Manipulation of colloidal particles by solute gradients in continuous-flow microfluidic devices

The controlled manipulation of colloidal particles (e.g., accumulation, removal, filtration, sorting, etc.) can play a key role in chemical and biological processes involved in numerous industrial applications such as water treatment, food processing, drug delivery, disease diagnosis and prevention. In the last decades, the fields of chemical engineering, bioengineering and biomedical engineering have seen a growing trend towards the intensification, integration and miniaturisation of processes and applications, in which bulk equipment and facilities are replaced by cheaper, more efficient, safer and smaller devices. The concept of ``lab-on-a-chip'' is indeed a paradigmatic example of this trend. As a result, the integration of colloid manipulation capability in microfluidic devices has become a critical requirement for several microfluidic applications, especially in the healthcare sector, such as for diagnostics and therapeutics. On the other hand, the field of microfluidics has not only offered opportunities for technology developments, but it has also turned over the new leaf in profoundly understanding the underlying concepts and theories governing the colloidal particle transport by enabling visualization and analysis of fluids and particles behaviour at the micron-length scale.

In the past few decades, various manipulation techniques requiring external forces such as electrophoresis (electrostatic), magnetophoresis (magnetic), thermophoresis (thermal), tensiophoresis (surface tension), optophoresis (optical), acoustophoresis (acoustic) and inertial migration (inertial) have been employed as a tool to perform particle operations, such as separation, filtration, trapping, focusing, and accumulation. However, their integration into portable devices for various applications remains a challenging task due to limited selectivity, low throughput, complex device design and a requirement for costly/bulky instrumentation. An innovative and successful particle manipulation strategy would have to be free from all of these downsides. Additionally, it would have to be fast (i.e., high-throughput), selective (i.e. targeting only the particles of interest), easy to integrate into a multifunctional device and, most importantly, one that does not rely on the use of external fields.

The need for new particle manipulation technologies has led to an increasing interest in the exploration of colloid transport by diffusiophoresis (DP) in a microfluidic environment. DP harnesses the chemical energy of a solute concentration gradient and converts it into the mechanical energy of the colloids, thereby propelling them through the liquid medium without applying any external field. Diffusiophoresis was first reported by Derjaguin and co-workers in the 1940s and further established by Anderson and Prieve in the 1980s, but in recent years there has been growing interest in exploring DP as a tool for understanding colloidal self-assembly, detection and healing of bone fractures, and to study the transport of latex particles, liquid drops, DNA, cells, etc. in confined geometries. Consequently, designing portable device platforms to decipher DP transport in dead-end geometries has allured researchers to unlock new opportunities in the healthcare sector (e.g. diagnostics, drug delivery), particle technology (e.g. focusing, accumulation, separation) and chemical engineering (e.g. oil extraction). In recent studies, transient salt concentration gradients have been successfully used to achieve enhanced particle transport into dead-end structures by diffusiophoresis and diffusioosmosis effects.

The aim of this project is to validate and optimise an innovative strategy to control the motion and spatio-temporal distribution of functional nanoparticles in novel microfluidic channels equipped with microgroove structures by salt-driven transport. In this research, a new mechanism for reversible trapping of sub-micron particles in dead-end geometries under steady-state solute gradients in a continuous flow setting is reported and investigated.

First, a microfabrication procedure based on soft/photo-lithography techniques is developed to manufacture microfluidic devices fitted with a microgrooved wall and capable to generate controlled salt concentration gradients past the grooves. The behaviour of sub-micron particles and their distribution within the channel and the grooves is investigated by fluorescence microscopy techniques. The effect of device geometry and operating conditions on the trapping performances, defined in terms of particles average concentration within the grooves and particles concentration at focusing point, are investigated and optimised.

The proposed particle manipulation strategy envisages opportunities for the exploitation of DP transport in soft matter and living systems for drug delivery, synthetic biology and on-chip diagnostics applications, where solute concentration gradients and flows in confined geometries are ubiquitous. An overview of some exemplary applications and future technology development work is outlined at the end of the thesis. In conclusion, the field of DP stands at a critical juncture to achieve large-scale applications and impact, and the studies at the microscale level form a solid foundation for future work to apply this transport mechanism for future technology development rather than limiting it to just a lab curiosity.