Contents

Hydrophobic vs hydrophilic (and super) surfaces

Pronunciation: /ˌhaɪdrəˈfoʊbɪk/ · /ˌhaɪdrəˈfɪlɪk/

A hydrophilic surface attracts water, so a drop spreads and its contact angle stays below 90°. A hydrophobic surface repels water, so the drop beads up above 90°; superhydrophobic surfaces go beyond 150° and let drops roll off easily.

Also called

Water repellent Water wettable Superhydrophobic Superhydrophilic

Related terms

Lotus effect Cassie-Baxter model Roll off / sliding angle

Topics

Wetting Contact angle Coatings
Book a 15 minute demo Read the contact angle measurement guide

At a glance

What the labels mean, where the thresholds come from, and how to tell which one you have.

What it is

The labels describe how water behaves on a solid, judged by the water contact angle θ measured through the drop.

  • Hydrophilic: θ below 90°, water spreads
  • Hydrophobic: θ above 90°, water beads
  • Superhydrophobic: θ above 150° with low hysteresis, drops roll off
  • Superhydrophilic: water spreads to an angle near zero, usually helped by roughness

The 90° and 150° lines are conventions, not physical boundaries.

Typical values

Water contact angles at room temperature, as a rough guide:

  • Very clean glass: below about 20°, often close to 0°
  • PTFE: about 108 to 115°
  • Lotus leaf and engineered superhydrophobic coatings: above 150°

Contamination and ageing shift these numbers, so measure your own sample. Our open dataset of Droplet Lab reference measurements lists measured values with conditions.

How it is measured

Place a small water drop on the surface, image it from the side and read the contact angle. For super surfaces, also tilt the sample and record when the drop moves. The sliding angle measurement guide covers that second step, and our free contact angle tool reads an angle from a drop photo.

Expert view

“Repellency and mobility are not the same thing: a high contact angle says a drop is repelled, but whether it rolls off is a separate question; the two can even be mutually exclusive.”

Dr. Alidad Amirfazli, Co-founder and Scientific Advisor, Droplet Lab

What the terms mean

Four wetting regimes, from complete spreading to drops that bounce and roll.

Superhydrophilic (θ near 0°)

Water spreads into a thin film within moments. Usually needs a high energy surface plus roughness or porosity that wicks the water. Used for antifogging and easy rinse surfaces.

Hydrophilic (θ below 90°)

Water wets the surface and the drop flattens. Typical of clean glass, metals with oxide layers and plasma treated polymers.

Hydrophobic (θ above 90°)

Water beads up but may still stick. Typical of PTFE, waxes, silicones and many untreated polyolefins.

Superhydrophobic (θ above 150°, low hysteresis)

The drop sits on a mix of solid tips and trapped air, so it barely touches the solid and rolls off at a small tilt. Needs a low energy chemistry and fine roughness together.

Classify a surface with the water contact angle measured through the drop. For super surfaces, add hysteresis or the roll off angle.

θ Measured (apparent) water contact angle, degrees
θY Young contact angle on the smooth version of the same material, degrees
θA, θR Advancing and receding contact angles, degrees
Δθ Contact angle hysteresis, θA − θR, degrees
α Roll off (sliding) angle, degrees of tilt

Key equations

Why roughness pushes a surface toward one extreme or the other.

Equation: cos θ* = r cos θY

Variable legend:

  • θ*: apparent contact angle on the rough surface (°)
  • r: roughness ratio, true area divided by projected area (r ≥ 1)
  • θY: Young contact angle on the smooth material (°)

Interpretation: Roughness amplifies what the chemistry already does. A hydrophilic material gets more hydrophilic, and a hydrophobic one more hydrophobic.

When to use: Use it when water penetrates the texture fully, with no air trapped beneath the drop.

Assumptions:

  • Roughness is small compared with the drop
  • Chemistry is uniform

Common mistakes:

  • Applying it when air is trapped in the texture
  • Expecting it to predict easy roll off; Wenzel drops tend to stick

Equation: cos θ* = f (cos θY + 1) − 1

Variable legend:

  • θ*: apparent contact angle (°)
  • f: fraction of the drop base that touches solid (0 to 1)
  • θY: Young contact angle on the smooth material (°)

Worked example: θY = 115° and f = 0.1. cos 115° = −0.423, so cos θ* = 0.1 × 0.577 − 1 = −0.942, and θ* ≈ 160°.

Interpretation: The less solid the drop touches, the closer the angle gets to 180°. This is how a surface with a 115° chemistry reaches the superhydrophobic range.

When to use: Use it for textured low energy surfaces where drops sit on top of the roughness.

Assumptions:

  • Air pockets are stable and do not fill with water
  • Texture is fine compared with the drop

Common mistakes:

  • Assuming the air layer survives pressure, impact or long contact
  • Reading a high static angle as proof of easy roll off

How to read it

What a measured water angle usually tells you about a surface.

A single static angle sorts surfaces into hydrophilic or hydrophobic. Claims of super behavior need a second number: hysteresis or roll off angle. Students can see this directly in experiment 8 on sliding and roll off angle on hydrophobic surfaces, and a study of superhydrophobic aspen leaves regrown after moth defoliation shows the same logic on natural surfaces.

From measured water angle to meaning

If you see It usually means Check next
θ below about 10° and the drop keeps spreading Superhydrophilic or very clean high energy surface Whether the angle rises over hours as the surface picks up contamination
θ between 10° and 90° Hydrophilic; water wets the surface Uniformity across the sample and drift while the drop sits
θ between 90° and 150° Hydrophobic; water beads but may pin Advancing and receding angles to see how strongly drops stick
θ above 150° and drops roll at a small tilt Superhydrophobic in the Cassie state Durability after abrasion, washing or drop impact
θ above 150° but the drop stays put when tilted Sticky high angle surface, not superhydrophobic Hysteresis; liquid may have entered the texture

Common misconceptions

Where hydrophobic and hydrophilic labels are most often misread.

90° is a sharp physical boundary

The 90° line is a convention tied to the sign of cos θ in the Young equation. Vogler (1998) argued from water structure and surface forces that a boundary near 65° is more meaningful for biological response, and Law (2014) reviews how these working definitions are used. State which convention you use.

A high contact angle means drops roll off

A drop can sit at 150° and still stay pinned when the sample is tilted. Superhydrophobicity needs a high angle and low hysteresis together. Measure the roll off angle as well as the static angle before calling a coating self cleaning.

Hydrophobic means the surface is dirty or oily

Many clean materials are hydrophobic by chemistry, including PTFE and silicones. Contamination can make a hydrophilic surface look hydrophobic, though, so a sudden rise in angle on glass or metal is worth checking. A case study of contact angle teaching across two university campuses shows how students meet this in practice.

Superhydrophobic surfaces repel water in every situation

The trapped air layer can collapse under pressure, impact or long immersion, and the surface then behaves like a sticky Wenzel surface. Hot plates show the opposite extreme, where a vapor layer lifts drops off any surface; our article on the Leidenfrost effect explains it.

Related terms

Where hydrophobic and hydrophilic sit among other wetting concepts.

Broader term

  • Wettability
  • Contact angle

Related terms

  • Lotus effect
  • Cassie-Baxter model
  • Wenzel model
  • Roll off / sliding angle
  • Contact angle hysteresis

Often confused with

  • Oleophobic
  • Water repellency (textile spray rating)

FAQ

Short answers to the questions people ask most about hydrophobic and hydrophilic surfaces.

How can I test if a coating is hydrophobic or hydrophilic?

Measure the static water contact angle. Place a small drop of pure water on the coating, photograph it from the side and read the angle through the drop. Below 90° the coating is hydrophilic, above 90° hydrophobic. Test several spots, and for superhydrophobic claims add a tilt test. The contact angle measurement guide explains each step.

What makes a surface superhydrophobic?

Two things together: a low energy chemistry that is already hydrophobic, and fine roughness that traps air under the drop. The drop then touches mostly air, its angle rises above 150°, and it rolls off at a small tilt. Lotus leaves combine wax with micro and nano bumps to do exactly this.

Is superhydrophilic the opposite of superhydrophobic?

In wetting terms, yes. Water spreads to an angle near zero instead of beading. There is no agreed threshold yet; many papers use an angle below about 10° with fast spreading. Both extremes rely on roughness. Labs choosing equipment for teaching these ideas can compare options in the best contact angle goniometer for teaching and academic labs.

References

Primary sources and further reading.

1. Law, K. Y. (2014). Definitions for hydrophilicity, hydrophobicity, and superhydrophobicity: getting the basics right. The Journal of Physical Chemistry Letters, 5(4), 686 to 688. https://doi.org/10.1021/jz402762h
2. Vogler, E. A. (1998). Structure and reactivity of water at biomaterial surfaces. Advances in Colloid and Interface Science, 74(1 to 3), 69 to 117. https://doi.org/10.1016/S0001-8686(97)00040-7
3. Drelich, J., Chibowski, E., Meng, D. D., and Terpilowski, K. (2011). Hydrophilic and superhydrophilic surfaces and materials. Soft Matter, 7(21), 9804 to 9828. https://doi.org/10.1039/C1SM05849E
4. Gao, L., and McCarthy, T. J. (2006). Contact angle hysteresis explained. Langmuir, 22(14), 6234 to 6237. https://doi.org/10.1021/la060254j
5. Contact angle measurement guide
6. Lotus effect
7. Cassie-Baxter model
8. Roll off / sliding angle