Device and method for mixing substances

#This invention relates to a device and method for mixing substances, particularly very viscous substances in small volumes.

#The mixing of liquids is essential for many industrial and laboratory processes, and has been addressed in the past, for example in the following publications:

#(1) Shraiman, B. I. & Siggia, E. D. Scalar turbulence, Nature, 405, 639-545 (2000).

#In order to understand the invention and to see how it may be carried out in practice, a preferred embodiment will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

#FIGS. 1a to 1c schematically illustrate three examples of a mixing tank suitable to be used in a mixing device according to the invention;

#FIG. 2 graphically illustrates a stress ratio as a function of a shear rate as obtained with the mixing tank of FIG. 1a;

#Referring to FIG. 1a, there is illustrated an example of a device 1 suitable to be used as a mixing device according to the invention. The device 1 comprises a cylindrical cup 2 (constituting a mixing tank) having upper and lower concentric plates 2a and 2b (i.e., parallel disks) for a liquid to be supplied into a spaced therebetween until a level d. The upper plate 2a is mounted for rotation being attached to a rotating shaft 4, and just touches the surface of the liquid to provide a swirling flow of the liquid between the disks. A special cover is used to minimize evaporation of the liquid. The space between the disks 2a and 2b thus presents a cavity for a closed continuous flow of a liquid thereinside with curvilinear trajectories of the flow.

#Following is an example illustrating the creation of a turbulent flow of a liquid in the tank 2. In the present example, the radii of the upper and the lower plates are R1=38 mm and R2=43.6 mm, respectively. The liquid used is a solution of 65% saccharose and 1% NaCl in water, viscosity ηs=0.324 Pa·s, as a solvent for added polymer, which is polyacrylamide (MW=18,000,000; Polysciences) at a concentration of 80 p.p.m by weight. The viscosity of the so-obtained solution is η=0.424 Pa·s at γ=1 s−1. The curvature ratio is quite high, d/R=0.263, to provide destabilization of the primary shear flow and development of the secondary vortical fluid motion at lower shear rates.

#For experimental purposes, the whole flow set-up 1 is mounted on top of a commercial viscometer (AR-1000 of TA-instruments) to measure precisely the angular velocity ω, of the rotating upper plate and the torque applied to it, to estimate the average shear stress in a polymer solution flowing inside the cup. The walls of the cup 2 are transparent, which allows Doppler velocimeter measurements by collecting light scattered from the crossing point of two horizontal laser beams. The flow is observed from below. The lower plate 2b of the cup is made from plexiglass, and a mirror (not shown) tilted by 45° is placed under the lower plate. The flow patterns are then captured by a CCD camera (photodetector) at the side, and the temperature is stabilized at 12° C. by circulating air in a closed box. The flow under the black upper plate is visualized by seeding the liquid with light reflecting flakes (1% of the Kalliroscope liquid). The liquid is illuminated by ambient light. The relaxation time, λ, estimated from the phase shift between the stress and the shear rate in oscillatory tests, was 3.4 s.