Gene Whyman

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.

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.

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.

Wetting transitions and depinning of the triple line.

Physical mechanisms of Cassie-Wenzel wetting transitions are discussed. The origin of the potential barrier separating the Cassie and Wenzel wetting states is clarified. It may contain contributions originating from the filling of hydrophobic pores and displacement of the triple line along the smooth portions of the relief. One- and two-dimensional scenarios of wetting transitions are considered. We demonstrate that the contribution to the potential barrier because of the displacement of the triple line is not negligible in both cases.

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.

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

Highlight The impact of cold plasma on the wetting, water absorption, and germination of beans (Phaseolus vulgaris) is reported. Plasma treatment accelerated the water absorption and germination of seeds.

Controlling drop bouncing using surfaces with gradient features

Drop hitting on superhydrophobic surfaces usually undergoes spreading and retraction stages before its complete rebound and there exists a minimum amount of time for the spreading and retraction processes. Impressively, it was recently shown that the so-called contact time can be significantly reduced by engineering surfaces with millimeter-scale tapered post arrays that allow the impinging drop to leave the surfaces in a pancake shape at the end of lateral spreading (pancake bouncing). Despite exciting progress, it remains elusive to rationally control the contact time and quantitatively predict the critical Weber number for the occurrence of pancake bouncing. Here, we experimentally demonstrated that the drop bouncing is intricately modulated by the surface morphology. Under the same centre-to-centre post spacing, surfaces with a larger apex angle could give rise to more robust pancake bouncing, which is characterized by significant contact time reduction, smaller critical Weber number, and wider Weber number range. We also developed simple harmonic spring models and theoretically revealed the dependence of timescales associated with the impinging drop and the critical Weber number for pancake bouncing on the surface morphology. The insights learned from this work will allow us to rationally design various surfaces for many practical applications.

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.

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.