We highlight some DIW challenges related to fluid mechanics and soft matter, such as extrudability and clogging, filament stability, post-deposition spreading, slumping, and buckling. Hopefully, it will inspire various fundamental studies in fluid mechanics. (including by us)

Take‑home. Very small holes clog; below ~3 grain diameters there’s no flow, and around 3–6 it’s intermittent. Large holes act like dense, fluid‑like jets with a steady rate. This is also why hourglasses keep time so well. @univofmaryland.bsky.social @ucsbengineering.bsky.social [4/4]

Why openings matter. Near the edge, grains dilate in a thin boundary layer. This boundary layer only extends 10–15 grain diameters, but it changes the discharge rate. Once you account for it, all the data collapse onto a single curve! [3/4]

What we find. We split the flow rate in two: the speed of the grains and how they pack at the opening. Gravity and the hole size set the speed. Away from the rim, packing settles to a near‑universal “free‑fall” value. We show this with 3D experiments and DEM simulations. [2/4]

5/ Read more :

🔗 Popular science article FR : www.espci.psl.eu/fr/actualite...
🔗 Full publication in PNAS: www.pnas.org/doi/10.1073/...

And a video entry to the Gallery of Fluid Motion at the APS DFD 2023: www.youtube.com/watch?v=V0RJ...

And bonus: the winning entry from @ramrajesh97 to the Gallery of Soft Matter @apsdsoft.bsky.social last year that shows some cool pictures with fiber suspensions

Capillary flows of particulate suspensions are already complex, but anisotropic fibers add even more layers of complexity. We are excited to further explore fiber dynamics in various situations. Feedback and suggestions are very welcome!

#3: Fiber coverage becomes uneven when increasing the volume fraction of fibers. Increased fiber overlap at higher concentrations reduces coating uniformity, despite fibers maintaining an isotropic orientation.

#2: Higher Weber numbers and increased fiber fractions enhance splashing events. We map this splash/no-splash transition.

#1: Increasing fiber concentration significantly reduces droplet spreading due to higher effective viscosity. Our findings extend existing spreading models of suspensions of spherical particles to account for fiber suspensions explicitly.

We studied droplets of non-Brownian fiber suspensions at varying volume fractions and fiber aspect ratios. Using high-speed imaging and absorption spectroscopy, we quantified droplet spreading, film thickness, and coating uniformity.