Realisation of the Brazil-nut effect in charged colloids without external driving

Abstract

Sedimentation is a ubiquitous phenomenon across many fields of science, such as geology, astrophysics, and soft matter. Sometimes, sedimentation leads to unusual phenomena, such as the Brazil-nut effect, where heavier (granular) particles reside on top of lighter particles after shaking. We show experimentally that a Brazil-nut effect can be realised in a binary colloidal system of long-range repulsive charged particles driven purely by Brownian motion and electrostatics without the need for activity. Using theory, we argue that not only the mass-per-charge for the heavier particles needs to be smaller than the mass-per-charge for the lighter particles, but that at high overall density, the system can be trapped in a long-lived metastable state, which prevents the occurrence of the equilibrium Brazil-nut effect. Therefore, we envision that our work provides valuable insights into the physics of strongly interacting systems, such as partially glassy and crystalline structures. Finally, our theory, which quantitatively agrees with the experimental data, predicts that the shapes of sedimentation density profiles of multicomponent charged colloids are greatly altered when the particles are charge regulating with more than two ion species involved. Hence, we hypothesise that sedimentation experiments can aid in revealing the type of ion-adsorption processes that determine the particle charge and possibly the value of the corresponding equilibrium constants.

arXiv comment13 pages, 4 figures. In ancillary files: SI and 2 SI videos, published manuscript with improved explanation on quantification of parameters

References in corpus (18)

• Structure of Electrorheological Fluids
• Day-night cloud asymmetry prevents early oceans on Venus but not on Earth
• Non-equilibrium sedimentation of colloids on the particle scale
• A highly magnetised and rapidly rotating white dwarf as small as the Moon
• Re-entrant melting and freezing in a model system of charged colloids
• Charge Regulation with Fixed and Mobile Charges
• Extended sedimentation profiles in charged colloids: the gravitational length, entropy, and electrostatics
• Charge Regulating Macro-ions in Salt Solutions: Screening Properties and Electrostatic Interactions
• Phase behavior of a deionized binary mixture of charged spheres in the presence of gravity
• Colloidal brazil nut effect in sediments of binary charged suspensions
• The sediment of mixtures of charged colloids: segregation and inhomogeneous electric fields
• A Case Study of Sedimentation of Charged Colloids: The Primitive Model and the Effective One-Component Approach
• Complex Fluids with Mobile Charge-Regulated Macro-Ions
• Sedimentation of binary mixtures of like- and oppositely charged colloids: the primitive model or effective pair potentials?
• Soft repulsive mixtures under gravity: brazil-nut effect, depletion bubbles, boundary layering, nonequilibrium shaking
• Colloid-oil-water-interface interactions in the presence of multiple salts: charge regulation and dynamics
• Colloidal Brazil nut effect in microswimmer mixtures induced by motility contrast
• Alternating strings and clusters in suspensions of charged colloids