The system includes a cup, a lid, a stir, and one or more bags. The cup is configured to hold fluids. The lid includes a top portion and a bottom portion configured to couple to and cover an open portion of the cup. The stir includes a knob, a longitudinal shaft, and a distal shaft extending outwardly from the longitudinal shaft. The knob is disposed on the top portion of the lid and coupled to the longitudinal shaft threaded through the bottom portion of the lid. The one or more bags are coupled along the length of the longitudinal shaft.

The invention generally relates to an apparatus and a method for mixing of liquids (50) or of particles with a liquid (50). In a volume of liquid (50), a thermal convection flow is generated at at least one surface of the volume of liquid by irradiating IR radiation (30) into the volume of liquid. Thereby it is possible to avoid a depletion zone at the surface and to more accurately measure interactions of the particles with the surface by means of surface-based measurement methods.