Manoharan The primary goal of the PI's proposed research is to investigate how colloidal particles self-assemble in confined and nonequilibrium systems, including particles trapped at liquid-liquid interfaces (e.g. emulsion droplets) and inside spherical containers. Although common in industrial formulations and fundamental condensed matter studies, these systems remain poorly understood, primarily because no existing experimental probes, including confocal microscopy, can yield real-space data with sufficiently fast acquisition times to resolve 3D dynamics. The PI proposes to use a powerful interferometric technique, Digital Holographic Microscopy (DHM), in concert with particle synthesis and algorithm development to overcome these limitations. Preliminary data show that the technique is capable of tracking several micrometer-sized colloidal particles with 30 nm spatial precision in all three dimensions on millisecond time scales. DHM may be able to yield the most complete physical picture to date of dynamics, interactions, and assembly in colloidal suspensions.
Intellectual Merit
The proposed research seeks to answer fundamental questions in colloid science by pursuing the following objectives: 1. Development and optimization of a Digital Holographic Microscope to investigate colloidal suspensions: A prototype of this instrument has been constructed and preliminary data are shown. The PI will develop new algorithms, data acquisition techniques, and instrumentation to make the technique as rapid and robust as possible. 2. Use of this instrument in the following experimental studies: ? Measuring the dynamics of particles trapped at planar liquid-liquid interfaces: determining the interactions governing the assembly of interfacially-bound particles. ? Measuring the dynamics and structure of particles trapped at spherical liquid-liquid interfaces: investigating the self-assembled structures of particles confined to an emulsion interface. ? Imaging self-assembly in dense colloids confined inside an interface: Determining the effect of a spherical droplet boundary on the structure and assembly of colloidal crystallites.
Broader Impacts
This work will benefit industrial product development by providing a rational framework for the formulation of multiphase colloidal systems. It will also yield insights into structure formation in complex fluids that may be useful in nanofabrication. In addition, an integrated education and outreach program will target 8th grade science students in the Cambridge school district, a diverse and urban school system with a majority of students from underrepresented minority groups and low-income households. Goals of the program are: 1. Developing a design module to engage at-risk 8th grade science students: The PI will work closely with a teacher from the Kennedy-Longfellow middle school to develop, implement, and assess an inquiry-based learning program that (a) meets Massachusetts standards for teaching physics and engineering in 8th grade and (b) targets all skill groups, including Special Education students. Harvard undergraduate volunteers from the new School of Engineering and Applied Sciences will be recruited as mentors and facilitators. The broader aim is to educate and engage more middle school students through mentorship, research, and open-ended design projects. 2. Undergraduate education through research and mentoring; graduate course development
The research supported by this grant involved understanding the assembly and interactions of colloidal particles on liquid interfaces. Colloids are suspensions of micrometer-scale (1/100 the size of a human hair) particles in a fluid. Common examples of colloidal suspensions include milk and paint. In this project we investigated what happens when these colloidal particles come into contact with an interface between two fluids, such as oil and water. These "multiphase" colloidal suspensions are not as well understood as single-phase colloids, despite their importance to a number of different industries and commercial products. In fact, many familiar emulsions, including ice cream, cosmetics, and emulsions found in crude oil recovery, are stabilized by microscopic solid particles that sit at the interface between oil and water, keeping the two from separating. Though the effect has been known for over 100 years, little is understood about how the particles find their way into the interface, and how they interact once they get there to keep the emulsion from separating. We studied this old problem using a new scientific instrument -- a microscope that takes holograms. The holograms yield 3D movies of individual particles as they burrow into an oil-water interface and move along the interface. The grant supported the development of this new instrument, as well as the scientific studies that we did with it. In the course of doing these studies, we developed a software package called HoloPy that makes it easy to use this new technique in other labs or other research projects. HoloPy can be freely downloaded and modified by anyone, including other researchers or companies. One key finding of our studies was the discovery that colloidal particles, once they reach an interface, can take a very long time to settle in -- months or even years. This was surprising, as we (along with many other researchers) had assumed that the particles would very quickly come to rest at the interface. The long timescales of the particle motion give some clues about why it is difficult to make a stable emulsion using particles, and how one might overcome these difficulties. This information could be useful for industrial researchers trying to figure out new ways to formulate products. Indeed, this study, which was published in Nature Materials, one of the most widely-read journals in the field, was also highlighted in trade magazines of the cosmetics and petroleum industries. The grant also supported the education and training of PhD students, undergraduates, and middle schoolers. Three PhD students in Applied Physics and Physics at Harvard were partially supported by the grant; two have graduated, and a third will graduate in the summer of 2013. We also created an outreach and education program in collaboration with the Cambridge Public School District. This program brought Harvard undergraduate volunteers to an 8th grade classroom, where they acted as mentors to middle schoolers working on an engineering design project. By working together, both groups of students learned about the fundamentals of engineering design, the practice of engineering, and how engineering relates to science. About 30 Harvard undergraduates and 200 8th graders participated in this project over the course of the grant. Survey results indicated that there was a strong synergy between the two groups, with both coming away as more motivated to learn science and engineering in the future.
