Controlling Evaporation with Particle Shape

Controlling Evaporation with Particle Shape

Ellipsoidal particles prevent coffee rings.

Dive In: Can We Stop Coffee Rings?

Remember those annoying dark rings that coffee or juice sometimes leave behind when they dry? We learned that these "coffee rings" happen because tiny particles in the liquid get dragged to the edge as water evaporates. But what if we didn't want a coffee ring? What if we wanted the particles to spread out evenly when the liquid dries? It turns out, scientists have found a clever trick: change the shape of the tiny particles floating in the liquid! Instead of round particles, if you use long, oval-shaped ones (like tiny footballs or M&Ms), the coffee ring effect can be stopped. This amazing discovery shows how much control we can have over liquids just by thinking about the shape of the tiny things inside them, which is super important for making better paints, inks, and even medicines!

The Science Scoop: Shape Matters for Particle Flow

This topic builds directly on the coffee ring effect and introduces the idea of how particle shape influences capillary flow and particle packing during evaporation.

1. Coffee Ring Recap: As we discussed, the classic coffee ring happens when water evaporates faster at the edges of a liquid drop, pulling water (and the round particles suspended in it) outwards. These round particles then get stuck and pile up at the edge, forming the ring.

2. The Role of Round Particles: Imagine tiny marbles. When they are pushed towards the edge of a drying drop, they can easily slide past each other and fit together to form a dense pile at the edge. This easy movement and packing allow the strong outward flow of liquid to carry them all to the perimeter.

3. Introducing Ellipsoidal Particles: Now, imagine those tiny particles are shaped like tiny, stretched-out footballs or elongated eggs (ellipsoids). When these oddly shaped particles are suspended in the liquid, they behave differently as the liquid evaporates.

4. "Log Jam" at the Edges: When the outward flow of water tries to push these ellipsoidal particles towards the edge, they don't easily slide past each other and pack neatly. Instead, because of their irregular shape, they tend to get tangled or jammed together before they even reach the very edge. They might rotate and block each other, creating a kind of "log jam" or "traffic jam" further inside the droplet, away from the very perimeter.

5. Reversed Flow (Capillary Flow Reversal): This "log jam" of particles actually creates a new problem for the water trying to flow outwards. It can make it harder for water to move outwards, and sometimes, it can even cause the water to start flowing inwards from the edges in some spots! This is called capillary flow reversal.

6. Even Spreading: Because the outward flow is either stopped or even reversed in places, and the particles get stuck in the middle, the particles don't all accumulate at the edge. Instead, they get deposited more evenly across the entire surface of the drying droplet, preventing the formation of a dark ring.

This seemingly simple change in particle shape has huge implications for industries like printing (making sure ink dries evenly), pharmaceuticals (ensuring medicines spread uniformly), and coatings (creating smooth, uniform paint layers).

For Educators: Teaching Tips

• Review Coffee Ring First: Start by briefly reminding students about the coffee ring effect.

• Analogy: Use the "marbles vs. footballs" or "round vs. long candies" analogy to explain how different shapes pack and move.

• Think Small: Emphasize that these particles are microscopic, but their tiny behavior has a big impact.

• Problem-Solving: Frame this topic as scientists solving a real-world problem (how to avoid coffee rings).

• Vocabulary: Introduce "ellipsoidal" (oval-shaped), "particle packing," and "capillary flow reversal" simply.

• Safety: Ensure careful handling of liquids and quick cleanup.

Experiment Time: Shape Shifters!

While making truly ellipsoidal particles for a classroom might be tough, you can use analogies to demonstrate the principle of particle shape affecting distribution.

Experiment 1: The "Marbles vs. Rice" Drop

• Materials: Two clear glass slides or plastic sheets, water, tiny plastic beads (like sprinkles or very small sugar beads – representing round particles), uncooked rice grains (representing ellipsoidal particles), eyedropper.

• Procedure:

  1. Place a small pile of plastic beads on one slide and a small pile of rice grains on the other.

  2. Add a drop of water to each pile, enough to just wet them without washing them away.

  3. Gently try to use the eyedropper to "push" the particles towards the edge of the water drop. Observe how they move and pack.

• Discussion: Which particles were easier to push and pile up at the edge? Which ones seemed to get stuck or scattered more in the middle? How does this relate to the idea that round things pack more easily than irregular shapes?

Experiment 2: Simulated Particle Jam

• Materials: A shallow tray or baking dish, a small funnel, a handful of round beads (marbles or large dried peas), a handful of elongated objects (uncooked pasta like penne or fusilli, or small Lego bricks).

• Procedure:

  1. Place the funnel upright in the center of the tray (to represent the "edge" where particles gather).

  2. Slowly pour the round beads onto the tray around the funnel. Observe how they move towards and collect around the funnel.

  3. Repeat with the elongated objects.

• Discussion: Which type of "particle" formed a denser, more organized pile around the funnel? Which ones tended to "jam" or scatter before reaching the funnel's tightest point? How does this help us understand why ellipsoids stop coffee rings?

Experiment 3: "Paint" with Different "Particles" (Observational)

• Materials: Two small clear plastic cups, water, a small amount of cornstarch (for roundish particles), a small amount of glitter (fine, irregular flakes can represent a different particle shape), stirring sticks, white paper.

• Procedure:

  1. Mix a small amount of cornstarch into water in one cup. Stir well.

  2. Mix a small amount of glitter into water in the other cup. Stir well.

  3. Using a spoon, carefully place a small drop of each mixture onto separate pieces of white paper.

  4. Let them dry completely. Observe the dried patterns.

• Discussion: Does one mixture form more of a "ring" than the other? Does the glitter seem more evenly distributed? (Glitter, being irregularly shaped, might resist forming a strong ring compared to the more uniform cornstarch particles, though this is a simplified analogy.)

Safety Note for Teachers: Remind students not to ingest any materials. Clean up spills promptly.

Learn More: Explore Further!

• For Young Learners:

  • Videos: Search YouTube for "science of paint" or "ink drying science for kids."

  • Books: Look for children's books on materials science, nanotechnology (very simple introductions), or everyday inventions.

• For Teachers & Parents (More In-Depth): 

  • Physics Today / American Physical Society (APS) News: Often feature accessible articles on recent research in soft matter physics, including particle self-assembly.

  • Research Articles: The original research on this topic was groundbreaking. Search for "ellipsoidal particles coffee ring effect" or "particle shape suppresses coffee rings" (e.g., in journals like Nature or Physical Review Letters).

  • Materials Science educational resources: Many university departments offer outreach materials on how material properties affect function.

References

• Yunker, P. J., Still, T., Yunker, M., & Lubensky, T. C. (2011). Irreversibility and self-organization in drying colloidal suspensions. Nature Materials, 10(2), 163-169. (This is a key foundational paper demonstrating how particle shape can influence deposition patterns).

• Li, X., Li, C., Zhu, W., Zhang, Q., & Wang, Z. (2014). Coffee ring suppression by ellipsoidal particles. Journal of Fluid Mechanics, 747, 182-194. (Another important paper specifically detailing the ellipsoidal particle effect).

• General fluid dynamics and materials science textbooks covering colloids, suspensions, and evaporation.