#################################################### # S. Ramakrishnan # requesting 5 days, at minimum=4 days # beamline 33ID, "Please description below for all 4 projects" #################################################### # top:/home/www/beamtime-requests/req00427.txt # UNICAT Member Beam Time Request #427 # created Thu Jul 31 15:53:37 CDT 2003 #################################################### beamline: 33ID collaboration: Yes collaborator_Pete: ON contact: ramakris@scs.uiuc.edu, 217-244-8358 days: 5 description: All experiments to be done on SB-USAXS Invstigators: S. Ramakrishnan, P. Mullick, V. Gopalakrishnan, W. E. Smith, A. Chan, J. A. Lewis and C. F. Zukoski 1) “Microstructure of Gels” : S. Ramakrishnan and V. Gopalakrishnan We are interested in studying the microstructure of gels and to link the underlying microstructure to macroscopic properties like Rheology. In our publications, we have reported on the microstructure of gels formed by adding polymer to a suspension of colloidal particles. Rheological measurement s of the gel elastic modulus were made and linked to the cluster sizes in the gels as measured by x-ray scattering. Another method, to induce gelation is to induce attractions between the particles by reducing the temperature of the system. The deeper one quenches the system, the stronger the attraction and hence stronger will be the gel formed. We have recently studied rheological properties of gels formed by cooling a colloidal suspension of hard spheres at different volume fractions. Our goal is to now link the rheological properties to the microstructure. We are interested in seeing how the microstructure changes as we go deeper into the gel at a fixed volume fraction. Does the cluster size change? What differences in the microstructure gives rise to the observed differences in the measured gel elastic moduli? These tests will serve as platforms to test fundamental theories of gelation and percolation. In our last trip, we were successful in testing out the new sample holder with which we could control temperature. We can now control temperatures from 0C to 50C by circulating water through the sample holder. Sample data was taken on a model system at a single volume fraction at 4 different temperatures. In this work, we plan to carry out a systematic study as a function of volume fraction and temperature and then link it to the rheological measurements. 2) “Nano particle halos : A new stabilization mechanism”: A. Chan and S. Ramakrishnan A new mechanism for regulating the stability of colloidal particles has been discovered by J. A. Lewis and co-workers at the University of Illinois. Negligibly charged colloidal microspheres, which flocculate when suspended alone in aqueous solution, undergo a remarkable stabilizing transition upon the addition of a critical volume fraction of highly charged nanoparticle species. Zeta potential analysis revealed that these microspheres exhibited an effective charge buildup in the presence of such species. Scanning angle reflectometry measurements indicated, however, that these nanoparticle species did not adsorb on the microspheres under the experimental conditions of interest. It is therefore proposed that highly charged nanoparticles segregate to regions near negligibly charged microspheres because of their repulsive Coulombic interactions in solution. This type of nanoparticle haloing provides a previously unreported method for tailoring the behavior of complex fluids. We propose to study the microstructure of the microspheres in the presence of the nanoparticles to understand the mechanism of this stabilization. Systematic experiments will be carried out in the which the microsphere volume fraction will be kept constant and the nanoparticle volume fraction varied and the resulting microstructure measured to understand the effect of the addition of nanoparticles. This data will also help in comparing with the computer simulations of the structure factor of a similar system by Prof. Eric Luijten (at UofI). Reference Valeria Tohver, James E. Smay, Alan Braem, Paul V. Braun, and Jennifer A. Lewis, “Nanoparticle halos: A new colloid stabilization mechanism”, PNAS 2001 98: 8950-8954. 3) “Structure of Fractal Aggregates”: W. E. Smith Anisotropic particle interactions will be investigated by using a model system involving colloidal fractal aggregates. The scattering data obtained will be used to gain insight into the thermodynamics and structural information due to the colloid-colloid interactions. The system will be studied as a function of concentration across phase boundaries. Also of interest will be the effect of changing electrostatic repulsions on both the structure and the thermodynamics. The system that will be studied will include fumed silica in water, THF and ethanol as solvents. Our aim is to develop a model fractal system and to provide a complete characterization of the resulting thermodynamics, microstucture and flow properties. This study will then extend the characterization techniques to different grades of fumed silica. Reference 1) Smith, W. E. and Zukoski C. F. “Structure and Thermodynamics of Fractal Aggregates”, in preparation for Langmuir. 4) “Structure and Thermodynamics of Thermoresponsive Particles and Gels”: P. Mullick Stimuli-responsive particles have potential applications in various areas like separation of bio-molecules, drug delivery and rheological property modifiers. Controlling particle size is key to these applications. In the case of temperature sensitive microgels, because the gel particles are composed of cross-linked polymer chains, the particle size is a function of temperature and solvent chemical potential. Previous experiments at the 33ID beamline at the UNICAT facility at the APS have shown that the particle size of these microgel particles is dependent on the number density of the particles as well as temperature. Increasing the number density leads to an increase in osmotic pressure which cause the particle to deswell. In this study we propose to study the effect of this deswelling transition on the structure factor of a mixture of the microgel particles with hard core colloidal particles. The polymeric microgel particles in such a mixture are expected to behave as depletants that have a tunable particle size. One can thus tune the range of the depletion attractions between the hard core particles leading to a rich variety of colloidal phase behavior. Our previous experiments at the UNICAT facility during the July 2003 cycle have shown distinct clustering of the hard core particles in the presence of microgel particles. In these experiments we probed the regime of high polymer loading and observed the clustering to be independent of the concentration of the polymer particles at such high loadings. In this proposed study we propose to traverse the phase space with extremely small loadings of polymer particles. These experiments will give insight into the structure of a mixture of hard and soft non-adsorbing particles. The system that will be studied will include Stober silica coated with poly(N-isopropylacrylamide) (polyNIPAM) and microgel particles composed of polyNIPAM. The solvent in all samples will be DI water. We hope to use the SB-USAXS technique to probe the changes in the structure factor of the colloidal suspension of the silica particles with increasing concentration of polyNIPAM microgel particles at different temperatures. equipment_required: SB-USAXS experiment: Please description below for all 4 projects foreign_nationals: hazards: minimumdays: 4 name: S. Ramakrishnan nonmembers: unacceptable_dates: October 10-18 z34ID_details: #REMOTE_HOST: zukoski4.scs.uiuc.edu #REMOTE_ADDR: 130.126.228.240 #CONTENT_LENGTH: 7583 #HTTP_REFERER: http://www.uni.aps.anl.gov/unireq.htm #HTTP_USER_AGENT: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.0)