46233-G7
Dynamics of Melting within Bulk Colloidal Crystals
Dynamics of Melting within Bulk Colloidal
1. Objectives
The varieties of phase transitions such as melting, freezing, glass transitions and solid-solid phase transitions exhibited by single-component systems are found throughout nature. Unfortunately, studying such phenomena within the bulk of a three-dimensional (3D) atomic or molecular crystal is impossible due to difficulty in tracking atoms or molecules. While it is easier to track colloidal particles in 3D, it is difficult to continuously change the volume fraction, which controls phase transition, in a single sample. The goal of this project is to employ thermally responsive colloidal spheres and realtime video light microscopy to experimentally investigate the melting transition in colloidal crystals in situ. The diameter, and therefore the volume fraction, of the microgel spheres can be precisely adjusted by tuning the temperature.
We proposed to synthesize temperature sensitive colloids and determine whether vibrational instability (the Lindemann criterion) and/or shear instability (the Born criterion) provide a comprehensive microscopic definition of melting by measuring particle motions using video light microscopy.
2. Results
2.1. Particle synthesis.
To create the thermally sensitive particles required for our experiments, we synthesized core-shell particles where the core is a fluorescence dyed silica particle and the corona is made out of N-isopropylacrylamide polymer (PNIPAm). PNIPAm have a lower critical solution temperature (LCST); they are hydrophilic below the LCST and hydrophobic above it. As a result, the thickness of the PNIPAm shell and therefore the volume fraction decreases with an increase in temperature. A core-shell particle with a fluorescently labeled core and unlabelled corona made locating the particle center easier and increased the spatial resolution [56]. We synthesized the fluorescent silica core using a modified Stober method with slight modification to incorporate the fluorescence dyes. The PNIPAm shell on fluorescent silica core was synthesized using atom transfer radical polymerization.
2.2. Measurement of Vibrational instability (The Lindemann criterion) and shear instability (the Born criterion)
The temperature sensitive colloids were loaded into hermetically sealed glass chamber and imaged particle motion using multi-photon optical microscopy. The colloids formed crystals at room temperature and melted into a liquid above a volume fraction of ~54.5%. In the crystal, the mean squared displacements (MSD) of the particles reached plateau at long time indicating the particles are caged formed by neighboring particles. The size of the cage increased with a decrease in volume fraction, eventually disappearing in the liquid phase. The Lindemann parameter (ratio of MSD and crystal lattice constant) increased with a decrease in volume fraction and reached ~12% near the melting volume fraction. The longitudinal and transverse modes, calculated from the particle displacement field, also decreased with a decrease in the volume fraction. The spring constants in the colloidal crystal decreased with a decrease in the volume fraction, eventually becoming very similar to each other near the melting point. Furthermore, the bulk modulus of the crystal became almost equal to the shear modulus near the melting point.
3. Conclusions
We used temperature sensitive colloidal particles to elucidate melting mechanism in colloidal crystals. The elastic constants became similar near the melting point. Our results show that both Lindemann and Born criterions are adequate in identifying the melting point in colloidal crystals.
4. Educational Outreach
The emerging concepts in self-assembly, diffusion, and the relationship between structure and dynamics from this project are incorporated in an existing course on soft materials. Furthermore, the highly visual and interactive nature of soft materials have been successful at attract and inspire undergraduates to high-level science by making complex ideas more tangible. The PI has supervised 3 undergraduate students through NSF Research Experience for Undergraduates (REU) program. Two of the participating students are now in graduate school.
