#################################################### # Subramanian Ramakrishnan # requesting 4 days, at minimum=3 days # beamline 33ID, Nanoparticle Halos: A New Stabilization Mechanism and Temperature Induced Gelation of COlloidal Suspensions # instrument 33ID-D SBUSAXS #################################################### # top:/home/www/beamtime-requests/req00576.txt # UNICAT Member Beam Time Request #576 # created Fri Apr 16 08:33:48 CDT 2004 #################################################### beamline: 33ID collaboration: Yes collaborator_Pete: ON contact: ramakris@scs.uiuc.edu days: 4 description: 1. “Nano particle halos : A new stabilization mechanism” A. Chan, S. Ramakrishnan, J. A. Lewis and C. F. Zukoski 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 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). In our previous runs, the volume fraction of silica was kept constant at 0.4 and the nanoparticle polystyrene volume fraction was varied from zero where the system is a gel to a value where the suspension becomes a fluid. Our aim in these measurements was to measure the structure factor of the suspensions and to see how the height of the first peak in the structure factor and the location of the first peak (q*) changes with added nanoparticle concentration. However the variations with increasing polystyrene concentrations were not systematic as expected. One would expect q* to shift higher wave vectors as one moves deeper into the gel. The reason for this nonmonotnoic shift we think is due to the sample procedure that we follow in loading the samples into the instrument. Not enough time was given for the samples to gel before it was loaded and the measurement started. Rheological measurements indicate, at least a few hours are needed for the structure to build up after it is broken up. Beam time is requested to continue our investigations of the effects of nanoparticle haloing on suspension microstructure using the newly perfected procedure to prepare samples. 2. Temparature Induced Gelation of Colloidal Suspensions S. Ramakrishnan and C. F. Zukoski We have recently carried out extensive rheological measurements in our laboratory to measure the rheological properties of D=90 nm silica gels. Our aim was to link the microstructure of the suspensions (measured on previous trips to Argonne) with the rheological properties. Statistical mechanical models were used to fit the structure factor to extract the interaction potential parameters. Our studies indicate that linking microstructure to rheology requires careful determination of the interaction energy. We propose to do this by investigating the low q behavior of S(q) using 45nm diameter particles. Previous studies have focused on 100 nm particles and the shift to smaller particles opens p the low q region for more detailed investigation. The plan is to perform S(q) measurements at a number of volume fractions starting at the high temperature hard sphere limit and decreasing temperature to the point where the particles are strongly attractive and the suspension gels. At each volume fraction, the suspension would go from being a liquid to a gel which has a finite yield stress and elastic modulus. Independent rheological experiments would be carried out to complete the link between microstructure and mechanical properties. equipment_required: I definitely need a temperature bath for heating and cooling. experiment: Nanoparticle Halos: A New Stabilization Mechanism and Temperature Induced Gelation of COlloidal Suspensions foreign_nationals: hazards: Samples are Silica particles suspended in Decalin. Polystyrene particles and polymers are added to the silica particles in suspension. instrument: other instrument_other: 33ID-D SBUSAXS minimumdays: 3 name: Subramanian Ramakrishnan nonmembers: unacceptable_dates: The dates which are not good for me are June 14, 15 : Funding committee meeting at UofI June 25-30 : North AMerican Membrane Society Conference July 17-22 : ACA conference Chicago z34ID_details: #REMOTE_HOST: zukoski4.scs.uiuc.edu #REMOTE_ADDR: 130.126.228.240 #CONTENT_LENGTH: 5096 #HTTP_REFERER: http://www.uni.aps.anl.gov/unireq.htm #HTTP_USER_AGENT: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; .NET CLR 1.0.3705)