Abraham Marmur

Soft contact: measurement and interpretation of contact angles

The measurement and interpretation of contact angles deceptively appear to be simple. This paper attempts to summarize the pitfalls in the field, and how to avoid them. First, the fundamental underlying theory that is necessary in order to properly measure and interpret contact angles is discussed, emphasizing recent developments. Then, the practical implications of these theoretical aspects are presented. In addition, the discussion highlights the missing pieces of the picture that need to be completed through future research.

From hygrophilic to superhygrophobic: theoretical conditions for making high-contact-angle surfaces from low-contact-angle materials.

The possibility of making high-contact-angle, rough surfaces from low-contact-angle materials has recently been suggested and demonstrated. A thermodynamic analysis of this possibility in terms of feasibility and stability is presented. It turns out that only roughness topographies that conform to a feasibility condition which is developed in the present paper can support this phenomenon. Even under conditions that support the phenomenon, the high-contact-angle state may not be stable, and transition from the heterogeneous (Cassie-Baxter) wetting regime to the homogeneous (Wenzel) regime with a lower contact angle may occur. In addition, it is suggested to use the general terms hygrophilic and hygrophobic (based on the Greek prefix hygro- that means liquid) to describe low- and high-contact-angle surfaces, respectively.

Superhydrophobic and Superoleophobic Nanocellulose Aerogel Membranes as Bioinspired Cargo Carriers on Water and Oil

We demonstrate that superhydrophobic and superoleophobic nanocellulose aerogels, consisting of fibrillar networks and aggregates with structures at different length scales, support considerable load on a water surface and also on oils as inspired by floatation of insects on water due to their superhydrophobic legs. The aerogel is capable of supporting a weight nearly 3 orders of magnitude larger than the weight of the aerogel itself. The load support is achieved by surface tension acting at different length scales: at the macroscopic scale along the perimeter of the carrier, and at the microscopic scale along the cellulose nanofibers by preventing soaking of the aerogel thus ensuring buoyancy. Furthermore, we demonstrate high-adhesive pinning of water and oil droplets, gas permeability, light reflection at the plastron in water and oil, and viscous drag reduction of the fluorinated aerogel in contact with oil. We foresee applications including buoyant, gas permeable, dirt-repellent coatings for miniature sensors and other devices floating on generic liquid surfaces.

Apparent contact angles on rough surfaces: the Wenzel equation revisited

Abstract The thermodynamic status of the Wenzel equation is discussed. It is explained and demonstrated by an example that the Wenzel equation may be incorrect. However, it is mathematically proven for sawtooth surfaces that the Wenzel equation becomes the correct one for the apparent contact angle as the drop size becomes infinitely larger than the scale of the roughness. It is shown, that under this condition the drop becomes axisymmetric even if the roughness of the solid surface is not. A complete proof for rough surfaces in general is also feasible, however the mathematical complexity of this proof is beyond the present scope.

When Wenzel and Cassie are right: reconciling local and global considerations.

The condition under which the Wenzel or Cassie equation correctly estimates the most stable contact angle is reiterated and demonstrated: these equations do hold when the drop size is sufficiently large compared with the wavelength of roughness or chemical heterogeneity. The numerical demonstrations somewhat mimic recent experiments that seemingly refuted the Wenzel and Cassie equations and show that these experiments were performed only for drops of sizes similar in order of magnitude to the wavelength of roughness or chemical heterogeneity. Under such conditions, the Wenzel and Cassie equations are a priori not expected to be valid. It is also explained that both the local equilibrium condition at the contact line and the global equilibrium condition involving the wetted area within the contact line are necessary and complementary.

Equilibrium and spreading of liquids on solid surfaces

Abstract Contact angle equilibrium and dynamic spreading of liquids on solid surfaces are reviewed and discussed. The thermodynamic status of the Young equation is critically reviewed. It is redeveloped rigorously and shown to apply locally, even for rough and heterogeneous though rigid and insoluble soolid surfaces, when written in terms of the local interfacial tensions. The line tension approach and the molecular approach to relate the local interfacial tensions to their values far way from the three-phase contact line are critically discussed, and the difficulties with some recent corrections of the young equation are explained. The problem of the shape of the fluid-fluid interface near the contact line is analyzed, and it is shown that a discontinuous transition has to occur between the solid surface and the fluid-fluid interface. Experimental data and theoretical models for dynamic spreading are discussed and compared. Special emphasis is put on the qualitative and quantitative effects of the primary film. The theoretical investigations are analyzed especially regarding their physical content rather than their hydrodynamic details. Three types of situations are distinguished: forced spreading, spontaneous spreading to an equilibrium contact angle and spontaneous spreading when no equilibriium contact angle exists. It is speculated that the primary film forms only in the latter situation.

Super-hydrophobicity fundamentals: implications to biofouling prevention

Abstract The theory of wetting on super-hydrophobic surfaces is presented and discussed, within the general framework of equilibrium wetting and contact angles. Emphasis is put on the implications of super-hydrophobicity to the prevention of biofouling. Two main lines of thought are discussed, viz. i) “mirror imaging” of the Lotus effect, namely designing a surface that repels biological entities by being super-hydrophilic, and ii) designing a surface that minimises the water-wetted area when submerged in water (by keeping an air film between the water and the surface), so that the suspended biological entities have a low probability of encountering the solid surface.

Superhydrophobic Tracks for Low-Friction, Guided Transport of Water Droplets

anti-fogging, [ 6 ] anti-icing, [ 7 ] buoyancy [ 8 ] and drag reduction. [ 9 ] By defi nition, a surface is superhydrophobic if the contact angle between a water drop and the surface at the solid/liquid/air interface is larger than 150 ° , and the contact angle hysteresis is small, i.e., drops readily slide or roll off when the surface is tilted slightly. [ 10–12 ] Here we explore the feasibility of using superhydrophobicity for guided transport of water droplets. We demonstrate a simple yet effi cient approach for droplet transport, in which the droplet is moving on a superhydrophobic surface, using gravity or electrostatic forces as the driving force for droplet transportation and using tracks with vertical walls as gravitational potential barriers to design trajectories. Although the slope of the platform is as small as a few degrees, the drops move at a considerable speed up to 14 cm s − 1 , even in highly curved trajectories. We further demonstrate splitting of a droplet using a superhydrophobic knife and drop-size selection using superhydrophobic tracks. These concepts may fi nd applications in droplet microfl uidics and lab-on-a-chip systems where single droplets with potential analytes are manipulated. [ 13–16 ]

Equilibrium contact angles: theory and measurement

Some fundamental problems concerning equilibrium contact angles are discussed. These include the modification of the Young equation for the intrinsic contact angle, the question of whether the actual contact angle is always identical with the intrinsic contact angle, the mechanism of contact angle hysteresis and the problem of measuring actual contact angles. New approaches to these problems are presented, and open questions are pointed out.

Partial wetting of chemically patterned surfaces: the effect of drop size.

Partial wetting of chemically heterogeneous substrates is simulated. Three-dimensional sessile drops in equilibrium with smooth surfaces supporting ordered chemical patterns are considered. Significant features are observed as a result of changing the drop volume. The number of equilibrated drops is found either to remain constant or to increase with growing drop volume. The shape of larger drops appears to approach that of a spherical cap and their three-phase contact line seems, on a larger scale, more circular in shape than that of smaller drops. In addition, as the volume is increased, the average contact angle of drops whose free energy is lowest among all equilibrium-shaped drops of the same volume appears to approach the angle predicted by Cassie. Finally, contrary to results obtained with two-dimensional drops, contact angle hysteresis observed in this system is shown to exhibit a degree of volume dependence in the advancing and receding angles. Qualitative differences in the wetting behavior associated with the two different chemical patterns considered here, as well as differences between results obtained with two-dimensional and three-dimensional drops, can possibly be attributed to variations in the level of constraint imposed on the drop by the different patterns and by the dimensionality of the system.