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Direct observation of impact propagation and absorption in dense colloidal monolayers

  1. Lucio Isaa,3
  1. aLaboratory for Interfaces, Soft Matter and Assembly, Department of Materials, ETH Zurich, 8093 Zurich, Switzerland;
  2. bDepartment of Mechanical and Process Engineering, ETH Zurich, 8092 Zurich, Switzerland;
  3. cEngineering and Applied Science, California Institute of Technology, Pasadena, CA 91125;
  4. dLaboratoire Quartz, Unité de Recherche EA-7393, Institut Supérieur de Mécanique de Paris - Supméca, 93400 Saint-Ouen, France
  1. Edited by David A. Weitz, Harvard University, Cambridge, MA, and approved October 3, 2017 (received for review July 10, 2017)


Single-particle characterization of the impact response has unveiled design principles to focus and control stress propagation in macroscopic granular crystalline arrays. We demonstrate that similar principles apply to aqueous monolayers of microparticles excited by localized mechanical pulses. By inducing extreme local deformation rates and tracking the motion of each particle with velocities that reach up to few meters per second, we reveal that a regime of elastic collisions, typically forbidden due to overdamping, becomes accessible. This provides insights on the stress propagation and energy absorption of dense suspensions upon fast deformation rates.


Dense colloidal suspensions can propagate and absorb large mechanical stresses, including impacts and shocks. The wave transport stems from the delicate interplay between the spatial arrangement of the structural units and solvent-mediated effects. For dynamic microscopic systems, elastic deformations of the colloids are usually disregarded due to the damping imposed by the surrounding fluid. Here, we study the propagation of localized mechanical pulses in aqueous monolayers of micron-sized particles of controlled microstructure. We generate extreme localized deformation rates by exciting a target particle via pulsed-laser ablation. In crystalline monolayers, stress propagation fronts take place, where fast-moving particles (V approximately a few meters per second) are aligned along the symmetry axes of the lattice. Conversely, more viscous solvents and disordered structures lead to faster and isotropic energy absorption. Our results demonstrate the accessibility of a regime where elastic collisions also become relevant for suspensions of microscopic particles, behaving as “billiard balls” in a liquid, in analogy with regular packings of macroscopic spheres. We furthermore quantify the scattering of an impact as a function of the local structural disorder.


  • ?1Present address: Physical and Theoretical Chemistry Laboratory, Department of Chemistry, University of Oxford, Oxford, OX1 3QZ, UK.

  • ?2I.B., J.C., W.-H.L., and S.J. contributed equally to this work.

  • ?3To whom correspondence may be addressed. Email: daraio{at}caltech.edu or lucio.isa{at}mat.ethz.ch.
  • Author contributions: I.B., C.D., and L.I. designed research; I.B., J.C., W.-H.L., and S.J. performed research; I.B., J.C., W.-H.L., S.J., and L.I. analyzed data; and I.B., J.C., S.J., C.D., and L.I. wrote the paper.

  • The authors declare no conflict of interest.

  • This article is a PNAS Direct Submission.

  • This article contains supporting information online at www.danielhellerman.com/lookup/suppl/doi:10.1073/pnas.1712266114/-/DCSupplemental.

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