Composite Norland Optical Adhesive (NOA)/silicon flow focusing devices for colloidal particle manipulation and synthesis

Microfluidic flow focusing devices are widely used to generate steep chemical concentration gradients at the interface between miscible or partially miscible streams. In this study, first we present an optimised protocol for the manufacturing of composite flow focusing devices, consisting of a micropatterned layer of Norland Optical Adhesive (NOA) glue bound to flat or microgrooved silicon substrates. Then, three different applications of these devices are demonstrated, namely (i) particle spreading and focusing in continuous flows past flat substrates, (ii) particle accumulation within the dead-end cavities of microgrooved substrates and (iii) synthesis of nano-sized liposomes. Colloidal particle spreading, focusing and accumulation were achieved through diffusiophoresis transport induced by salt concentration gradients at the interface between electrolyte streams. Epi-fluorescence microscopy was adopted to characterise the spatio-temporal distribution of silica and polystyrene nanoparticles in the devices with flat or microgrooved surfaces. The effects of particle zeta potential and groove thickness on particle dynamics were investigated. 1,2-di-(9Z-octadecenoyl)-sn-glycero-3-phosphocholine (DOPC) liposomes were generated by hydrodynamic focusing and characterised via dynamic light scattering. Liposome populations with controlled narrow size distributions could be achieved by adjusting the flow rate conditions in the devices. This work demonstrates that composite NOA/silicon flow junction devices offer a competitive alternative to conventional PDMS chips and can support a wide range of microfluidic applications, including nanoparticle synthesis, characterisation and filtration, drug encapsulation and biochemical analysis.