Triplet correlations in two-dimensional colloidal model liquids

Three-body distribution functions in classical fluids have been theoretically investigated many times, but have never been measured directly. We present experimental three-point correlation functions that are computed from particle configurations measured by means of video-microscopy in two types of quasi-two-dimensional colloidal model fluids: a system of charged colloidal particles and a system of paramagnetic colloids. In the first system the particles interact via a Yukawa potential, in the second via a potential Γ/r. Varying the particles density in the charged system, or, the interaction strength Γ in the magnetic system, one can systematically explore how triplet correlations behave if the coupling between the particles changes. We find for both systems very similar results: on increasing the coupling between the particles one observes the gradual formation of a crystal-like local order due to triplet correlations, even though the system is still deep inside the fluid phase. These are mainly packing effects as is evident from the close resemblance between the results for the two systems having completely different pair-interaction potentials. To demonstrate that triplet correlations are significant not only locally, but also when integrated over the whole volume we consider the Born-Green equation and show that in a strongly interacting system this equation can be satisfied only with the full triplet correlation function but not with three-body distribution functions obtained from superposing pair-correlations (Kirkwood superposition approximation). PACS numbers: 61.20.-p, 61.20.Ne, 82.70.Dd Submitted to: J. Phys.: Condens. Matter