Nanoparticles near walls organize based on slip speed.

When soft gel particles pack tightly together into a jammed suspension, the whole mixture can slide along smooth surfaces. Scientists suspected that a tiny, sub-micrometric layer right next to the boundary allows this sliding to happen. However, directly seeing what happens in that tiny space has been difficult. To solve this, researchers used a special imaging technique called total internal reflection fluorescence microscopy to track nanoparticles near a glass boundary. They found that particle speeds near the wall are nonlinear, showing that particles move unevenly. These speeds level off into a steady slip velocity very close to the surface, matching larger-scale measurements. Slip speed also changes how particles organize by altitude, showing that near-wall heterogeneity drives the sliding.

Why it matters

This helps engineers design better pumps and coatings for thick fluids like paints or biological slurries. Knowing why these fluids slip near walls can prevent clogs and improve manufacturing.

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