Michael Nosonovsky

From superhydrophobicity to icephobicity: forces and interaction analysis

The term “icephobicity” has emerged in the literature recently. An extensive discussion took place on whether the icephobicity is related to the superhydrophobicity, and the consensus is that there is no direct correlation. Besides the parallel between the icephobicity and superhydrophobicity for water/ice repellency, there are similarities on other levels including the hydrophobic effect/hydrophobic interactions, mechanisms of protein folding and ice crystal formation. In this paper, we report how ice adhesion is different from water using force balance analysis, and why superhydrophobic surfaces are not necessary icephobic. We also present experimental data on anti-icing of various surfaces and suggest a definition of icephobicity, which is broad enough to cover a variety of situations relevant to de-icing including low adhesion strength and delayed ice crystallization and bouncing.

The rose petal effect and the modes of superhydrophobicity

The wetting of rough surfaces remains a subject of active investigation by scientists. The contact angle (CA) is a traditional parameter used to characterize the hydrophobicity/philicity of a solid surface. However, it was found recently that high CAs can coexist with strong adhesion between water and a solid surface in the case of the so-called ‘rose petal effect’. Several additional parameters have been proposed to characterize the interaction of water with a rough solid surface, including the CA hysteresis, the ability of water droplets to bounce off a solid surface, the tilt angle needed to initiate the flow of a droplet, and the normal and shear adhesion. It is clear now that wetting is not characterized by a single parameter, since several modes or regimes of wetting of a rough surface can exist, including the Wenzel, Cassie, lotus and petal. Understanding the wetting of rough surfaces is important in order to design non-adhesive surfaces for various applications.

Why Superhydrophobic Surfaces Are Not Always Icephobic

We discuss mechanical forces that act upon a water droplet and a piece of ice on a rough solid surface and the difference between dewetting and ice fracture. The force needed to detach a water droplet depends on contact angle (CA) hysteresis and can be reduced significantly in the case of a superhydrophobic surface. The force needed to detach a piece of ice depends on the receding CA and the initial size of interfacial cracks. Therefore, even surfaces with very high receding CA may have strong adhesion to ice if the size of the cracks is small.

Towards Optimization of Patterned Superhydrophobic Surfaces

Experimental and theoretical study of wetting properties of patterned Si surfaces with cylindrical flat-top pillars of various sizes and pitch distances is presented. The values of the contact angle (CA), contact angle hysteresis (CAH) and tilt angle (TA) are measured and compared with the theoretical values. Transition from the composite solid–liquid–air to the homogeneous solid–liquid interface is investigated. It is found that the wetting behaviour of a patterned hydrophobic surface depends upon a simple non-dimensional parameter, the spacing factor, equal to the pillar diameter divided by the pitch. The spacing factor controls the CA, CAH and TA in the composite interface regime, as well as destabilization and transition to the homogeneous interface. We show that the assumption that the CAH is a consequence of the adhesion hysteresis and surface roughness leads to the theoretical values of the CAH that are in a reasonably good agreement with the experimental values. By decreasing the spacing factor, the values of CA=170°, CAH=5° and TA=3° are achieved. However, with further decreasing of the spacing factor, the composite interface destabilizes.

Multiscale Dissipative Mechanisms and Hierarchical Surfaces

In the introduction chapter, the subjects and definitions of the surface science and tribology are discussed, as well as their relations to the concepts of hierarchy, mesoscale, energy dissipation and biomimetics. 1.1 Surfaces and Surface Free Energy Surface science is defined as the study of physical and chemical phenomena that occur at the interface of two phases (solid–liquid, solid–gas, solid–vacuum) or of different substances of the same phase (solid–solid, liquid–liquid) [6]. Various properties of matter (e.g., the density, ρ) can change rapidly at the interface. It is therefore convenient to assume that the interface is a geometrically two-dimensional surface in a sense that every point at the interface can be characterized by only two parameters. In reality, every interface has a nonzero thickness and the bulk properties change gradually at the interface; however, the thickness is so small compared to the two other dimensions that it can often be neglected. An important characteristic of every surface or interface is the surface free energy, γ . In the bulk of the body, chemical bonds exist between the molecules and certain energy has to be applied in order to break the bonds. The molecules that do not form the bonds have higher potential energy than those that form the bonds. Molecules at the surface do not form bonds at the side of the surface and thus they have higher energy. This additional energy is called surface or interface free energy and is measured in the energy per area units, that is, in the SI system, J/m2 or N/m. In order to create an interface (e.g., to form a vapor bubble inside boiling water), the energy should be applied which is equal to the area of the interface multiplied by the interface free energy. For the stable existence of the interface it is required that the free energy of formation of the interface be positive, so that accidental fluctuations do not result in the dispersion of one material into the other. The opposite example of an interface, which does not offer opposition to the dispersion, is that between two gases or between miscible liquids [6]. Any system tends to achieve a position that

Model for solid-liquid and solid-solid friction of rough surfaces with adhesion hysteresis

Mechanisms of energy dissipation during solid-solid and solid-liquid friction are discussed. A conservative van der Waals adhesion force, when combined with surface imperfectness, such as deformation, leads to adhesion hysteresis (AH). When an asperity slides upon a substrate, the substrate is subjected to a loading-unloading cycle, and energy is dissipated due to the AH. Another mechanism, which leads to energy dissipation, involves energy barriers between metastable states due to surface roughness. Both mechanisms are fundamental for sliding and result in both solid-liquid and solid-solid friction.

Scale effects in friction using strain gradient plasticity and dislocation-assisted sliding (microslip)

Scale effects in dry friction at macro- to nanoscale are considered. According to the adhesional model of friction, the friction force depends on the real area of contact and the average shear strength of asperity contacts during sliding. The scale dependence of the so-called geometrically necessary dislocations causes enhanced hardness with decreasing scale. In the case of plastic contacts, enhanced hardness results in a decrease in the real area of contact. The average shear strength at the interface is associated with dislocation-assisted sliding (microslip) and increases with decreasing scale, from geometrical considerations. In cases of single-asperity and multi-asperity elastic or plastic contact, the scale dependence of the real area of contact and shear strength results in scale-dependent friction. Comparisons of the model with experimental data are also presented.

Multiscale effects and capillary interactions in functional biomimetic surfaces for energy conversion and green engineering

Biological surfaces (plant leaves, lizard and insect attachment pads, fish scales, etc.) have remarkable properties due to their hierarchical structure. This structure is a consequence of the hierarchical organization of biological tissues. The hierarchical organization of the surfaces allows plants and creatures to adapt to energy dissipation and transition mechanisms with various characteristic scale lengths. At the same time, an addition of a micro-/nanoscale hierarchical level, for example of surface roughness, can change qualitatively the properties of a system and introduce multiple equilibriums, instability and dissipation. Thus, small roughness has a large effect. In particular, a small change of surface roughness can lead to a large change in the capillary force. The capillary effects are crucial for small-scale applications. Multiscale organization of the biomimetic surfaces and their adaptation to capillary effects make them suitable for applications using new principles of energy transition (e.g. capillary engines) and environment-friendly technologies (e.g. self-cleaning oleophobic surfaces).

Wetting Transitions in Two-, Three-, and Four-Phase Systems

We discuss wetting of rough surfaces with two-phase (solid-liquid), three-phase (solid-water-air and solid-oil-water), and four-phase (solid-oil-water-air) interfaces mimicking fish scales. We extend the traditional Wenzel and Cassie-Baxter models to these cases. We further present experimental observations of two-, three-, and four-phase systems in the case of metal-matrix composite solid surfaces immersed in water and in contact with oil. Experimental observations show that wetting transitions can occur in underwater oleophobic systems. We also discuss wetting transitions as phase transitions using the phase-field approach and show that a phenomenological gradient coefficient is responsible for wetting transition, energy barriers, and wetting/dewetting asymmetry (hysteresis).

Scale effects in dry and wet friction, wear, and interface temperature

Scale effects in tribology at the macroscale to nanoscale are considered. The coefficient of dry friction depends on the real area of contact and the shear strength due to adhesion and two- and three-body deformation. The real area of contact depends on the surface topography and elastic modulus for elastic contact, and on the hardness for plastic contact. The surface topography is scale dependent, on the basis of a fractal model or an empirical rule. The hardness is scale dependent on the basis of the strain gradient plasticity. The adhesional shear strength is scale dependent on the basis of a dislocation-assisted sliding model. The two-body deformation component of the coefficient of friction is scale dependent due to the scale dependence of the average asperity slope. The real area of three-body contact is scale dependent due to the scale dependence of the probability for a particle to be trapped at the interface and shear strength. In the presence of a liquid film the measured value of the coefficient of friction is different from the coefficient of dry friction due to the meniscus contribution. The meniscus force is scale dependent, since it depends on the number of contacts and summit radius of the asperities, which are scale dependent, on the basis of the surface topography. The scale dependence of other parameters of tribological importance, such as the wear coefficient, which depends on the scale dependent hardness, and the interface temperature rise, which depends on the scale dependent mean contact size, is also considered.