Microscopic interactions in soft colloidal systems can be engineered and quantitatively measured. In my thesis, two model systems were studied: DNA-coated emulsion droplets and pNIPAM microgels. Programmable adhesion between DNA-coated droplets was studied as a function of salt concentration. By modifying electrostatic screening, salt controls DNA binder packing within adhesion patches and influences droplet deformation. In addition to previously reported ring and disk morphologies, a new intermediate adhesion geometry, termed the wide-annulus, was identified. An extended free-energy model incorporating this morphology successfully captures the observed transitions between adhesion states.
To enable quantitative measurements of colloidal interactions, a systematic error-analysis framework for line optical tweezers was developed. The framework accounts for axial particle motion, Brownian dynamics during image acquisition, and static imaging noise. Correcting for these effects enables reliable extraction of pair interaction potentials and improves measurement accuracy.
This methodology was applied to pNIPAM microgels. Direct measurements of microgel pair interactions showed a dependence of interaction softness on internal particle architecture. A Brush-Hertzian model was introduced to describe the distinct mechanical contributions of the polymer corona and elastic core, providing a link between microgel internal structure and interaction potentials.
Together, these results demonstrate how colloidal interactions can be both engineered and directly measured, providing insight into the behaviour of soft and responsive materials.