China
Africa
Explore chapters and articles related to this topic
Nature of Solid Surfaces
Published in Andrew Terhemen Tyowua, Liquid Marbles, 2018
A non-wetting state is witnessed when a liquid drop sits almost spherically on the surfaces of a solid with an equilibrium contact angle value above 90° with relatively low hysteresis. The surface is said to be ‘hydrophobic’ when the liquid is water and ‘oleophobic’ when the liquid is oil. Surfaces that are both hydrophobic and oleophobic are said to be ‘omniphobic’. ‘Superhydrophobic’ and ‘superoleophobic’ surfaces are those in which the equilibrium contact angle made by the water and oil drops, respectively, is >> 90° (usually ≥ 150°). Similarly, surfaces that are both superhydrophobic and superoleophobic are said to be ‘superomniphobic’. These surfaces have a relatively low surface energy. Their creation by scientists has been inspired by nature (biological species), plants, and animals alike, whose surfaces remain largely non-wetting when in contact with liquids and hence the term ‘bioinspired non-wetting’ surfaces. The surfaces of the lotus (Nelumbo nucifera) leaf and butterfly (Parnassius glacialis) wings, Figure 2.8, which are completely non-wetted by water drops, are typical examples of these surfaces.
Finishing Processes and Recent Developments
Published in Asis Patnaik, Sweta Patnaik, Fibres to Smart Textiles, 2019
Andrew D. Hewitt, Andrew J. Hebden
The wetting behaviour of a textile is dependent on its surface energy and the surface tension of the liquid. When the surface energy is higher than the liquid’s surface tension, then good wetting behaviour and a low contact angle is observed. When the surface energy is lower than the liquid’s surface tension, then the textile exhibits poor wetting behaviour and a large contact angle is observed (Figure 11.6). Water has a higher surface tension than many oils, so a fabric that is proven to be hydrophobic may not be oleophobic. Finishes that are hydrophobic and oleophobic are often referred to as stain-resistant or soil-resistant finishes, as they prevent a wide range of potentially staining liquids from wetting the fabric.
Surface engineering for anti-fingerprint applications
Published in Surface Engineering, 2022
Lakshmi Gopal, Tirumalai Sudarshan
In industry, oleophobic and/or omniphobic coatings are typically applied to glass through vacuum and/or vapour deposition processes. An energy source, such as an electron beam, vaporizes dielectric materials that are vacuum sealed to the outer layer of the glass. After the required thickness of dielectric materials is obtained, the oleophobic evaporative materials are deposited on the surface, to form a permanent chemical bond with the underlying dielectric material. These layers are extremely thin monolayers that offer negligible resistance to light, thereby enabling maintenance of transparency (Figure 4).
Foaming honey: particle or molecular foaming agent?
Published in Journal of Dispersion Science and Technology, 2022
Andrew T. Tyowua, Adebukola M. Echendu, Stephen G. Yiase, Sylvester O. Adejo, Luter Leke, Emmanuel M. Mbawuaga, Bernard P. Binks
Applications of liquid foams are ubiquitous. In the food industry, liquid foams are used as food (e.g. whipped ice cream and beer foam) or as precursors of novel food products. In cosmetics, liquid foams are used for shaving and bathing. Liquid foams are also components of some fire extinguishers. However, these applications are greatly impeded by the physical separation of the gaseous and the liquid phases, driven by the relatively large liquid–air interfacial tension. In aqueous and nonaqueous liquids like oils, the physical separation of the gaseous and the liquid phases is commonly prevented using either a particle (so-called Pickering foams) or a molecular (proteins, polymers, surfactants) foaming agent. Pickering,[1] Ramsden[2] or Ramsden–Pickering[3] aqueous and oil foams are devoid of complete phase separation for several months or years. The foams are so named after Ramsden[4] and Pickering[5] who first used small solid particles to stabilize foams and emulsions. By contrast, surfactant-stabilized aqueous and oil foams typically collapse completely within a few minutes to a few days of their preparation. The particles self-adsorb on the surfaces of the gas bubbles during foaming, preventing coalescence which ultimately leads to phase separation. Particle adsorption is dictated by the most favorable balance between the solid–air (also known as particle surface free energy), solid–liquid and liquid–air interfacial tensions, tuned by wettability.[6] Particle wettability is quantified by the three-phase contact angle θ. The balance between the three interfacial tensions is achieved with particles of relatively low surface free energy for which θ > 90°. Such particles are said to be hydrophobic or oleophobic when the liquid is water or oil, respectively. Once adsorbed, particle detachment is impossible due to the relatively large energy requirement (order of 103kBT),[6] where kB is Boltzmann’s constant and T is the absolute temperature. This can be compared to surfactants which are in dynamic equilibrium (adsorbing and desorbing from the surfaces of the gas bubbles). The dynamic equilibrium nature of surfactants makes their foams more prone to drainage, disproportionation, coalescence and ultimately phase separation compared to foams of small solid particles.