Polymers & Hydrogels: Stretch, Squish, and Shine!¶

Welcome to Polymers & Hydrogels, where you’ll explore materials that can behave like solids and liquids—sometimes at the same time. You’ll make water-filled hydrogels and play with oobleck to discover shear-thinning and shear-thickening behavior.

What You’ll Learn¶

What polymers are and how monomers link into long chains
How hydrogels form 3D networks that hold up to 99% water
What viscosity means and how it changes under stress
The difference between shear-thinning and shear-thickening fluids

Supplies Checklist¶

Sodium alginate solution (colored optional)
Calcium chloride solution (crosslinker)
Syringe or dropper
Beaker or cup for CaCl₂ bath
Cornstarch
Water
Bowl or tray for oobleck
Food coloring (optional)

Alginate and calcium — All set: sodium alginate (left) and calcium chloride bath (right).

Question

Why does calcium chloride cause alginate chains to crosslink into a gel?

What Is a Polymer?¶

A polymer is a material made of many repeating molecular “links” (monomers) bonded into long chains or networks.

Polymer chains — Monomers (one unit) link to form polymers (many units).

Polymers can be linear or crosslinked. Crosslinking creates a network that can trap water—forming a hydrogel.

Making Hydrogels¶

Hydrogels are 3D crosslinked polymer networks—water gels—that can hold up to 99% water!

Step 1: Prepare Your Bath¶

Pour 100 mL calcium chloride solution into a beaker.
Swirl gently to remove bubbles.

Step 2: Draw Up Alginate¶

Fill a syringe or dropper with sodium alginate solution (colored if you like).

Step 3: Create Hydrogel Art¶

Inject drops, lines, or shapes of alginate into the CaCl₂ bath.
Wait 30 s for hydrogels to form.
Scoop out your gels and arrange them on a tray.

Question

How would increasing Ca²⁺ concentration affect gel firmness?

Viscosity & Flow¶

Viscosity measures how easily a fluid flows. Water has low viscosity; honey is high viscosity.

Viscosity diagram — Viscosity vs. shear stress: Newtonian vs. non-Newtonian fluids.

Shear-Thinning vs. Shear-Thickening¶

Hydrogels: shear-thinning (become runnier under stress)
Oobleck: shear-thickening (become more solid under stress)

Oobleck: Cornstarch & Water¶

Oobleck is a simple non-Newtonian fluid made from cornstarch and water.

Make Oobleck¶

Mix 2 parts cornstarch with 1 part water in a bowl.
Stir until smooth; add water sparingly.

Oobleck mixing — Oobleck ready for squeezing and poking.

Test Its Behavior¶

Press slowly: it flows like a liquid.
Punch quickly: it behaves like a solid!

Oobleck demo — Squeeze vs. strike: oobleck’s surprising response.

Question

What molecular interactions cause oobleck to thicken under impact?

Think & Discuss¶

Hydrogel hardness: What changes if you add more crosslinker?
Oobleck uses: Where might we use shear-thickening fluids in real life?
Polymer design: How would you engineer a hydrogel for drug delivery or soft robotics?

MSE Research Spotlight¶

Hydrogel research — UM researchers developing amazing use cases for hydrogels.

Real MSE teams at Michigan are exploring: - Biocompatible hydrogels for tissue engineering
- Smart gels that respond to temperature or pH
- Self-healing materials that repair when damaged

Design Challenge¶

Can you create a gradient hydrogel (firmer one end, softer the other)?
- Vary alginate or CaCl₂ concentration along a tube
- Measure firmness with your fingers or a simple force gauge

Reflection

What applications could benefit from a gradient hydrogel?

Summary¶

You learned what polymers and crosslinking are.
You made and tested hydrogels (shear-thinning) and oobleck (shear-thickening).
You connected classroom demos to real MSE research.

Head back to the Outreach Homepage for more demos!