(416b) Fluid Dynamics Modeling for a Novel High Throughput Pipette Viscometer

High throughput research methods enable combinatorial approaches for rapid exploration of wide parameter spaces when developing new chemistries and materials. Robotic automation of the processes facilitates the high rate of data gathering, including the resulting physical and chemical properties of the new materials. In the development of fluidic materials, viscosity measurements are often a critical screening step, providing data necessary to understand both performance and manufacturing attributes related to flow behavior. Examples of such materials include agrochemicals, paints, paper coatings, pressure sensitive adhesives, elastomers, thermosets, silicones, water soluble polymers and cellulosic systems, food thickeners, de-icers, cooling emulsions, polishing suspensions, skin creams, shampoos, cleaning solutions, liquid detergents, and many more consumer products.

This presentation will describe a novel high throughput system for measuring viscosity of fluids under different conditions of shear rate, temperature, etc. This method utilizes the transient flow of complex fluids in robot-controlled pipettes, and is easily incorporated into the high throughput sequencing when transferring fluid from one set of vessels to another using microtiter plate formats. The high throughput advantages include small sample volumes (less than 1ml) and high sample rates (greater than 100 samples per hour) through parallel operation. The viscosity determination is based on mass and pressure measurements during both aspiration and dispensing of the fluid. The viscosity screening tool has been successfully utilized for a multitude of complex fluids including oils, paints, solvents, and detergents.

Numerical methods involving computational fluid dynamics (CFD), as well as analytical methods, are used to understand and characterize the capabilities and limitations of this viscosity measurement technique. The numerical tools are an essential component of the viscosity determination, especially when working with complex, non-Newtonian fluids. The transient CFD approach uses a moving-mesh method to capture the specified piston displacement and a volume-of-fluid method to track the fluid-air interface in the pipette. The fluid rheology is specified and the air is treated as a compressible ideal gas. Capillary and gravitational forces are included in the momentum equation. The pressure profiles in the pipette, along with the liquid aspiration rate and volume, are monitored as a function of time. The analytical method involves an interdependent mass and pressure balance at each time step. It can be set up using a spreadsheet, with individual time steps represented on each row of the spreadsheet.

The results obtained from the CFD and spreadsheet methods are in almost perfect agreement with each other for constant viscosity fluids. They also closely match the experimental data involving fluids with standard viscosities. The advantage of the spreadsheet method is its computational speed, whereas the CFD approach allows the analysis of complex rheologies. This capability is essential in evaluating the experimental data for non-Newtonian fluids.