Abstract
Interactions of diffusing particles are governed by hydrodynamics on different length and timescales. The local hydrodynamics can be influenced substantially by simple interfaces. Here, we investigate the interaction dynamics of two micron-sized spheres close to plane interfaces to mimic more complex biological systems or microfluidic environments. Using scanned line optical tweezers and fast 3D interferometric particle tracking, we are able to track the motion of each bead with precisions of a few nanometers and at a rate of 10 kilohertz. From the recorded trajectories, all spatial and temporal information is accessible. This way, we measure diffusion coefficients for two coupling particles at varying distances h to one or two glass interfaces. We analyze their coupling strength and length by cross-correlation analysis relative to h and find a significant decrease in the coupling length when a second particle diffuses nearby. By analysing the times the particles are in close contact, we find that the influence of nearby surfaces and interaction potentials reduce the diffusivity strongly, although we found that the diffusivity hardly affects the contact times and the binding probability between the particles. All experimental results are compared to a theoretical model, which is based on the number of possible diffusion paths following the Catalan numbers and a diffusion probability, which is biased by the spheres' surface potential. The theoretical and experimental results agree very well and therefore enable a better understanding of hydrodynamically coupled interaction processes.
Highlights
The evolution of life over billions of years has been accompanied by many safety mechanisms.[1]
The hydrodynamic coupling of two particles was investigated by means of two adjacent static optical traps.[5,6]
4.1 Contact times between particles nearby surfaces In the experiments, the mean distance hhi of the particles to the wall was varied by steering the height hp of the piezo table
Summary
The evolution of life over billions of years has been accompanied by many safety mechanisms.[1]. Living cells vary the size of their compartments and the distance to interfaces such as membranes and thereby change the local viscous drag.[2] In this way, the hydrodynamic coupling of two particles was investigated by means of two adjacent static optical traps.[5,6] Here, position correlation functions turned out to be a versatile analysis tool to gain insights into hydrodynamic coupling. Optical traps were used to induce colloidal interactions and to investigate the influence of nearby surfaces.[7,8,9,10] The tracking of the colloids translational motion allows to determine interaction parameters such as relative diffusion coefficients and interparticle contact times In this context, the influence of nearby cell membranes on the particle motion were measured by means of magnetic[11] and optical tweezers.[12]
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