Non-Spherical Particles in Inkjet Printing

Non-Spherical Particles in Inkjet Printing

Applications of coffee ring research.

Dive In: Why Do Some Printed Pictures Look Better Than Others?

Have you ever printed a picture from a computer, only to find that some parts look blurry, uneven, or like the colors are blotchy? Sometimes, when an ink droplet dries, the tiny color particles inside it don't spread out nicely. Instead, they clump up around the edges, creating an unwanted "coffee ring" effect, just like that dried coffee stain on the table! Scientists who work on inkjet printers need to solve this problem to make sure your photos and documents look crisp and clear. One brilliant solution they've found comes directly from our understanding of how liquids dry: by changing the shape of the tiny color particles in the ink! This shows how a seemingly simple kitchen science observation can lead to big improvements in the technology we use every day.

The Science Scoop: Smarter Ink Through Particle Design

This topic directly applies the principles of the coffee ring effect and the impact of particle shape on evaporation-driven deposition to the real-world application of inkjet printing.

1. The Inkjet Printing Challenge: Inkjet printers work by squirting tiny droplets of liquid ink onto paper. This ink contains color-producing particles (pigments) suspended in a liquid. For a crisp, uniform image, these pigment particles need to dry evenly across the entire area where the ink droplet landed.

2. The Unwanted Coffee Ring: However, without careful design, the coffee ring effect often happens. As the liquid in the ink droplet evaporates, it evaporates fastest at the edges. This creates an outward flow of liquid, dragging the tiny pigment particles along with it. These particles then get stuck and pile up at the perimeter of the drying droplet, leaving a darker, denser ring and a lighter, less colorful center. This leads to blurry images, jagged lines, and uneven colors in your print.

3. Round Particles are the Problem: Most traditional pigments are made of roughly spherical (round) particles. As we learned, round particles easily slide past each other and pile up at the edges when pushed by the evaporating liquid's flow.

4. The Solution: Non-Spherical Particles! This is where coffee ring research becomes incredibly valuable. Scientists discovered that if the pigment particles are shaped differently – specifically, if they are non-spherical, like elongated ellipsoids (tiny oval or rod shapes) or even irregular flakes – they behave differently during evaporation.

   • "Jamming" Effect: These non-spherical particles don't slide past each other as easily. As the liquid tries to push them towards the edges, they tend to get tangled or "jam" together before they reach the very perimeter. They can block each other and resist the outward flow.

   • Flow Reversal/Even Distribution: This jamming effect either slows down the outward flow significantly or, in some cases, can even create tiny inward flows of liquid that help distribute the particles more evenly across the entire drying area.

   • Uniform Deposit: The result is that the pigment particles are deposited much more uniformly throughout the dried ink spot, rather than being concentrated at the edges. This eliminates or significantly reduces the coffee ring effect, leading to much sharper lines, more vibrant and even colors, and overall higher-quality printed images.

This application of fundamental fluid dynamics and particle physics shows how basic scientific curiosity about a coffee stain can lead to major advancements in everyday technology. It also highlights the importance of materials science and particle engineering.

For Educators: Teaching Tips**

• Review Connections: Start by reminding students about the coffee ring effect (Topic 1) and how particle shape can influence it (Topic 9).

• Real-World Problem: Frame this as a challenge engineers faced and how science helped solve it.

• Visual Aid: Show examples of printed images with and without coffee rings (you can even intentionally create some with highly diluted ink).

• Analogy: Reinforce the "round marbles vs. elongated objects" analogy for packing and flow.

• Vocabulary: Introduce "pigment," "inkjet printer," "uniform," and "non-spherical particles."

• Impact: Discuss how this affects the quality of photos, documents, and even printed electronics.

Experiment Time: Make Your Own "Ink" and See the Effect!

While we can't easily create perfectly spherical or ellipsoidal pigment particles in a classroom, we can simulate the principle using readily available materials to observe how particle shape affects deposition.

Experiment 1: Homemade "Ink" with Different "Particles"

• Materials: Two small clear plastic cups, water, a small amount of fine cornstarch (represents more spherical particles), a small amount of fine glitter or finely crushed dried herbs like oregano (represents non-spherical/irregular particles), stirring sticks, white paper or coffee filters, eyedropper or small spoon.

• Procedure:

  1. In one cup, mix a teaspoon of cornstarch with about 1/4 cup of water. Stir well to make a cloudy liquid.

  2. In the second cup, mix a teaspoon of glitter (or crushed herbs) with about 1/4 cup of water. Stir well.

  3. Using an eyedropper or spoon, place a small drop of the cornstarch mixture onto a piece of white paper.

  4. Place a separate small drop of the glitter mixture onto another spot on the same paper.

  5. Let both drops dry completely (this may take a while).

  6. Once dry, observe the patterns left by each drop.

• Discussion: Which drop left a more noticeable "ring" at the edge? Which one seemed to leave its particles more spread out in the middle? How does the "shape" of your cornstarch particles compare to the glitter/herb particles, and how did that affect the dried pattern? (Cornstarch, being more uniform and granular, should show a stronger ring than the irregular glitter).

Experiment 2: "Packing" Different Shapes

• Materials: A clear, shallow tray or plate, a handful of small, round beads (like tiny plastic beads or sugar beads), a handful of elongated small objects (e.g., uncooked rice grains, short pasta like orzo or tubettini).

• Procedure:

  1. Place a small pile of the round beads in the center of the tray. Gently try to push them outwards towards the edge using a ruler or a finger. Observe how easily they pack together and pile up.

  2. Repeat with the elongated objects. Try to push them towards the edge.

• Discussion: Which type of "particle" was easier to push and pile neatly at the "edge"? Which one seemed to get stuck or create more chaotic piles, resisting uniform movement? How does this simulate what happens inside a drying ink drop?

Safety Note for Teachers: Remind students not to ingest any experimental materials. Keep the area clean to avoid slip hazards.

Learn More: Explore Further!

• For Young Learners:

  • Videos: Search YouTube for "how inkjet printers work for kids" or "science of paint and ink."

  • Books: Look for children's books on inventions, everyday technology, or how things are made.

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

  • HP or Canon (printer companies) educational materials: Sometimes they have resources explaining ink technology.

  • MIT News / Harvard Gazette (university news sites): Often feature articles about new materials science research, including advancements in inks and coatings.

  • Research Articles: Search for "coffee ring suppression inkjet printing," "non-spherical particles ink," or "colloidal suspensions printing." (e.g., in journals like ACS Nano or Advanced Functional Materials).

  • TED-Ed: Sometimes has lessons on materials science or physics of everyday objects.

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. (While not specifically about inkjet, this foundational paper demonstrated the impact of particle shape on drying 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. (This paper directly addresses the use of non-spherical particles to suppress the coffee ring effect).

• General materials science, colloid chemistry, and printing technology resources.