100 Nano-Stories: Young’s Law (Part 1)!

Episode #89: Hydrophobicity + Contact Angle + Surface Tension!

Carlos Manuel Jarquín Sánchez
6 min readMay 7, 2021

Preface! ✨

It’s your favorite material science & nanotechnology enthusiast! Today, we will cover the mechanisms of hydrophobicity from Young’s Law with a bit of help from surface tension and mathematics!

There is no article to prepare you this time, reader! Let’s jump into today’s topic! 😄

Young’s Law Explained! 💡

Surface Tension: Definitions! 🔑

Young’s Law is the contact angle of a liquid or vapor touching the surface of solid material and determines how wet an object can get. The angle is normally measured from the angle between the liquid and the solid.

Surface Tension can be defined as the strongest intermolecular forces between molecules of a liquid (water) at the surface only. Because the intermolecular forces are the strongest, the water molecules will minimize their surface area.

Surface Tension occurs because of the cohesion of water. Cohesion is the attraction between water molecules to each other.

Surface Area means that it is a measure of the total area that the surface of a 3-dimensional object covers/occupies.

These 3 dimensions are:

  • Length (L)
  • Width (W)
  • Height (H)

But why are the intermolecular forces on the surface the strongest? How does it work? 🤔

Surface Tension: Mechanism! 🔑

In water, the intermolecular forces are hydrogen bonds. If you notice the circle on the right, the hydrogen bonds in the water molecule are “pulling” on the water molecule in all directions, which increases its surface area (because it’s being pulled in all directions, so it’s like getting stretched!).

But on the left circle, the hydrogen bonds are pulling from all sides except above. This lack of “pull” from any hydrogen bonds of the molecules on the surface is known as surface tension. Because there is no “pull” on the water molecules on the surface, the molecules at the surface of the water will get closer to each other, reducing surface area.

Solvents! 🔑

Wait, Carlos, what do solvents have to do with Young’s Law and Surface Tension? 🤔

The types of solvents used in aerogels can help determine what outcome will result in Young’s Law.

A solvent is a liquid/dissolving medium where a substance/solid (a.k.a. solute) will dissolve in the liquid to form a solution/homogenous mixture.

Example Of Solvent

However, the reason why solvents are important to explain Young’s Law is that they all have different properties and reactions to the surface tension or surface energy. These solvents are called pure solvents and aqueous solvents.

Pure Solvents are just the solvent itself (methanol, ethanol, etc.)

Aqueous Solvents are when the solvent is water (The solute can be whatever).

Applications (Critical Surface Tension)! 🔑

In aqueous solutions, you have to worry about the critical surface tension because if you surpass the critical surface tension, the water on top of the aerogel will be adsorbed completely.

(γc)Math symbol for Critical Surface Tension of the aerogel.

The surface tension for the aqueous solvent is labeled as (γ).

If the surface tension of the aqueous solvent (γ) is greater than the critical surface tension of the aerogel (γc), the aqueous solvent will be able to wet the gel a little bit. 👌🏽

If the surface tension of the critical surface tension of the aerogel (γc) is greater or equal to the surface tension of the aqueous solvent (γ), the aqueous solvent will be able to wet the gel completely, and the solvent will undergo adsorption. 💥

Adsorption means that particles/molecules of something will collectively add up on the surface of a material. In some way, you could say the particles are “resting” on the material.

Young’s Law! 🔑

As I mentioned earlier, Young’s Law is the contact angle of a liquid or vapor touching the surface of solid material and determines how wet an object can get. The angle is normally measured from the angle between the liquid and the solid.

θ → Mathematical symbol for contact angle.

If the contact angle is greater than 90 degrees (90°), the liquid/water in the aerogel will not be able to spread, and the aerogel is impervious/hydrophobic to water.

If the contact angle is less than 90 degrees (90°), the liquid/water in the aerogel will spread, and the aerogel is hydrophilic to water, and the water will adsorb until the contact angle is equal to 0 degrees (0°).

Hydrophobicity Achieved!

In an aerogel, the more of the (concentrated) solvent you add, it can actually decrease the contact angle and reduce hydrophobicity in the aerogel.

One of the most common solvents in aerogels is methanol (CH₃-OH) and ethanol (C₂H₅-OH).

The maximum concentration of methanol you can achieve without lowering the contact angle below 90° is 39% of pure concentrated methanol in the solvent. The maximum concentration of ethanol you can achieve without lowering the contact angle below 90° is 35% of pure concentrated ethanol in the solvent.

Closing Thoughts! 💭

Author’s Note:

This article is explaining the concepts that you need to have down to understand how the physics and mathematics behind Young’s Law! Don’t worry, Young’s Law isn’t complicated, but there are many properties that can affect the outcome of the adsorption/hydrophobicity on the surface of the aerogel! 🙏🏽

See you tomorrow to explain the Physics and Mathematics of Young’s Law! ✌🏽

Vocabulary! 📓

Young’s LawThe contact angle of a liquid or vapor touching the surface of solid material and determines how wet an object can get. The angle is normally measured from the angle between the liquid and the solid.

Young’s Law (Mechanism)If the contact angle is greater than 90 degrees (90°), the liquid/water in the aerogel will not be able to spread, and the aerogel is impervious/hydrophobic to water. If the contact angle is less than 90 degrees (90°), the liquid/water in the aerogel will spread, and the aerogel is hydrophilic to water, and the water will adsorb until the contact angle is equal to 0 degrees (0°).

Surface TensionThe strongest intermolecular forces between molecules of a liquid (water) at the surface only. Because the intermolecular forces are the strongest, the water molecules will minimize their surface area.

SolventA liquid/dissolving medium where a substance/solid (a.k.a. solute) will dissolve in the liquid to form a solution/homogenous mixture.

Homogenous → A substance/mixture that is uniform/is the same everywhere (appearance and composition). A homogeneous mixture can be coffee or water.

Covalent BondsTwo atoms share their electrons equally.

Pure SolventsIt’s just the solvent itself (methanol, ethanol, etc.)

Aqueous SolventsThe solvent is water (The solute can be whatever like salt, sugar, etc.).

Adsorption Particles/molecules of something will collectively add up on the surface of a material. In some way, you could say the particles are “resting” on the material.

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© 2021 by Carlos Manuel Jarquin Sanchez. All Rights Reserved.

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