Since the advent of quantum mechanics, the field of physics has been divided into two separate fields: classical physics and quantum physics. While classical physics deals with the motion of everyday objects in the macroscopic world, quantum physics explains the strange behavior of tiny elementary particles in the microscopic world.

Many solids and liquids are composed of particles that interact with each other at close distances, resulting in the formation of “quasiparticles”. Quasiparticles are stable excitations that behave like weakly interacting particles. The concept of quasiparticles was introduced by Soviet physicist Lev Landau in 1941 and has since become an important tool in the study of quantum matter. Some well-known examples of quasiparticles include Bogolyubov quasiparticles in superconductors, excitons and phonons in semiconductors.

Studying new bulk phenomena from the perspective of quasiparticles has provided insight into a wide variety of physical parameters, particularly superconductivity and superfluidity, and more recently the famous example of Dirac quasiparticles in graphene. Until now, however, the observation and use of quasiparticles has been limited to quantum physics: in the classical condensed medium, the collision rate is often too high to allow such excitation of long-lived particles.

Figure 1. Left: experimental measurement of colloidal particles moving in a thin microfluidic channel. The particles form stable hydrodynamically bonded pairs that move at the same speed (arrows). These pairs are the basic quasiparticles of the system. Right: Simulation of a hydrodynamic crystal showing a pair of quasiparticles (yellow and orange particles at far left) emitted in a hydrodynamic crystal, leaving behind a supersonic Mach cone of excited quasiparticles. The colors indicate the magnitude of the couple’s arousal, the white background their speed.

However, the standard view that quasiparticles are only quantum matter has recently been questioned by a group of researchers at the Soft and Living Matter Center (CSLM) at the Institute of Basic Sciences (IBS) in South Korea. They investigated a classical system of microparticles moving in a viscous flow in a thin microfluidic channel. When particles are drawn into the flow, they disrupt the streamlines around them, thereby exerting hydrodynamic forces on each other.

Remarkably, the researchers found that these far-reaching forces cause particles to organize in pairs (Figure 1, left). This is because the hydrodynamic interaction violates Newton’s third law, which states that the forces between the two particles must be equal in magnitude and in opposite directions. Instead, the forces are “anti-Newtonian” because they are equal and in the same direction, thus balancing the pair.

A large population of paired particles implied the fact that these are the long-lived fundamental excitations in the system – its quasiparticles. This hypothesis was confirmed when the researchers modeled a large two-dimensional crystal made up of thousands of particles and studied its motion (Figure 1 on the right). Hydrodynamic forces between particles cause the crystal to vibrate similar to thermal phonons in an oscillating solid.

These paired moieties diffuse throughout the crystal and stimulate the formation of other pairs through a chain reaction. Quasiparticles move faster than the speed of phonons, and each pair therefore leaves behind an avalanche of newly formed pairs, similar to the Mach cone that forms behind a supersonic jet (Figure 1 on the right). Finally, all these pairs collide with each other, which eventually leads to the melting of the crystal.