Observation of Brownian Motion in Liquids at Short Times: Instantaneous Velocity and Memory Loss

• Published in: Science (American Association for the Advancement of Science) Vol. 343; no. 6178; pp. 1493 - 1496
• Main Authors: Kheifets, Simon, Simha, Akarsh, Melin, Kevin, Li, Tongcang, Raizen, Mark G.
• Format: Journal Article
• Language: English
• Published: United States American Association for the Advancement of Science 28-03-2014; The American Association for the Advancement of Science
• Subjects: Barium; Brownian motion; Fluid mechanics; Hydrodynamics; Instantaneous velocity; Kinetics; Liquids; Mechanics (Physics); Motion; Observation; Particle mass; Quantum statistics; Quantum tunneling; Titanates; Velocity
• ISSN: 0036-8075; 1095-9203
• DOI: 10.1126/science.1248091

Measurement of the instantaneous velocity of Brownian motion of suspended particles in liquid probes the microscopic foundations of statistical mechanics in soft condensed matter. However, instantaneous velocity has eluded experimental observation for more than a century since Einstein's prediction of the small length and time scales involved. We report shot-noise–limited, high-bandwidth measurements of Brownian motion of micrometer-sized beads suspended in water and acetone by an optical tweezer. We observe the hydrodynamic instantaneous velocity of Brownian motion in a liquid, which follows a modified energy equipartition theorem that accounts for the kinetic energy of the fluid displaced by the moving bead. We also observe an anticorrelated thermal force, which is conventionally assumed to be uncorrelated.