Microfluidic Emulsification and Force Transmissions in Colloidal Systems

  • Jun Dong

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

By combining super-resolution imaging with colloidal droplets which are labelled with a solvatochromic dye, we quantitatively obtain the force distribution within the colloidal system and explore the relationship between local structure and mechanical properties in colloidal gels and glasses.

In this thesis, first we demonstrate methods of producing monodisperse oil-in-water colloidal droplets by using microfluidic devices. One approach is to reduce the dimension of channels to a value that is comparable to the desired droplet size and combining with Norland Optical Adhesive (NOA) as the device material. Alternatively, we use a PDMS device with a novel opposed-flow geometry, where we observe a second order jetting transition, jet radius following power-law scaling. By tuning applied pressures continuously, we can produce droplets down to sub-micron.

Colloidal gels are prepared by adding non-absorbing polymers. Structures of gels vary slightly upon deeper quench, with a decrease of both number of neighbours and contacts. The probability distributions of contact force decay following a Gaussian distribution at large forces. The average force of the colloidal gels is approximately 5nN. No percolated force chains are detected in the elastic gels, but only some short chains. We detect higher population of local structures in particles forming force chains than that of the bulk emulsions, implying local structure is crucial for force chain formation.

We investigate three dense emulsions with volume fraction up to 0.60, aiming to identify force-bearing neighbours and force networks in colloidal glasses. However due to low polydispersity, small regions of crystals are found in all dense emulsions. We notice at low volume fraction about 0.54, the shape of force distribution is also Gaussian-like, however as the volume fraction increases, it becomes close to exponential which is similar to experimental measurements of contact forces in jammed granular matter.
Date of Award24 Mar 2020
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorC P Royall (Supervisor)

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