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# Subramanian Ramakrishnan <ramakris@scs.uiuc.edu>
# requesting 3 days, at minimum=2 days
# beamline 33ID, "Microstructure of Gels"
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# top:/home/www/beamtime-requests/req00373.txt
# UNICAT Member Beam Time Request  #373
# created Wed Mar 12 09:17:59 CST 2003
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beamline:
	33ID

collaboration:
	Yes

collaborator+Pete:
	on

contact:
	ramakris@scs.uiuc.edu

days:
	3

description:
	Working with mixtures of hard spheres and nonabsorbent polymers we have established how polymer molecular weight and concentration alter suspension microstructure. Some of the key findings of our previous work on colloid-polymer suspensions have been the development of enhanced scattering at low q when a suspension gels.  This upturn might have two origins. First, it might be due to cluster formation or it might be due to randomly distributed heterogeneities.  Preliminary analysis suggest that in suspensions driven to gel by the presence of small non adsorbing polymers is accompanied by the formation of dense, incompressible regions of size approximately 2-5 particle diameters with randomly distributed heterogeneities (necessary to conserve the average particle density in the suspension), with a correlation length of 4-8 particle diameters.  These studies were carried out at a fixed particle volume fraction near 0.4.To better understand gel microstructures, the above question, we propose to perform a series of experiments where the particle volume fraction is varied over a wide range.  The goal of this work is to understand how the cluster size and correlation lengths vary as gels are formed.   This would provide us with a comprehensive set of data to test the different theories in literature.  A second goal of this project is to relate the observed microstructure with the mechanical properties measured with a rheometer.  These experiments will be carried out in our laboratory on identical samples where flow microstructures have been determined.
	
	 In a second set of experiments we are interested in understanding the role of depletion attractions in the presence of additional attractions.  In particular, we will work with systems where the depletion interaction range is 0.06-1.0 particle diameters and observe changes in suspension microstructure as we increase the strength of a very short-range attraction produced by decreasing the suspension temperature.  The short range attraction is produced by driving the octadecyl hairs on the particle surface through their theta point. We propose to study the effect of temperature on the microstructure of our model colloid polymer system for a variety of conditions.  For these experiments we have developed a scattering cell holder that allows us to vary temperature with a circulating water bath.  The beam time will allow us to test the capabilities of this new cell holder.
	
	Reference
	1) S. A. Shah, S. Ramakrishnan, Y. L. Chen, K. S. Schweizer and C. F. Zukoski, %93Scattering Studies of the Structure of Colloid-Polymer Suspensions and Gels%94, submitted to Langmuir.
	
	2) S. A. Shah, Y. L. Chen, S. Ramakrishnan, K. S. Schweizer and C. F. Zukoski, %93Microstructure of Dense Colloid-Polymer Suspensions and Gels%94, submitted to Journal of Physics Condensed Matter.

equipment+required:
	SBUSAXS

experiment:
	Microstructure of Gels

foreign+nationals:
	Canadian Citizen

hazards:
	All samples will be prepared at UofI.

minimumdays:
	2

name:
	Subramanian Ramakrishnan

nonmembers:
	

unacceptable+dates:
	June 15-18 : I have an ACS conference in Atlanta.


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