A COUNTER-ROTATING COUETTE APPARATUS TO STUDY DEFORMATION OF A SUBMILLIMETER SIZED PARTICLE IN SHEAR-FLOW

We describe a new counter-rotating Couette apparatus that has been dev eloped for deformation studies of single sub-millimeter sized particle s in shear flow. New features are the adaption to the low viscosities of water-based systems and temperature control of the device. The inne r to outer radius ratio of the cylinders used is 0.9785 and the height to width aspect ratio of the gap is 4.0, while the inner radius is 10 0 mm. Because of the limited particle size a high mechanical accuracy of the Couette geometry is necessary. The swing of the inner cylinder is less than 2 mu m and that of the outer cylinder less than 4 mu m. W e achieved this by carefully choosing the design parameters of the aer ostatic bearing and the coupling between cylinder and motor unit. Furt hermore, special drive units give a shear rate resolution of 0.018 s(- 1), while the maximum shear rate is 100 s(-1). For a liquid viscosity on the order of 1 mPas the effective maximum shear rate is 30 s(-1). W e have shown that deformations as small as (L-B)/(L+B) approximate to 0.01 of giant bilayer vesicles (typical radius 10 mu m) with length L and width B can be observed with our device. (C) 1998 American Institu te of Physics. [S0034-6748(98)03803-9].