness exponent ζ for water (ice)/rough polymer systems was located within 0.55-0.63. For epoxy glue/rough polymer systems somewhat lower values of the exponent, 0.42 < ζ < 0.54, were established. The obtained values of ζ were close for the Cassie and Wenzel wetting regimes, different liquids, and different substrates' topographies. Thus, the above values of the exponent are to a great extent universal. The switch of the exponent, when the roughness size approaches to the correlation length of the defects, is also universal.

On the role of the line tension in the stability of Cassie wetting.

The effect of line tension may increase or decrease the potential barrier separating the Cassie and the Wenzel wetting states, depending on the sign of the line tension and the topography of a relief. The formation of the barrier separating the Cassie and Wenzel wetting states on reentrant topographies, which demonstrate pronounced superhydrophobicity and superoleophobicity, is treated. Various topographies giving rise to high apparent contact angles are discussed. It spite of the fact that the line tension is regarded as a weak effect, it may be essential for stabilizing (destabilizing) the Cassie wetting. The effect may be crucial to understanding the stability of the Cassie-like wetting of inherently hydrophilic nanoscaled reliefs.

Superhydrophobicity of lotus leaves versus birds wings: different physical mechanisms leading to similar phenomena.

Remarkable water repellency of birds' feathers and lotus leaves is discussed. It is demonstrated that physical mechanisms of superhydrophobicity of birds' feathers and lotus leaves are very different. The topography of lotus leaves is a truly hierarchical one, whereas birds' feathers manifest pseudohierarchical relief, where various scales do not interact. The pronounced stability of the Cassie state observed on birds' feathers is due to the high value of critical pressure necessary for their total wetting, which is on the order of magnitude of 100 kPa. This high value allows feathers to withstand large dynamical pressure of rain droplets and remain dry under the rain. The energy barrier separating the Cassie state from the complete wetting situation calculated for a feather is also very high, allowing the increased stability of superhydrophobicity.

How to make the Cassie wetting state stable?

Wetting of rough hydrophilic and hydrophobic surfaces is discussed. The stability of the Cassie state, with air trapped in relief details under the droplet, is necessary for the design of true superhydrophobic surfaces. The potential barrier separating the Cassie state and the Wenzel state, for which the substrate is completely wetted, is calculated for both hydrophobic and hydrophilic surfaces. When the surface is hydrophobic, the multiscaled roughness of pillars constituting the surface increases the potential barrier separating the Cassie and Wenzel states. When water fills the hydrophilic pore, the energy gain due to the wetting of the pore hydrophilic wall is overcompensated by the energy increase because of the growth of the high-energetic liquid-air interface. The potential barrier separating the Cassie and Wenzel states is calculated for various topographies of surfaces. Structural features of reliefs favoring enhanced hydrophobicity are elucidated.

Shape, vibrations, and effective surface tension of water marbles.

The surface of water "marbles" obtained with hydrophobic lycopodium and polyvinylidene fluoride particles was investigated first with environmental scanning electron microscopy. The shape of water marbles was studied both experimentally and theoretically. The mathematical model describing the deformation of marbles by gravity is proposed. The model allowed the calculation of the effective surface tension of marbles and gives 0.09 J/m2 for marbles coated with PVDF and 0.06 J/m2 for marbles coated with lycopodium. The effective surface tensions of marbles calculated independently by the horizontal vibration of marbles were in semiquantitative agreement with the above values (0.07 J/m2 for marbles coated with PVDF and 0.055 J/m2 for marbles coated with lycopodium).

Oblate spheroid model for calculation of the shape and contact angles of heavy droplets.

A model is proposed for calculation of the shape and contact angle of droplets placed on solid substrates, which is based on the oblate spheroid approximation. Illustrative examples of calculation of various geometrical characteristics of the droplets are presented. Calculated contact angles show their actual independence of the droplet weight in a broad range of volumes. The useful application to the goniometry is proposed.

Characterization of rough surfaces with vibrated drops.

Wetting transitions were studied with vertically-vibrated drops on various artificial and natural rough substrates. Alternative pathways of wetting transitions were observed. The model of wetting transition is presented. Multiple minima of the Gibbs free energy of a drop deposited on a rough surface explain alternative pathways of wetting transitions. We demonstrate that a wetting transition occurs when the constant force resulting from vibrations, Laplace and hydrostatic pressure acts on the triple line. It is shown that the final wetting states are mainly the Cassie impregnating wetting state with water penetrating the pores in the outer vicinity of the droplet or the Wenzel state with water inside the pores under the droplet whereas the substrate ahead the drop is dry.

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.

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.

Resonance Cassie-Wenzel wetting transition for horizontally vibrated drops deposited on a rough surface.

The transition between the Cassie and Wenzel wetting regimes has been observed under horizontal vibrations of a water drop placed on the rough micrometrically scaled polymer pattern. The observed transition has a distinct resonance character. The resonance frequencies as established experimentally coincide with the calculated eigenfrequencies of capillary-gravity standing waves on the drop surface. The resonance Cassie-Wenzel transition is related to the displacement of the triple line caused by both the inertia force and the increase in the Laplace pressure. This strengthens the idea that the Cassie-Wenzel wetting transition is most likely a 1D affair stipulated by the triple-line behavior. The study of the vibrated drop deposited on the rough surface supplied valuable information concerning the Cassie-Wenzel wetting transition.

Cassie-Wenzel wetting transition in vibrating drops deposited on rough surfaces: is the dynamic Cassie-Wenzel wetting transition a 2D or 1D affair?

The transition between the Cassie and Wenzel wetting regimes has been observed under vertical vibration of a water drop placed on a rough micrometrically scaled polymer pattern. The transition takes place under the constant force per unit length of the triple contact line, not under constant pressure. A study of the vibrating drop deposited on the rough surface supplied valuable information concerning the Cassie-Wenzel wetting transition.

Environmental scanning electron microscopy study of the fine structure of the triple line and cassie-wenzel wetting transition for sessile drops deposited on rough polymer substrates.

The wetting of rough honeycomb micrometrically scaled polymer substrates was studied. A very strong dependence of the apparent contact angle on the drop volume has been established experimentally. The environmental scanning electron microscopy study of the fine structure of the triple line is reported first. The triple line is not smooth and prefers grasping the polymer matrix over air holes. The precursor rim surrounding the drop has been observed. The revealed dependence of the apparent contact angle on the drop volume is explained by the transition between the pure Cassie and combined Wenzel-Cassie wetting regimes, which is induced by capillarity penetration of water into the holes of relief.