  Particle-particle interactions are of paramount importance in every
multi-body system as they determine the collective behaviour and coupling
strength. Many well-known interactions like electro-static, van der Waals or
screened Coulomb, decay exponentially or with negative powers of the particle
spacing r. Similarly, hydrodynamic interactions between particles undergoing
Brownian motion decay as 1/r in bulk, and are assumed to decay in small
channels. Such interactions are ubiquitous in biological and technological
systems. Here we confine two particles undergoing Brownian motion in narrow,
microfluidic channels and study their coupling through hydrodynamic
interactions. Our experiments show that the hydrodynamic particle-particle
interactions are distance-independent in these channels. This finding is of
fundamental importance for the interpretation of experiments where dense
mixtures of particles or molecules diffuse through finite length, water-filled
channels or pore networks.
