physics

Wetting transition

Wetting transition is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Wetting transition rather than just read about it. In short: A wetting transition (Cassie–Wenzel transition) may occur during the process of wetting of a solid (or liquid) surface with a liquid. The transition corresponds to a certain change in contact angle, the macroscopic parameter characterizing wetting.

Key takeaways

Wetting transition belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Wetting transition to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Wetting transition from memory before moving on to harder problems.

Reference excerpt

A wetting transition (Cassie–Wenzel transition) may occur during the process of wetting of a solid (or liquid) surface with a liquid. The transition corresponds to a certain change in contact angle, the macroscopic parameter characterizing wetting. Various contact angles can co-exist on the same solid substrate. Wetting transitions may occur in a different way depending on whether the surface is flat or rough.

Flat surfaces When a liquid drop is put onto a flat surface, two situations may result. If the contact angle is zero, the situation is referred to as complete wetting. If the contact angle is between 0 and 180°, the situation is called partial wetting. A wetting transition is a surface phase transition from partial wetting to complete wetting.

Rough surfaces The situation on rough surfaces is much more complicated. The main characteristic of the wetting properties of rough surfaces is the so-called apparent contact angle (APCA). It is well known that the APCA usually measured are different from those predicted by the Young equation. Two main hypotheses were proposed in order to explain this discrepancy, namely the Wenzel and Cassie wetting models. According to the traditional Cassie model, air can remain trapped below the drop, forming "air pockets". Thus, the hydrophobicity of the surface is strengthened because the drop sits partially on air. On the other hand, according to the Wenzel model the roughness increases the area of a solid surface, which also geometrically modifies the wetting properties of this surface. Transition from Cassie to Wenzel regime is also called wetting transition. Under certain external stimuli, such as pressure or vibration, the Cassie air trapping wetting state could be converted into the Wenzel state. Apart from external stimuli, intrinsic contact angle of the liquid (below or above 90 degree), liquid volatility, structure of cavities (reentrant or non-reentrant, connected or unconnected) are known to be important factors determining the rate of wetting transition. Complex structures such as hierarchical structures, and concave structures are reported to prevent wetting transition. It is well accepted that the Cassie air trapping wetting regime corresponds to a higher energetic state, and the Cassie–Wenzel transition is irreversible. However, the mechanism of the transition remains unclear. It was suggested that the Cassie–Wenzel transition occurs via a nucleation mechanism starting from the drop center. On the other hand, recent experiments showed that the Cassie–Wenzel transition is more likely to be due to the displacement of a triple line under an external stimulus. The existence of so-called impregnating Cassie wetting state also has to be considered. Understanding wetting transitions is of a primary importance for design of superhydrophobic surfaces.

Worked examples

Example 1 — a first encounter with Wetting transition

Start with the simplest possible case. Write down what Wetting transition claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Wetting transition before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Wetting transition ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Wetting transition

In research: Wetting transition appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Wetting transition in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Wetting transition is common in secondary-school and first-year university syllabi. It links to neighbouring topics Fluid mechanics, Surface science, so understanding it makes those chapters shorter.
In everyday life: Look for Wetting transition outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Wetting transition in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Wetting transition means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Wetting transition out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Wetting transition in simple terms?

Why does Wetting transition matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Wetting transition?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Wetting transition.