Liquid Repellency

Research Goals

Develop and characterize SOCAL, Chemical and Physical Superhydrophobic Surfaces
- Assess drag force profiles for various surface chemistries and treatments
- Examine surface interactions
Apply Landau-Levich Dip-coating theory to better understand Oleoplaning Droplets
- Understand fundamental physics of surface interaction between droplets and coated/treated surfaces, including assessing drag profiles
- Determine differences relative to "traditional" superhydrophobia

Combine structural superhydrophobia and oleoplanning to determine limiting parameters under constraints and parameters

Structured Superhydrophobia

Conventionally hydrophobic flat surfaces (eg. non-stick fluorinated coatings) have interactions overwhelmingly governed by drag mechanics on the surface. The greater contact area of an exposed droplet on the surface of a substrate the greater the drag. Compare two types of at superhydrophobic surfaces: the conventional Teflon vs. Slippery Omniphobic Covalently Attached Liquid (SOCAL) surfaces. On the left, the surface is functionalized with a thin layer of Teflon and exhibits contact angle hysteresis of Δθ ≳ 20°. However the SOCAL surface exhibits significantly lower drag force and lower contact angle hysteresis, Δθ ≲ 5°.

This distinction indicates that the surfaces exhibits fundamentally distinct drag mechanics. While the fluorinated surface has a nanometric stationary (bonded) fluorinated layer between the substrate and a dispensed droplet, the droplet still experiences surface roughness. In the SOCAL case, the thin nano-polymer brushes slide over one another between the droplet and the substrate reducing the experienced interaction with the surface and thereby the work needed to overcome surface features.

By contrast, microstructured ("lotus effect") surfaces produce superhydrophobia by reducing the effective area of interaction between the droplet and the surface; dispenses liquids on the surface attempt to maximize their area to volume ratio by taking on a spherical shape demanding lower surface interaction. The microstructures on the surface support the maintenance of a stable air layer between the structures , which helps buoy the droplet atop the structures (the so called "Cassie-Baxter state") thereby increasing the effective contact angle. In the case of hierarchical structures, where a structured surface is functionalized as in the case of fluorination or SOCAL polymer brushes, the primary origin of liquid repellency changes: now, the microstructures dominate the interaction, which is seen by comparable drag forces (Fd) between the Teflon and SOCAL surface treatment.

SOCAL_SH.mp4

Interactions on Structured Surfaces scale with surface fraction

The fraction of exposed area interacting with liquid (surface fraction, ϕ) regulates the lotus-effect surfaces.

A droplet suspended atop the microstructures drags over only the surface fraction of the total area. The impacts of local drag on each microstructure ("pinning") effectively controls the experienced drag forces.

Local pinning environment of the retreating droplet. The surface tension interface can be seen from the clinging of the droplet to the edges of the micro-posts. Liquid is seen left behind the droplet on top of the micro-posts.

Shown the graph above, Fd ↛ 0 as U → 0, but rather settles on a Fd,avg.
The model of drag being proportional to the root of the surface fraction proved to be apt, as can be seen in the plot of experimentally measured Fd compared to ϕ. Note: two fit lines are shown, for SOCAL and for fluorinated surfaces.

SLIPSexample.mp4

Slippery Omniphobic surfaces (SLIPS)

In SLIPS, the interaction science differs considerably. To prepare a sample, substrates were functionalized with Teflon-like fluorinated coating of sufficient thickness through controlled vapour deposition in order to provide a stable chemical environment minimizing any substrate effects, and then coated with a fluorinated oil.

Dip-coating gives insight into steady-state interaction mechanics

In lubricated surfaces like these, the droplet experiences a buoying force upward from the interfacial tension of the lubricant-water interface which reduces droplet's the interaction with the surface. The droplet is only buoyed when there is a stable film of lubricant beneath it. The stability of the film is related to the chemical environment of the substrate, the thickness of the lubricant layer, and the speed of the droplet.

Landau-Levich theory offers insight that, in the limit of low fluid velocities, a stable film ought to scale as U2/3. This can be expressed in terms of the Capillary Number which depends on viscosity and surface tension over the interface of the immiscible fluids (in this case oil film and water droplet). This is in fact what is seen. When a droplet rides on a stable film, this is called oleoplanning.

As theory would predict, at low velocities (in the range of 25 um/s to 0.6 mm/s) the film thickness beneath the droplet, as measured thin film interference techniques (reflectometry and ellipsometry) scales as as U2/3.

By introducing surface microstructures, it becomes clear which regime of liquid repellency a droplet experiences: lotus-effect or oleoplanning.

Shown above, two distinct regions become apparent: one where structures dominate the interaction with the droplet and another where the lubricant film does. As speed decreases (~Ca2/3) the smallest thickness above the substrate that the droplet's get go is that of the micro-posts. So the scaling asymptotes off to the point where the hfilm ≈ hpost.

If posts are significantly larger than the lubricant thickness in this speed regime it would be expected that the lubricant never scales according to Landau-Levich theory, instead as the post height, which is observed.

Viscous resistance governs drag force in SLIPS

Drag is the product of viscous dissipation in the libricant layer. Work is done in the transient region (preceding and following the stable film region) to overcome viscous stress of the lubricant layer.

Experimental measurement of drag force profile (see Force vs. Time graph) demonstrates similar peak and average forces, but distinctly shows that Fd → 0 as U → 0, unlike in the superhydrophobic case where the droplet remains "pinned" in place.

When experiment is compared to the model, a good relationship is observed (shown by the linear trend where hfilm > hpost). Below the point where hfilm ≈ hpost , dispersion of data points is seen. This is the likely result of energy interactions being dominated by posts and not lubricant layer. Additionally, from the measurement limitations of the experimental apparatus (rotating stage), small changes in radius of rotation will offset speed, and therefore Capillary number, spreading the data.

Experimental Apparatus

A rotating stage provides tangential velocity is v = R*w, depending on radius from the center of rotation. Small errors (Δ) in the lever arm position can influence the droplet's tangential velocity, spreading data.

Summary

Publications

PhysRevLett.120.244503.pdf

oleoplaning_2017_05_05.pdf

Note: published information and released material was created or collected by the author, and is used with permission.

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