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This study was carried out to investigate the break up of nanoparticle clusters in a liquid using an in-line rotor stator. Two types of fumed silica particles were dispersed in distilled water: Aerosil 200 V, which is hydrophilic, has a primary particle size of 12 nm and Aerosil R816 which is based on Aerosil 200 V and surface modified to render it hydrophobic. The article reports on the rheology of the dispersions, particle size analysis, the effect of concentration, and processing conditions such as the rotor speed, that is, the specific power input, and flow rate, that is, the residence time.

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In-line rotor-stators are used for a wide range of power intensive dispersion applications, including the breakup of immiscible liquid droplets or agglomerates. This study, performed within the DOMINO project at BHR Group, aimed at studying the performance of three different rotor-stator head designs for deagglomeration processes. A given test system, nanoscale silica particles-in-water, was used to identify the mechanism and kinetics of break-up and determine the smallest attainable size. Three rotorstator head designs used were the GPDH-SQHS and EMSC screens from Silverson and Ytron Z-Lab from Ytron. These in-line rotor-stators were used in the recirculation loop of a stirred tank with a total dispersion volume of 100 litres. Power input and residence time were varied by changing the rotor speed and dispersion flow rate. Breakup was found to occur through erosion regardless of the operating conditions or rotor-stator design. The smallest fragments obtained were aggregates, rather than primary particles, and these were of a mean diameter of 150-200 nm; also independent of the operating conditions or rotor-stator head design. With a given rotor-stator operated at a given flow rate, increasing the rotor speed and hence the power input increased the break up kinetics. For a given design at a given specific power input, whilst the break up rate per tank turnover decreased when the flow rate was increased, the total processing time could be reduced. There were differences in the volume of the mixer head and chamber volumes; in addition, a smaller flow rate range could be covered with the Ytron design. Comparison of the different designs was therefore not straightforward. It could however be shown that the rotor-stator designs with a high number and small size of holes and/or gaps have a faster break up rate.

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