Cellulose Color Films and Drying Humidity

ISEF Category: Materials Science

Subcategory: Electronic, Optical, and Magnetic Materials  ·  Difficulty: Intermediate  ·  Setup: School Lab  ·  Time: 1 to 2 Months

The Hook

A thin film can change color without dye. The color comes from structure, not pigment, like the way a peacock feather or soap bubble shifts in light. You can make that happen with cellulose nanocrystals from cotton. Then you can test how drying humidity changes the reflected color.

What Is It?

Structural color happens when a material’s tiny physical pattern controls light. The pattern acts like a filter. Some wavelengths bounce back, while others cancel out or pass through. That means the color you see depends on spacing, order, and thickness, not on a chemical pigment.

Cellulose nanocrystals are tiny rod-like pieces you can get from plant-based cellulose. When they self-assemble while a film dries, they can form a layered structure called a cholesteric or helical arrangement. That structure reflects one main color, often with a metallic look. Think of it like a stack of very thin mirrors, where the spacing decides which color gets reflected most strongly.

Drying humidity matters because water leaves the film at a different rate when the air is wetter or drier. That changes how the crystals line up before the structure locks in place. If you can control that drying step, you can test how the final peak reflectance wavelength shifts. That gives you a clear link between a processing condition and an optical result.

Why This Is a Good Topic

This is a strong science fair topic because you can change one clear variable, drying humidity, and measure one clear outcome, peak reflectance wavelength. The project connects to display films, security labels, sensors, and bio-based materials. You can learn how to make a calibration plan, collect repeatable color data, and compare groups with real statistics. The result can be tested with school-level tools if you plan carefully.

Research Questions

• How does drying humidity affect the peak reflectance wavelength of cellulose-nanocrystal films?
• What is the effect of drying humidity on the color uniformity across a cellulose-nanocrystal film?
• Does the thickness of the film change how strongly drying humidity shifts the reflected color?
• To what extent does drying humidity affect the repeatability of the same film recipe across trials?
• Which humidity range produces the narrowest reflectance peak in cellulose-nanocrystal films?
• How does the reflectance spectrum change when cellulose-nanocrystal films dry on different substrates?

Basic Materials

• Cotton balls or purified cellulose source.
• Mild acid hydrolysis or school-approved cellulose nanocrystal prep materials.
• Beakers or glass jars.
• Stirring rods.
• Digital kitchen scale with 0.1 g accuracy.
• pH paper or pH meter.
• Humidity sensor or hygrometer.
• Sealed containers or a small humidity chamber.
• Glass slides or clear plastic substrates.
• Dropper or pipette set.
• Ruler or calipers.
• Smartphone camera with fixed settings.
• White light source.
• White and black background cards.
• Computer with spreadsheet software.

Advanced Materials

• Transmission or reflection spectrometer.
• Polarizing microscope.
• Scanning electron microscopy access for structure imaging.
• Controlled-environment chamber with adjustable relative humidity.
• Spin coater or doctor blade for film casting.
• UV-Vis spectrophotometer.
• Profilometer for thickness measurements.
• Image analysis target and color standard card.
• Analytical balance.
• Desiccator.
• Cleanroom wipes and dust control supplies.

Software & Tools

• ImageJ: Measures color intensity, film uniformity, and spot-to-spot variation from images.
• Python: Fits calibration curves and compares humidity groups with statistics.
• Google Sheets: Organizes trials, plots trends, and checks repeatability.
• R: Runs deeper statistical tests and creates publication-style graphs.
• ColorPick Eyedropper: Records RGB values from photos when you need a simple color readout.

Experiment Steps

1. Define the exact optical outcome you will measure, such as peak wavelength, RGB shift, or reflectance intensity.
2. Choose the one drying condition you will change first, then keep every other part of the film recipe the same.
3. Plan a way to hold humidity steady during drying, so each sample sees a known environment.
4. Build a measurement plan that compares each film against the same light source, background, and camera or spectrometer setup.
5. Set controls that separate real structural color changes from thickness, cracking, or uneven drying.
6. Decide how you will summarize each group, then test whether the humidity trend is consistent across repeats.

Common Pitfalls

• Letting film thickness vary from sample to sample, which can shift color even when humidity stays the same.
• Measuring color under changing room light, which makes photo-based results drift between trials.
• Using cotton material with inconsistent purity, which changes how well the nanocrystals self-assemble.
• Ignoring edge effects, which can make the center and border of the same film look like different materials.
• Treating a cracked or cloudy film as a valid result, which hides whether the structure formed correctly.

What Makes This Competitive

A competitive project goes past simple color photos. You would map humidity to a real optical metric, then back that up with repeat trials and strong controls. You could compare several substrates, drying rates, or film thicknesses and test which factor matters most. If you add spectral data and careful statistics, your project starts to look like materials research instead of a classroom demo.

Project Variations

• Test how film thickness changes the humidity-color relationship for the same cellulose recipe.
• Compare cotton-derived cellulose nanocrystals with another plant source, such as wood pulp, to see whether the color shift behaves the same.
• Measure whether substrate type, like glass versus plastic, changes the final peak reflectance wavelength at the same drying humidity.

Learn More

• PubMed: Search for review articles on cellulose nanocrystals, structural color, and cholesteric films.
• Google Scholar: Look for recent peer-reviewed papers on humidity effects in self-assembled cellulose films.
• MIT OpenCourseWare: Search materials science and optics lectures for background on light, interference, and ordered structures.
• USDA Forest Service Research and Development: Find papers and reports on cellulose, plant fibers, and bio-based materials.
• NASA NTRS: Search for studies on structural color, thin films, and optical materials used in sensing.