#################################################### # Subramanian Ramakrishnan # requesting 3 days, at minimum=2 days # beamline 33ID, Scattering from Anisotropic Particles and Gels # instrument 33ID-D USAXS #################################################### # top:/home/www/beamtime-requests/req00696.txt # UNICAT Member Beam Time Request #696 # created Sat Dec 04 17:41:39 CST 2004 #################################################### apsrun: 2005-01 beamline: 33ID collaboration: Yes collaborator_Pete: ON contact: ramakris@scs.uiuc.edu days: 3 description: Structure and Rheology of Anisotropic Particles E. Mock, S. Ramakrishnan and C. F. Zukoski Anisotropic colloidal particles are encountered in a number of commercial and industrial products. From paints and inks to cosmetics and shampoos to processing of advanced ceramics and coal slurries and photonic band gap crystals, the engineer or chemist seeks to control the rheology and the microstructure of these suspensions. The limiting factor in the study of these materials so far has been to synthesize them in the nanometer range so that techniques like USAXS can be used to look at the microstructure. Traditional polymer synthesis techniques have resulted in dumbbell shaped particles which are of the order of a few microns and hence move out of the window of measurement in scattering techniques. Also, these particles are so big that one runs into sedimentation problems. We have recently synthesized nanometer size polystyrene particles (350nm diameter) which are dumbbell shaped. A sample SEM micrograph is shown in Figure1. These particles would be ideally suited for study by USAXS. Our goal is to assemble these anisotropic particles into useful structures and to link the underlying microstructure to macroscopic properties such as rheology (viscosity, yield stress and elastic modulus). We have recently developed a theory [1] to predict the flow properties of spherical particles and this theory has been successful in predicting the experimental measurements. We would like to extend these predictions to anisotropic particles. A key input to the theory is the structure factor of the colloidal suspensions which can be directly measured by USAXS. The polystyrene particles will be dispersed in glycerol at different volume fractions following the procedure outlined by Lumma et. al. [2]. Suspending polystyrene particles in glycerol makes them amenable to be studied by X-ray scattering. The following are the list of experiments we plan to perform. 1) Dilute Solution Scattering: Samples with <0.05: These set of runs will help us determine the scattering from a single particle and help us set the form factor. 2) Scattering from concentrated Suspensions: Once the form factor is determined, the volume fraction of the particles will be increased from dilute till concentrated ( ~ 0.5) and the static structure factor measured. Complementary measurements of the flow curve are being performed in the laboratory with an aim of correlating the flow curves with the scattering pattern. Once the structure is determined, our goal would be to analyze the scattering pattern with theoretical models: to pick an interaction potential and to see if we can extract interaction potential parameters which best describe the thermodynamics of the system. This would then set the stage for prediction of the macroscopic properties such as yield stress and elastic modulus. Reference 1) S. Ramakrishnan, Y. L. Chen, K. S. Schweizer and C. F. Zukoski, “Elasticity and Clustering in Concentrated Depletion Gels”, Phys. Rev. E, 70, 040401 (2004). 2) D. Lumma, L. B. Lurio, M. A. Borthwick, P. Falus and S. G. J. Mochrie, “Structure and Dynamics of Concentrated Dispersions of Polystyrene Latex Spheres in Glycerol: Static and Dynamic X-ray Scattering”, Phys. Rev. E, 62 (6), 8258 (2000). Figure1 2. Temparature Induced Gelation of Colloidal Suspensions S. Ramakrishnan, V. Gopalakrishnan, E. Mock and C. F. Zukoski Extensive rheological measurements in our laboratory have been carried out on suspensions composed of 90 nm diameter, D, silica particles where flow properties can be tuned by changing temperature. Our aim is to predict flow properties from measured structure factors, S(q). Our preliminary work indicates that in predicting the flow properties of gels, it is the large qD values of S(q) that are important. This occurs in a region where measurements cannot be made. However, we have also demonstrated that if we can fit S(q) for qD<10 using analytical m1odels such as the adhesive hard spheres, mechanical properties can be predicted. Our initial results suggest the curve fitting was extremely sensitive to the low q region of S(q). In our last runs at Argonne we performed experiments with 45nm particles and measured S(q) for four different volume fractions (0.1, 0.15, 0.2 and 0.3) at two different temperatures of 10C and 8C. The suspensions gel at 8C. Due to time constraints and equipment problems we were not able to run more samples and at different temperatures deeper into the gel. We would like more beam time to further our study by running more samples up to a volume fraction of 0.45 and to temperatures as low as 0C which are deeper into the gel. We already have rheological data in our laboratory of the elastic modulus and yield stress of these gels. equipment_required: SBUSAXS; water bath with temperature control experiment: Scattering from Anisotropic Particles and Gels foreign_nationals: hazards: instrument: 33ID-D USAXS instrument_other: minimumdays: 2 name: Subramanian Ramakrishnan nonmembers: unacceptable_dates: z34ID_details: #REMOTE_HOST: zukoski1.scs.uiuc.edu #REMOTE_ADDR: 130.126.228.237 #CONTENT_LENGTH: 5569 #HTTP_REFERER: http://www.uni.aps.anl.gov/unireq.htm #HTTP_USER_AGENT: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; .NET CLR 1.0.3705)