Colloidal Gels and Depletion

Gianguido Cianci, Gary Hunter, Tsondue Samphel, Tiffany Soo, and Eric Weeks

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    With this project, we wish to investigate the behaviour of Colloidal Gels, an interesting example of a class of materials known simply as Gels - "a semi-solid colloidal system consisting of a solid dispersed in a liquid" [1].
    In this context, semi-solid refers to the gels ability to hold its shape or resist (slight) deformation, despite its low solid concentration - as little as 0.1% solid for coagulated blood. Jell-O® or yogurt are good everyday examples of a gel.
    Somehow then, in gels, a small amount of solid manages to support the weight of the whole system and thus maintain its shape. The solid-like character of these materials is invariably due to some sort of solid network that acts a little like a sponge, imprisoning the liquid within its structure. There exists a multitude of materials which can establish these networks with a wide range of structural complexity. In our case, the solid scaffolding's basic units are microscopic spherical particles called colloids; they form a gel because they are sticky (see below for details).
    We have succeeded in making one such colloidal gel and we are currently studying the dependence of some of its properties on synthesis conditions as well as temperature and most interestingly, on its age.

What are we looking for?

    We're just getting started, but we're curious about how the gel changes when we change the stickiness of the colloidal particles. We're also looking at the motion of the particles in the gel, again as we vary the stickiness. For a more detailed explanation of what we see, and why the particles are sticky, see below.

What are we looking at? - The System

    We use confocal microscopy to track fluorescently dyed PMMA (polymethylmethacrylate) particles in three dimensions. The particles are suspended in a density matched solvent so as to avoid sedimentation and floating.
    Also, we dissolved some polystyrene (PS) at various concentrations. Once in the solvent, the polystyrene collapses into a tight bundle with roughly spherical symmetry. We are thus effectively left with two types of particle in suspension: a large species and a small one. Preliminary measurements have set the size ratio of PMMA to PS of the order of 1:20 with the radius of the PMMA particles at 1.1E-6 m.

What do we see? - Preliminary Data

Colloidal Gels are special

    On the right, you can see our sample as we see it. In fact, this is only a snapshot of what we see - we typically take 3D data over time.
   Click on the right to see a movie of our data. (2D section at constant z)

    The reason (we think) our gels are special is the nature of the solid they contain. Unlike chemical gels such as Jell-O®, our basic network building block is not a microscopic molecule but a colloidal particle, big enough so that we can see it with optical microscopy. The relevant length-scales and time-scales in these systems are very different from those of chemical gels and so are the experimental techniques required to study them.

Enlarge -- Movie
Fig.1: 3D Snapshot of a Colloidal Gel.
Notice the isolated trimer close to the centre of the picture. It was diffusing around the viewing volume during the experiment.

3D network

Click here to use an applet to rotate the image of the colloidal gel.

Why are the particles sticky? - Depletion Forces

Enlarge
Fig.3:    The lack of Brownian kicks between the particles is the origin of the Depletion Force. Note that the small species as well as the solvent molecules are not pictured here.
   The force that is keeping these colloids together is often referred to as the Depletion force [2] and is present whenever two species of different sizes are suspended in a solvent in the right proportion. Again, we have PMMA colloids and Polystyrene globules as our two species. This force is an attractive one, and therefore we often think of it as a "stickiness".

    Both species receive extremely frequent "Brownian kicks" from the diffusing solvent molecules and thus they too diffuse around the volume. But the smaller species feels these kicks much more than the PMMA and therefore moves around faster. As a result polystyrene particles also give Brownian kicks to the PMMA.
    However, if two colloids get so close to each other as to prevent the polystyrene from diffusing between them (ie. if the polystyrene is depleted in the region between the two colloids with respect to the rest of the solvent) then, on average, there is an unbalance in the kicks the colloids receive. In fact there are no small species between the colloids to kick them apart while there are still some around them to kick them together. The idea is illustrated above. In Fig.3a we see an isolated PMMA colloid receiving random kicks from solvent and polystyrene. Fig.3b shows the lack of kicks between two particles which therefore have a tendency to stick together.

Acknowledgments

    We thank Jaci Conrad and Tony Dinsmore for lots of helpful advice on making these gels, and Aonghus Lawlor for telling us about Jmol.

References

[1] - Oxford English Dictionary, Second Edition (1991) (Electronic version)
[2] - S. Asakura and F. Oosawa, J. Polym. Sci. 33 , 183 (1958) (The seminal "depletion paper")
For more information, please contact:
G. Cianci <gcianci(at)emory.edu> or E.Weeks <weeks(at)physics.emory.edu>