Micro-rheometry

Our goal is to develop micro and nano-fluidic tools that measure rheological properties of quantity-limited complex fluids and that probe the regime where the fluid's structural length scales are comparable to that of the flow. These tools are especially needed by the pharmaceutical, polymers, and medical testing industries to the probe rheological properties of fluids which are difficult and expensive to produce.

Our approach to the development of micro and nano-fluidic devices is to start with existing concepts and geometries of rheometry and to then think small. This approach ensures that we are measuring fundamental materials properties rather than quantities that are experiment and geometry specific. Examples include:

• Development of micro-fluidic devices that generate well-defined flow fields to probe interfacial properties such as interfacial tension and interfacial viscosity.
• Development of a multi-sample capillary rheometer that is based on a traditional rheometer but uses approximately 1000 times less material.
• Development of nano-scale coulter counters which can measure the properties of individual nano-particles and the effects of confinement on polymer flow.
• Development of a dynamic rheometer for solution characterization that uses traditional oscillating parallel plates, but is constructed in a MEMS platform.

We have developed a multi-sample micro-capillary rheometer,M2R,which is capable of measurements over a broad range of temperatures, viscosities and shear rates. The instrument is inherently simple as the flow is generated by external gas pressure and the shear rate is measured through optical tracking of the flow front. In the current implementation, the required mass of each sample is approximately 20 mg, which is significantly less than traditional rheometers, further it measures four samples simultaneously. The device shown above contains no moving parts and the flow geometry is simply defined by slits cut into standard grade stainless steel shims. We find excellent agreement between the results obtained with the M2R and those from standard rheometers.

Figure 2 shows viscosity measurements that cover a range from high temperature polymer melts down to very light room temperature oils. In order to conduct measurements at viscosities below ~1 Pa s, we must account for the capillary forces which act to pull the fluids into the flow channels. We successfully measured these forces by removing the input gas pressure and by tilting the device so that a fluid in one slit travels upwards while an identical fluid in another flows downwards,at a greater rate. From the difference in flow speed, we can measure the capillary force and thus correct our data. We also demonstrate procedures for correcting data for shear-thinning and entry/exit flows. This instrument will be particularly useful in cases of multiple samples, limited material quantity, when flow heating is unacceptable and when optical access is useful. More generally, the techniques employed here pave the way for the development of polymer melt microfluidics. We envision that mixing, processing and measurement operations can all occur in these devices.

We have developed a new method to analyze the motions of a gel-interface under shear flow. Fluid flow near a deformable solid is ubiquitous in nature and technology. Blood flow through vessels, lubrication of cartilage in joints,microfuidic valves, and coating and printing processes are all examples of systems governed by the interaction between a flowing fluid and an elastic solid. Similar interfacial phenomena also occur in the multiphase flow of complex fluids. The rheological properties of such soft interfaces are inaccessible to conventional rheometry, creating the need for non-invasive and localized techniques. The new method utilizes particle-tracking for analyzing the motion of a soft-interface under shear flow. Here, we measure the motion of a sheared fluid-gel interface for which mechanical noise plays a role analogous to temperature. We measure the linear response and extract the rheological properties of the gel,laying the foundation for a non-Brownian optical microrheology.

• Our micro and nano-fluidic devices will be the next generation of tools to measure rheology, nanoparticle size and charge, and provide critical tests of theories of polymer confinement in channels.
• We developed a multi-sample capillary rheometer that is based on a traditional rheometer but uses approximately 1000 times less material.
• Our methods to measure rheological properties on nano-liter volumes of fluids will provide a critical characterization tool in the development of monoclonal antibody therapies, which are produced as concentrate visco-elastic solutions in sub-ml quantity.
• Customers include Stanford University (ring polymers), Michigan State University (nano-composites), University of Maryland(photo-active polymers) and Lord Corporation (magnetic suspensions).