Clustering of self-propelled particles
From Wikipedia, the free encyclopedia
Many experimental realizations of self-propelled particles exhibit a strong tendency to aggregate and form clusters,[1][2][3][4][5] whose dynamics are much richer than those of passive colloids. These aggregates of particles form for a variety of reasons, from chemical gradients to magnetic and ultrasonic fields.[6][7] Self-propelled enzyme motors and synthetic nanomotors also exhibit clustering effects in the form of chemotaxis.[7][8] Chemotaxis is a form of collective motion of biological or non-biological particles toward a fuel source or away from a threat, as observed experimentally in enzyme diffusion[9][10][11] and also synthetic chemotaxis[12][13][14] or phototaxis.[14] In addition to irreversible schooling, self-propelled particles also display reversible collective motion, such as predator–prey behavior and oscillatory clustering and dispersion.[15][16][17][18][19][20]
This clustering behavior has been observed for self-propelled Janus particles, either platinum-coated gold particles[1] or carbon-coated silica beads,[2] and for magnetically or ultrasonically powered particles.[5][6] Clustering has also been observed for colloidal particles composed of either an embedded hematite cube[3] or slowly-diffusing metal ions.[4][15][16][17][18] Autonomous aggregation has also been observed in anatase TiO2 (titanium dioxide) particles.[21] Clustering also occurs in enzyme molecule diffusion.[9][10][11][22] Recently, enzymes such as hexokinase and alkaline phosphatase were found to aggregate in the presence of their substrates.[23] In all these experiments, the motion of particles takes place on a two-dimensional surface and clustering is seen for area fractions as low as 10%. For such low area fractions, the clusters have a finite mean size[1] while at larger area fractions (30% or higher), a complete phase separation has been reported.[2] The dynamics of the finite-size clusters are very rich, exhibiting either crystalline order or amorphous packing. The finite size of the clusters comes from a balance between attachment of new particles to pre-existing clusters and breakdown of large clusters into smaller ones, which has led to the term "living clusters".[3][4][15][16][17][18]
