Physical ageing of the contact line on colloidal particles at liquid interfaces

From Soft-Matter
Revision as of 15:30, 10 February 2012 by Annawang (Talk | contribs)

Jump to: navigation, search

Spring 2012 entry - Anna Wang


David M. Kaz, Ryan McGorty, Madhav Mani, Michael P. Brenner, and Vinothan N. Manoharan (2012) Physical ageing of the contact line on colloidal particles at liquid interfaces Nature Materials, Nature Materials advance online publication December 2011,


It is commonly accepted that it is energetically favourable for particles to bind to fluid fluid interfaces. The particle sits at an equilibrium position in between the two bulk phases, as predicted by Young's law. But how does a particle reach this equilibrium position from the bulk phase? In this paper, the authors examine the dynamics of how a particle reaches this position and find that the process is surprisingly slow and has parallels in ageing of glassy systems.

Experimental setup

The experiments are performed in a custom-made trough, which allows for a pinned aqueous subphase and an oil superphase. Polystyrene microspheres (1.9um diameter) start in the aqueous phase and are pushed gently upwards towards the interface using radiation pressure from out-of-focus optical tweezers.

Digital Holographic Microscopy

Figure 1. Digital holographic microscopy is used to image the microspheres

Digital Holographic Microscopy is used to capture the three-dimensional trajectory of the microspheres as they approach the interface at up to 5000 frames/sec. A high spatial precision of 2nm is achieved by refractive index matching the aqueous phase (60% glycerol, 40% water) to the decane. This minimises reflections within the sample, which would make hologram analysis much more complicated.


Approach ==

The approach of the particles towards the interface can be accounted for entirely by considering radiation pressure from the optical tweezers, and the opposing drag force as the particle moves. The particle's axial motion then plateaus momentarily and at low salt concentrations, the particles do not rise any further (Fig 1b.).


For high salt concentrations (~100mM NaCl) the particle then rises sharply, which is interpreted as the particle breaching the interface. From here, the interfacial tension from the newly-formed three-phase contact line then drives the particle's motion upwards.


The dynamics of the particle as it breaches the interface are shown in Figure 1d. The rise is logarithmic in time, and the particle does not reach the reported equilibrium contact angle even after the tens of seconds the experiment lasts for (and in fact, would take months if we extrapolate from the logarithmic time dependence).


The initial velocity is 1000 times smaller than hydrodynamics can account for, and a model taking into account enhanced viscous drag do not predict logarithmic behaviour. Instead the authors look to the analogy to ageing in non-equilibrium systems, and use a model based on the microscopic processes to predict the motion of the particles.

Arrhenius model

As the contact line moves across the particle, it is