Coffee-Ring vs. Marangoni Flow in Droplets

ISEF Category: Physics and Astronomy

Subcategory: Mechanics  ·  Difficulty: Intermediate  ·  Setup: Home Setup  ·  Time: 1 to 2 Months

The Hook

A drying drop can leave a bright ring, a smooth spot, or a strange swirl. That pattern comes from tiny flows inside the liquid, not just from evaporation. You can watch those flows change when you tweak surfactant concentration. That makes this project feel like a physics puzzle you can see with your own eyes.

What Is It?

When a droplet dries, particles inside it do not stay still. Water leaves faster from the edge than from the center, so liquid often rushes outward to replace it. That outward flow carries particles to the rim and leaves the classic coffee-ring stain.

But a second flow can fight back. Surfactants are molecules that lower surface tension, which is the skin-like pull at a liquid surface. If surface tension changes across the drop, fluid can move from low tension to high tension. That motion is called Marangoni flow. Think of it like two invisible conveyor belts inside one tiny drop. One belt pushes material to the edge, and the other can pull it inward or spread it more evenly.

Your job is to map where each pattern wins. You can change surfactant concentration, then compare the final stain shape and particle distribution. If you do this well, you can turn a simple droplet into a phase diagram, which is a map of what pattern appears under each condition.

Why This Is a Good Topic

This is a strong science fair topic because you can change one variable, measure a clear outcome, and make real images from simple materials. The project connects to printing, coatings, diagnostics, and any process that depends on how particles dry on a surface. You can learn experimental design, image analysis, and how to separate cause from noise without needing a professional lab.

Research Questions

• How does surfactant concentration change the fraction of particles that end up in the coffee ring?
• What is the effect of particle type, such as turmeric, mica powder, or milk-protein latex, on the final stain pattern?
• Does substrate material, such as glass, plastic, or coated paper, shift the boundary between ring formation and more uniform drying?
• To what extent does droplet volume change the deposition pattern at the same surfactant concentration?
• Which surfactant concentration produces the most uniform particle distribution across the dried spot?
• How does adding a small amount of salt change the competition between coffee-ring flow and Marangoni flow?

Basic Materials

• Household surfactant source, such as dish soap or diluted shampoo.
• Turmeric powder, mica powder, or milk mixed with water as particle proxies.
• Clear glass slides, plastic sheet, or microscope slides.
• Micropipette, transfer pipette, or medicine dropper for consistent droplet size.
• Phone with camera.
• Phone microscope clip lens or inexpensive macro lens attachment.
• White LED desk lamp.
• Black cardstock or matte background.
• Metric ruler or calibration slide.
• Permanent marker for labeling samples.
• Small disposable cups or sample wells.
• Digital kitchen scale with 0.1 g accuracy.

Advanced Materials

• Laboratory surfactant standards, such as SDS or Tween solutions.
• Colloidal particles with known size distribution, such as polystyrene beads or silica beads.
• Spin-coated or plasma-cleaned glass slides.
• Analytical balance.
• Contact angle goniometer.
• Optical microscope with camera attachment.
• Stage micrometer for image calibration.
• Controlled humidity chamber or desiccator.
• Image analysis software for particle mapping.
• Surface tensiometer.
• Pipettes with low-volume tips.

Software & Tools

• ImageJ: Measures ring width, stain area, and particle intensity from droplet images.
• Python: Automates image cleanup, thresholding, and phase-diagram plotting.
• Google Sheets: Organizes sample conditions, replicates, and summary statistics.
• QGIS: Helps if you want to build a visual heat map of pattern types across conditions.
• NIH Image to CSV: Exports pixel data from images for simpler analysis.

Experiment Steps

1. Define the pattern outcome you will score, such as ring, mixed ring, or uniform deposit.
2. Choose one variable to change first, then hold the others fixed so you can isolate cause and effect.
3. Plan a way to classify each dried droplet with the same scoring rules every time.
4. Build an image calibration method so your phone photos turn into comparable measurements.
5. Set up replicate trials and a data table before you collect a single droplet.
6. Design a phase diagram that plots concentration against final deposit pattern.

Common Pitfalls

• Using uneven lighting across photos, which makes stain brightness look different even when the pattern did not change.
• Changing droplet size from trial to trial, which blurs the link between surfactant concentration and final deposit shape.
• Mixing powders that clump, which creates fake rings and uneven particle settling.
• Scoring patterns by eye without fixed rules, which makes borderline samples hard to compare.
• Ignoring substrate cleaning, which lets dust or grease change wetting and distort the drying flow.

What Makes This Competitive

A stronger project goes beyond saying that surfactant changes the stain. You can build a clear phase diagram, quantify ring strength with image analysis, and compare more than one particle type or surface. You can also test whether the pattern boundary shifts under different humidity or substrate conditions. That kind of careful analysis gives your project depth, not just pretty pictures.

Project Variations

• Compare dish soap, shampoo, and hand soap as surfactant sources to see whether formulation changes the drying pattern.
• Swap turmeric for mica powder or milk-protein latex to see whether particle shape or surface chemistry changes ring formation.
• Test glass, plastic, and paper surfaces to see how wetting and adhesion shift the coffee-ring boundary.

Learn More

• PubMed: Search review articles on coffee-ring effect, Marangoni flow, and drying colloids to find background physics and methods.
• NASA NTRS: Search for droplet evaporation and particle transport studies related to coatings and space environments.
• NIH ImageJ documentation: Learn how to measure ring width, area, and pixel intensity from your droplet photos.
• MIT OpenCourseWare: Look for fluid mechanics and interfacial phenomena lecture notes that explain surface tension and flow.
• Journal of Colloid and Interface Science: Search for peer-reviewed papers on evaporating droplets and deposition patterns.