Contents

Sessile drop method

Pronunciation: /ˈsɛsaɪl drɒp ˈmɛθəd/

The sessile drop method measures the contact angle of a liquid drop resting on a flat solid, from an image of the drop's side profile. It is the most widely used way to characterize how well a liquid wets a surface.

Also called

Sessile drop technique Sessile drop test Static contact angle measurement

Related terms

Contact angle

Topics

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

At a glance

What the sessile drop method measures, and why it is the default wetting test.

What it is

A small drop sits on a level sample. A camera records its silhouette against a bright background. Software finds the drop edge and the baseline where liquid meets solid, then reads the angle between them.

  • Gives the contact angle in degrees, read through the liquid
  • Works on most flat, rigid solids
  • Can record static, advancing and receding angles

Why it matters

The sessile drop angle is the everyday number behind coating adhesion, cleaning checks, membrane wetting and surface energy work. It needs little sample and no immersion, so it suits both research and quality control. Teaching labs use it as a first wetting experiment, and the surface science teaching curriculum hub builds whole lab tracks around it.

Worked example

A small water drop is 1.0 mm high with a base diameter of 4.0 mm. If the drop is a spherical cap, tan(θ/2) = h/a.

  • Base radius a = 2.0 mm, so h/a = 1.0 / 2.0 = 0.5
  • θ/2 = arctan 0.5 = 26.6°
  • θ = 53.1°, a moderately hydrophilic surface

You can try a profile fit on your own drop image with our free contact angle tool.

How it is measured

Place the drop, capture the side profile, fit the edge and read the angle on both sides of the drop. Sample preparation, drop placement and reporting are covered step by step in the contact angle measurement guide.

Expert view

“Wettability is set by the top five to ten ångström of a surface: what the liquid actually sees. It doesn't matter what lies beneath; that outermost layer is what you are really measuring.”

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

What the terms mean

The three angles a sessile drop can give you, and the symbols used to describe it.

Static angle

The drop is placed and left at rest. The angle you read is one value inside the hysteresis range, and it depends on how the drop was deposited.

Advancing angle (drop grows)

Liquid is added through the needle until the contact line moves outward. The angle just as the line advances is the advancing angle, the upper limit of the range.

Receding angle (drop shrinks)

Liquid is withdrawn until the contact line moves inward. The angle just as the line recedes is the receding angle, the lower limit of the range.

The contact angle is drawn at the contact point, between the baseline and the tangent to the drop profile, on the liquid side.

θ Contact angle, measured through the liquid, degrees
h Drop height above the baseline, mm
a Base (contact) radius of the drop, mm
Bo Bond number, gravity relative to surface tension, dimensionless
γ Surface tension of the liquid, mN/m

Key equations

The geometry behind a sessile drop reading and the check for when it holds.

Equation: tan(θ/2) = h / a

Variable legend:

  • θ: contact angle (°)
  • h: drop height (mm)
  • a: base radius (mm)

Interpretation: For a drop small enough to be a sphere section, height and base width alone fix the contact angle.

When to use: Use it as a quick check on small drops, or to sanity test an automated reading.

Assumptions:

  • The drop is a spherical cap, so gravity is negligible
  • The drop is symmetric around its vertical axis
  • The baseline is placed correctly

Common mistakes:

  • Applying it to large, flattened drops
  • Measuring the base from a blurred or reflected edge

Equation: Bo = Δρ g a² / γ

Variable legend:

  • Δρ: density difference between liquid and surrounding gas (kg/m³)
  • g: 9.81 m/s²
  • a: base radius (m)
  • γ: surface tension (N/m)

Interpretation: Bo much below 1 means surface tension dominates and the drop stays nearly spherical. Near or above 1, gravity flattens it.

When to use: Use it to decide whether a circle or polynomial fit is enough, or whether a full Young-Laplace fit is needed.

Assumptions:

  • One characteristic length describes the drop

Common mistakes:

  • Mixing mm and m in the length term
  • Ignoring that water has a capillary length of about 2.7 mm at room temperature

Equation: Δp = γ (1/R₁ + 1/R₂)

Variable legend:

  • Δp: pressure jump across the drop surface (Pa)
  • R₁, R₂: principal radii of curvature (m)
  • γ: surface tension (N/m)

Interpretation: Inside a sessile drop the pressure grows with depth, so curvature changes along the profile. Fitting this equation reproduces the real drop shape.

When to use: Use it for axisymmetric drop shape analysis of larger drops, where a spherical cap no longer fits.

Assumptions:

  • Axisymmetric drop
  • Uniform surface tension and density

Common mistakes:

  • Applying it to a drop pinned unevenly on a patterned surface

How to read it

What a sessile drop result usually tells you, and what to check before you trust it.

A sessile drop angle is only as good as the surface and the image behind it. Comparing your values with a published set, such as the Droplet Lab reference measurement dataset, helps you spot a reading that is out of family.

From sessile drop result to meaning

If you see It usually means Check next
Left and right angles differ by several degrees The sample is tilted or not uniform under the drop Stage level, baseline position, local contamination
The angle falls steadily over a minute Evaporation, absorption or spreading into the surface Humidity, porous substrate, time of reading
Different values on repeat drops of the same sample Real variation across the surface Cleaning, handling marks, coating coverage
Static angle near 150° or above with a rough surface An apparent angle on a textured, water repellent surface Advancing and receding angles, or the sliding angle

How it is measured

The method in four short steps. The contact angle measurement guide has the full procedure.

1

Level the sample

Place a clean, flat sample on a level stage so the drop sits symmetrically.

2

Deposit the drop

Lower the drop gently onto the surface from a needle so it does not splash or spread.

3

Capture the profile

Record a sharp side image of the drop against an even, bright background.

4

Fit and read the angle

Detect the edge and baseline, fit the profile and read the angle on both sides.

Common misconceptions

Where sessile drop results are most often misread.

The static sessile drop angle is the Young angle

A static drop can rest at any angle between its advancing and receding angle. The value depends on how it was placed and how long you waited. On real surfaces, report which angle you measured, and add advancing and receding values where hysteresis matters.

A bigger drop always gives a better reading

Large drops are easier to see but flatten under gravity once they approach the capillary length. Then a simple circle fit underestimates or overestimates the angle. Keep the drop size consistent, and use a Young-Laplace fit when drops are large.

One drop describes the whole sample

A drop only samples the few square millimetres under it. Coatings, plasma treated parts and cleaned glass often vary across the surface. Several drops at different positions show whether the surface is uniform.

Any camera setup gives the same answer

Lighting, focus, magnification and the viewing angle all shift where the edge and baseline are found. A camera looking slightly down on the drop hides the true contact point. Teaching labs weighing setups can compare options in our guide to the best contact angle goniometer for teaching and academic labs.

Related terms

Where the sessile drop method sits among other wetting methods.

Broader term

  • Contact angle
  • Drop shape analysis (DSA)

Related terms

  • Goniometer
  • Advancing and receding contact angle
  • Contact angle hysteresis
  • Drop volume
  • Polynomial fitting method

Often confused with

  • Captive bubble method
  • Pendant drop method
  • Tilting plate

FAQ

Short answers to the questions people ask most about the sessile drop method.

What is the difference between the sessile drop and pendant drop methods?

A sessile drop rests on a solid and gives the contact angle between the liquid and that solid. A pendant drop hangs from a needle and gives the surface tension of the liquid from its shape. Both rely on imaging the drop profile, and both can use a Young-Laplace fit.

What instrument do I need for the sessile drop method?

You need a level stage, a way to dispense a small drop, even backlighting and a camera with edge fitting software. A classic optical goniometer does this, and so does the Dropometer smartphone drop shape analysis kit, with automatic edge and baseline detection. Its method is validated in peer review against a laboratory instrument.

Is the sessile drop method accurate on superhydrophobic surfaces?

It is harder. Near 150° and above, the contact point is hard to locate and small baseline errors shift the angle a lot. The drop may also roll away before it settles. Report advancing and receding angles or the sliding angle as well, since a static angle alone says little about these surfaces.

References

Primary sources and further reading.

1. Kwok, D. Y., and Neumann, A. W. (1999). Contact angle measurement and contact angle interpretation. Advances in Colloid and Interface Science, 81(3), 167 to 249. https://doi.org/10.1016/S0001-8686(98)00087-6
2. Huhtamäki, T., Tian, X., Korhonen, J. T., and Ras, R. H. A. (2018). Surface-wetting characterization using contact-angle measurements. Nature Protocols, 13(7), 1521 to 1538. https://doi.org/10.1038/s41596-018-0003-z
3. Rotenberg, Y., Boruvka, L., and Neumann, A. W. (1983). Determination of surface tension and contact angle from the shapes of axisymmetric fluid interfaces. Journal of Colloid and Interface Science, 93(1), 169 to 183. https://doi.org/10.1016/0021-9797(83)90396-X
4. Chen, H., Muros-Cobos, J. L., and Amirfazli, A. (2018). Contact angle measurement with a smartphone. Review of Scientific Instruments, 89(3), 035117. https://doi.org/10.1063/1.5022370
5. Contact angle measurement guide
6. Contact angle
7. Goniometer