Experimental Soft Condensed Matter Group
Harvard University, Prof. D. A. Weitz

The Physics of Colloids in Space (PCS) project

This NASA-sponsered research project involves the study of colloidal suspensions. These experiments are being done in the microgravity environment of the International space station. They focus on the physics of colloidal particles, including nucleation and growth of binary colloidal alloys crystals, the structure and properties of colloidal particles with attractive interactions (depletion interactions) induced by the addition of non-absorbing polymer, and the behavior of large scale fractal aggregates and gels. All samples will be studied by means of a multi-purpose light scattering apparatus.

The present project focuses on the growth mechanism, the dynamics and the basic physical properties of the materials. In the future, we will exploit this knowledge in attempts to fabricate novel materials with unique physical properties. For example, the binary colloid crystal alloy might be fabricated using particles not only of different sizes, but also of different materials. The long-term goal of this research is to establish a new field of "Colloidal Engineering", the making of new materials using colloidal particles as precursors. These experiments are performed in microgravity to eliminate completely the detrimental effects of sedimentation and convection. This will allow high-concentration, index-matched samples to be studied without any sedimentation. Furthermore, it will enable the growth of much larger scale fractal objects than has been feasible previously; and it will ultimately enable the growth of binary colloidal crystals comprised of particles of different materials, and hence, different densities.

The samples listed below will be installed in the light scattering apparatus which will be launched on the Endeavor shuttle on flight STS-100 – 9^th International Space Station Flight (6A). The apparatus containing the samples will be installed in the U.S. Laboratory (Destiny) section of the ISS. Currently, the launch is scheduled for April 19,2001 and the recent successful installation of the U.S. Laboratory may indicate that there will be no last-minute delays of this flight. Approximately, 2 ½ weeks later, the experiment will be turned on for the first time. Over the next seven months, the apparatus will be turned on approximately for 2200 hours.

Sample Details (Final)

Samples for PCS

    Fractal Gels
             1 Polystyrene
                1 Silica

    Binary Alloy Crystals
                1 AB6
                    1 AB13

    Colloidal Glass
                1 that may form crystals on earth

     Colloid + Polymer Samples
                1 near critical point in two-phase region
                1 gel-like sample
                   1 crystal sample

Binary Colloidal Alloys

It has long been known that monodisperse colloidal spheres suspended at high concentrations in a liquid can, due to natural Brownian motion, crystallize, i.e. form arrays with long-ranged order. More recently, the crystallization behavior of binary mixtures of particles of two different sizes has been investigated. Under certain conditions, 'hard sphere' particles (colloidal polymethylmethacrylate, PMMA) at size ratio 0.58 form both the AB2 and the AB13 superlattice structures. AB2 consists of a simple hexagonal arrangement of large A particles, with the smaller B particles filling all the interstices between the A layers. AB13 is a remarkably complex structure. Icosahedral clusters of 13 small B particles are body centered in a simple cubic lattice of A particles; the icosahedra are rotated by 90 degrees between adjacent cubic subcells so that the unit cell of the structure consists of eight subcells consisting of 112 particles. These two structures were identified both by static light scattering ('powder light crystallography) and by electron microscopy of the dried suspensions.

For the space experiments, binary lattices designed to produce crystal structures of the AB6 and the AB13 form will be observed under micro-gravity conditions. Once in orbit, the samples will be homogenized by rapidly spinning each sample cell and suddenly stopping it. Once the sample is homogenized, the nucleation and growth of the binary colloidal alloy crystallites will be monitored using static light scattering. Scattering from the crystal nuclei will appear at small angles relative to the incident beam. By measuring the dependence upon scattering vector q, the size of the nuclei, and their growth over time, can be determined, during the earliest stages of the growth process. As more crystallites nucleate, correlation between the particle positions can develop, complicating the interpretation of the data but also providing more information about the evolution of the crystals. Finally, as the crystals become larger, the scattering moves to very small angles and Bragg scattering will appear. The width of these Bragg reflections will provide additional information about the size of the crystallites. As the crystals grow the Bragg peaks will become narrower.

Depletion Interactions

The phase behavior of colloidal particles with an attractive interaction induced by depletion has been studied for several different systems, including charge-stabilized polystyrene spheres, emulsion droplets, and PMMA particles. The samples being sent to the International Space Station are all made with PMMA colloids and polystyrene polymer. The addition of polymer molecules to a colloidal suspension induces an effective attractive interaction between the particles, called the depletion attraction. This interaction is highly controllable, as the radius of gyration of the polymer sets its range and the polymer concentration controls the strength. Thus, the depletion attraction is an ideal way of exploring the phase behavior of attractive particles. As the strength of the attraction is increased, the particles can crystallize. Unlike the more familiar form of colloidal crystals, these crystals result from an attractive interaction at much lower volume fractions. Thus, their structure, morphology and growth kinetics can be significantly different, and these will all be measured in the microgravity environment.

As the strength of the attractive interaction is increased by increasing the polymer concentration, the fluid-solid coexistence extends over an increasing range of colloid concentrations. However, the approach to the ultimate equilibrium structure becomes obscured by the kinetics of the phase behavior. As the polymer concentration is increased, the strength of the attractive interaction becomes so large that the colloidal particles form a gel-like structure. This is characterized by a fractal structure at short length scales and a liquid-like ordering at larger length scales, resulting in a ring of intense light scattering at low angles. Ultimately, these gel-like structures should anneal into a crystalline order; however, under normal gravity the gel cannot support its own weight and ultimately collapses, leading to macroscopic phase separation, obscuring the true equilibrium behavior. When the potential is weaker, crystallization of the particles can be induced by the depletion. The structure and morphology of these crystals may differ significantly from those formed at high concentration by purely repulsive interactions; however, this behavior is obscured by sedimentation and has not yet been studied.

In the space experiments, the colloid-polymer mixtures will be homogenized using the same procedure used for the binary colloid mixtures. Immediately after they are homogenized, the small angle scattering will be monitored to measure the formation of the gel-like structure. The small angle scattering will exhibit a peak at a wave vector corresponding to the repeat size that characterizes the gel. This peak will evolve in time as the gel forms. Once the gel forms, the peak should remain stationary until the gel begins to collapse as it equilibrates. Finally, the position and intensity of the peak should provide information about the evolution of the structure and should enable us to follow the equilibration of the gel into the final structures, whatever they may be.

Fractal gels

Numerous studies of gels formed by aggregating colloidal particles by inducing an attractive interaction between the particles through the addition of salt to the solution have shown that such gels are fractal. Recently, our group observed that colloidal polystyrene gels continue to evolve long after the gel forms. For example, small angle scattering has shown that such gels, which gel in approximately one day, may continue to evolve for another 2 weeks. Dynamic light scattering performed at small scattering angles on these samples reveals that very slow processes are occurring and these processes become increasingly slow during this period of time. The precise cause of this behavior is unknown.

As a part of this project we will study two fractal gels. The colloidal polystyrene gel will have a lower volume fraction and should grow larger aggregates. It will be interesting to see if such a gel exhibits the same behavior as those previously studied. The silica colloidal gel will test the question of whether or not the aging behavior is unique to polystyrene gels.

These space experiments are somewhat different from those described above in that they can only be performed once. After the sample is gelled, the process cannot be reversed as shearing reverses the processes studied in the other samples. The fractal gel samples will be contained in two separate chambers, one for colloid and one for salt solution, until combined with a special mixing device at the beginning of the experiment. After combination, the samples will be studied using primarily small angle static and dynamic light scattering.

The Experimental Apparatus

The experiments will be conducted in a specially constructed apparatus that accommodates eight cells containing one of the samples listed above. The apparatus is equipped to perform seven different measurements:

· Static Light Scattering (SLS) – A narrow beam of 532 nm laser light traverses the sample and the scattered light is collected by special optics equipped with single mode fibers that guides the light to avalanche photodiodes (APDs). These, in turn, produce a TTL pulse for approximately 70% of the photons; so, the scattered intensity is measured by counting these pulses. An SLS measurement consists of measuring the time-averaged scattered intensity at several different angles. The instrument can access scattering angles through a range of 11° to 169°. In addition, the sample cell can be rotated to improve ensemble averaging of gel-like samples.

· Dynamic Light Scattering (DLS) – Using the SLS optics, DLS measurements calculate the time autocorrelation function of the scattered light.

· High Angle Scattering or Bragg Scattering (HAS) – A large diameter (8 mm) 532 nm laser beam is shown through the sample cell, and the scattered light is focused on a special screen. A CCD camera then images the screen. Using this camera, we can obtain images of Bragg rings, individual Bragg spots (if the crystals are large enough) and azimuthally averaged scattered intensities as a function of angle.

· Low Angle Static Scattering (LAS) – Using the same laser as for Bragg Scattering, light scattered at low angles passes clear of Bragg screen thanks to a hole centered around the optical axis and is collect by a second CCD camera. LAS measurements are time-averaged measurements of the angular dependence of the scattered light.

· Low Angle Dynamic Scattering (LAD) – The optics for small angle scattering are designed to image speckles onto the pixels of the CCD sensor. Software then autocorrelates the light from the individual speckles to perform dynamic light scattering at very low angles.

· Rheology – Using the DLS capability combined with oscillatory motion of the sample cell, some rheological measurements can be performed.

· Imaging – Two color cameras, one operating at 1X and one at 10X magnification, allow direct, color imaging of the samples. This is particularly useful and impressive with crystalline samples.