We discuss in this review how the roughness of a solid impacts its wettability. We see in particular that both the apparent contact angle and the contact angle hysteresis can be dramatically affected by the presence of roughness. Owing to the development of refined methods for setting very well-controlled micro- or nanotextures on a solid, these effects are being exploited to induce novel wetting properties, such as spontaneous filmification, superhydrophobicity, superoleophobicity, and interfacial slip, that could not be achieved without roughness.

Wetting and Roughness

Research into extreme water-repellent surfaces began many decades ago, although it was only relatively recently that the term superhydrophobicity appeared in literature Here we review the work on the preparation of superhydrophobic surfaces, with focus on the different techniques used and how they have developed over the years, with particular focus on the last two years We discuss the origins of water-repellent surfaces, examining how size and shape of surface features are used to control surface characteristics, in particular how techniques have progressed to form multi-scaled roughness to mimic the lotus leaf effect There are notable differences in the terminology used to describe the varying properties of water-repellent surfaces, so we suggest some key definitions

Progess in superhydrophobic surface development.

Protein adhesion plays a major role in determining the biocompatibility of materials. The first stage of implant integration is the adhesion of protein followed by cell attachment. Surface modification of implants (surface chemistry and topography) to induce and control protein and cell adhesion is currently of great interest. This communication presents data on protein adsorption (bovine serum albumin and fibrinogen) onto model hydrophobic (CH3) and hydrophilic (OH) surfaces, investigated using a quartz crystal microbalance (QCM) and grazing angle infrared spectroscopy. Our data suggest that albumin undergoes adsorption via a single step whereas fibrinogen adsorption is a more complex, multistage process. Albumin has a stronger affinity toward the CH3 compared to OH terminated surface. In contrast, fibrinogen adheres more rapidly to both surfaces, having a slightly higher affinity toward the hydrophobic surface. Conformational assessment of the adsorbed proteins by grazing angle infrared spectroscopy (GA...

Interpretation of Protein Adsorption: Surface-Induced Conformational Changes

While the behaviour of large amounts of liquid is dictated by gravity, surface forces become dominant at small scales. They have for example the remarkable ability to make droplets stick to their substrates (even if they are inclined), which is a practical issue in many cases (windshields, window panes, greenhouses, or microfluidic devices). Here we describe how this problem can be overcome with super-hydrophobic materials. These materials are often developed thanks to micro-textures, which decorate a solid surface, and we describe the way such textures modify the wettability of that solid. We conclude by showing the unusual dynamics of drops in a super-hydrophobic situation.

https://iopscience.iop.org/article/10.1088/0034-4885/68/11/R01/pdf

Non-sticking drops

Superhydrophobic surfaces: From natural to biomimetic to functional

In this work we present a reliable technique for the production of large areas of high aspect-ratio patterns and describe their use as model super-hydrophobic systems. The high thickness and straight sidewalls possible with SU-8 were used to generate dense patterns of small pillars. These photoresist patterns could be used directly, without the need for micromoulding. A method is given allowing resist thickness to be varied over a wide range and a bottom antireflective layer was used to simplify patterning on reflective substrates. This patterning technique allows rapid testing of wetting theories, as pattern size and depth can be varied simply and samples can be produced in sufficient numbers for laboratory use. We show how the static contact angle of water varies with pattern height for one sample-pattern and how static and dynamic contact angles vary with dimension using high aspect-ratio patterns.

/pdf/the-use-of-high-aspect-ratio-photoresist-su-8-for-super-30e3k3gvzh.pdf

The use of high aspect ratio photoresist (SU-8) for super-hydrophobic pattern prototyping

Super water-repellent surfaces occur naturally on plants and aquatic insects and are created in the laboratory by combining micro- or nanoscale surface topographic features with hydrophobic surface chemistry. When such types of water-repellent surfaces are submerged they can retain a film of air (a plastron). In this work, we report measurements of the terminal velocity of solid acrylic spheres with various surface treatments settling under the action of gravity in water. We observed increases in terminal velocity corresponding to drag reduction of between 5% and 15% for superhydrophobic surfaces that carry plastrons.

/pdf/terminal-velocity-and-drag-reduction-measurements-on-4khbkehoxt.pdf

Terminal velocity and drag reduction measurements on superhydrophobic spheres

Electrowetting on micro-patterned layers of SU-8 photoresist with an amorphous Teflon (R) coating has been observed. The cosine of the contact angle is shown to be proportional to the square of the applied voltage for increasing bias. However, this does not apply below 40 V and we suggest that this may be explained in terms of penetration of fluid into the pattern of the surface. Assuming that the initial application of a bias voltage converts the drop from Cassie-Baxter to Wenzel regime, we have used this as a technique to estimate the roughness factor of the surface.

/pdf/electrowetting-on-superhydrophobic-su-8-patterned-surfaces-4ws80mgh5n.pdf

Electrowetting on superhydrophobic SU-8 patterned surfaces

Recent reports using particle image velocimetry and cone-and-plate rheometers have suggested that a simple Newtonian liquid flowing across a superhydrophobic surface demonstrates a finite slip length. Slippage on a superhydrophobic surface indicates that the combination of topography and hydrophobicity may have consequences for the coupling at the solid--liquid interface observed using the high-frequency shear-mode oscillation of a quartz crystal microbalance (QCM). In this work, we report on the response of a 5 MHz QCM possessing a superhydrophobic surface to immersion in water--glycerol mixtures. QCM surfaces were prepared with a layer of SU-8 photoresist and lithographically patterned to produce square arrays of 5 mum diameter circular cross-section posts spaced 10 microm center-to-center and with heights of 5, 10, 15, and 18 microm. Non-patterned layers were also created for comparison, and both non-hydrophobized and chemically hydrophobized surfaces were investigated. Contact angle measurements confirmed that the hydrophobized post surfaces were superhydrophobic. QCM measurements in water before and after applying pressure to force a Cassie-Baxter (non-penetrating) to Wenzel (penetrating) conversion of state showed a larger frequency decrease and higher dissipation in the Wenzel state. QCM resonance spectra were fitted to a Butterworth-van Dyke model for the full range of water-glycerol mixtures from pure water to (nominally) pure glycerol, thus providing data on both energy storage and dissipation. The data obtained for the post surfaces show a variety of types of behavior, indicating the importance of the surface chemistry in determining the response of the quartz crystal resonance, particularly on topographically structured surfaces; data for hydrophobized post surfaces imply a decoupling of the surface oscillation from the mixtures. In the case of the 15 microm tall hydrophobized post surfaces, crystal resonance spectra become narrower as the viscosity-density product increases, which is contrary to the usual behavior. In the most extreme case of the 18 microm tall hydrophobized post surfaces, both the frequency decrease and bandwidth increase of the resonance spectra are significantly lower than that predicted by the Kanazawa and Gordon model, thus implying a decoupling of the oscillating surface from the liquid, which can be interpreted as interfacial slip.

Decoupling of the liquid response of a superhydrophobic quartz crystal microbalance

Bistable azimuthal nematic alignment textures have been created in micrometer-scale channels for which one sidewall is smooth and straight and the other possesses a symmetric sawtooth morphology. The optical textures have been observed during dynamic switching between the two stable states in response to dual frequency ac waveform driving of a highly dispersive nematic liquid crystal. The switching processes involves collapsing of filamentlike director reorientation (tilt-wall) loops and the associated motion and annihilation of surface defects along and close to the edge at the sawtooth sidewall. The predictions from both the n-director-based Ericksen-Leslie theory and the Q-tensor theory are in good agreement with the experimental observations.

Carl Evans

Papers

The use of high aspect ratio photoresist (SU-8) for super-hydrophobic pattern prototyping

Terminal velocity and drag reduction measurements on superhydrophobic spheres

Electrowetting on superhydrophobic SU-8 patterned surfaces

Decoupling of the liquid response of a superhydrophobic quartz crystal microbalance

Defect trajectories and domain-wall loop dynamics during two-frequency switching in a bistable azimuthal nematic device.