Bachelor Assignment: Measuring Colloidal Energy Landscapes
Introduction
Interactions between colloidal particles (e.g. silica, polystyrene or PMMA latex spheres) are key determinants of their self-assembly into composite particles, aggregates or crystals, and macroscopic behaviors like phase transitions or flow mechanics. However, both theoretical expressions for the forces and experimental methods for measuring them have shortcomings. One approach for measuring the interaction free energy ∆G(r) between two colloids makes use of the Boltzmann probability:
P(r)=Poexp(-∆G(r)/kT).
Using optical microscopy and image processing software, it can essentially be ‘counted’ how frequently each distance r is found. However demonstrations of this principle are relatively scarce.
Meanwhile, knowing the colloidal interactions between two dissimilar spheres is important for the design of e.g. photocatalysts or diagnostic tracers. For example, the coating of a large (+) sphere with small (-) spheres, can be achieved via electrostatic attraction (see Figure). However as the adsorption progresses, the central particle will become less attractive while the adsorbing spheres will increasingly repel each other. The final state will then depend on the strength of the electrostatic interactions.
Left: fluorescence microscopy snapshot of 1 μm size spheres adsorbed onto a 5 μm sphere. The non-adsorbed 1 μm spheres are diffusing freely. Right: Time-projection image from a 5 minute video of the same experiment.The dark outer ring could be indicative of the electrostatic repulsion between the diffusing 1 μm spheres.
Research Objectives
The main goal of the BSc assignment is to demonstrate the capabilities of measuring the ∆G(r) between two dissimilar charged colloidal spheres. Insights into the dependence of ∆G(r) on the aqueous ion composition are available from colloid science, while state-of-the-art microscopy for live particle imaging and some analysis software are present in the PCF group. A possible challenge is the occurrence of interactions much stronger than the kT-scale; this could be mitigated by changing pH, salinity or particles (size, surface chemistry).
Learning Objectives
In addition to the standard learning objectives for a Bachelor’s project (research planning, academic writing, data presenting, etc.), you will:
· Increase your practical and theoretical skills on working with colloids (if needed, a personal ‘mini course’ will be included)
· Learn how to work with a Confocal Scanning Laser Microscope
· Use and (co-) develop image analysis software
Contact Information
· Daily Supervision: Dr. Michel Duits
· Supervision: Prof. Dr. Frieder Mugele
References
1. Royall, CP, Louis, A A., & Tanaka, H. Measuring colloidal interactions with confocal microscopy. The Journal of chemical physics, 127(4) (2007).
2. Adamczyk, Z, & Warszyński, P. Role of electrostatic interactions in particle adsorption. Advances in Colloid and Interface Science, 63 (1996), 41-149.