# # top.uni.aps.anl.gov:/home/www/beamtime-requests/req00180.txt # UNICAT Member Beam Time Request #180 # created Thu May 24 11:36:02 CDT 2001 # collaboration: Yes collaborator+Pete: on contact: syedshah@uiuc.edu days: 3 description: Formation of gels is often desired for a number of applications widely used in the coatings, food and pharmaceutical industries. The addition of non-adsorbing polymer to a stable suspension of particles is a useful way of inducing attractions via the depletion mechanism. Depending on the strength and the range of these attractions, systems often undergo non-equilibrium phase transitions like gel transitions. The strength and range of the depletion attraction is proportional to the polymer concentration and radius of gyration respectively. The structure of these flocculated suspensions is inherently linked to the rheological properties of these systems. In the first part of our study we intend to systematically alter both the strength and range of the attractions and investigate the effects of these parameters on the suspension microstructure as we traverse across the gel transition boundary. Experiments will be conducted both in the one-phase fluid region (before gelation) and well into the gel phase. We intend to use the USAXS technique to probe subtle changes in the suspension structure factor as the gel boundary is traversed. Knowledge of these effects on the flow properties of the system will help establish links between the suspension microstructure and mechanics. These studies will be compared to model predictions of the suspension microstructure to elucidate the origin of gel transitions. In the second study we intend to investigate the depletion effect using thermosensitive microgels. Thermosensitive microgel particles undergo volume phase transitions and size changes with small changes in temperature. Dynamic light scattering studies indicate that the particle diameter ranges from 300 nm to 150 nm as the temperature increases 20 oC to 32 oC. This size change can be used as a precise control of volume fraction for a fixed number density of polymer. Transitions from fluids with well-defined zero shear rate viscosities to suspensions with well-defined zero frequency elasticities accompany the onset of iridescence suggesting an order-disorder transition. Furthermore, these microgel particles can be used as non-adsorbing depletants in a stable colloidal suspension to induce depletion attraction. Controlled manipulation of the microgel size can induce a variety of thermoreversible phase transitions. Changes in suspension structure factor over these transitions would be investigated using the USAXS measurements. This will help elucidate the link between the microstructure and rheology of these thermosensitive microgel suspensions. equipment+required: Temperature control experiment: Determination of changes in suspension microstructure during phase transitions induced by the addition of non-adsorbing polymer foreign+nationals: Syed Ali Shah (British) Prashant Mullick (Indian) Both are UNICAT members with clearance to use the APS facility. hazards: Solvents used - Toluene, water Particles - Silica , Poly N-isopropylacrylamide, Polystyrene All precautions will be taken to safely transport the solvents according to DOT regulations. minimumdays: 2 name: Syed Ali Shah nonmembers: station: 33ID-D unacceptable+dates: August 1st - September 10th, 2001 #REMOTE_HOST: zukoski3.scs.uiuc.edu #REMOTE_ADDR: 130.126.228.239 #CONTENT_LENGTH: 3399 #HTTP_REFERER: http://www.uni.aps.anl.gov/unireq.htm #HTTP_USER_AGENT: Mozilla/4.75 [en] (Win98; U)