Nanoparticle manipulation and forced spreading within microscale acoustofluidic droplet systems

Lab-on-a-chip and micro-total-analysis systems are vital analytical tools used in both the biotechnology and nanotechnology industries. One of the most important aspects of this type of system is the ability to reliably control and manipulate the system itself or the sub-components located within the system such as cells. Audible frequency acoustofluidic actuation can provide a number of potential benefits for microfluidic procedures and is thusly investigated within this thesis. This thesis concentrates on two fundamental facets of manipulation of a microscale droplet system. The first aspect involves a previously undiscovered mechanism allowing manipulation of particles sized down to the nanoscale. The oscillatory motion of the fluid causes a time averaged linear relationship between particle and fluid flow. The intricate interplay between the hydrodynamic focussing and steady streaming effects must be controlled to optimise particle handling. The reduction in particle size has been achieved by minimising the strength of the acoustic streaming function and optimising the multiple-pass hydrodynamic focusing that acts on the particles. The magnitude of the scale reduction presented is quite significant, as particles as small as 190nm in diameter have been manipulated. The other aspect elucidates the harmonic fluidic motion’s ability to modify the wettability of a microscale droplet. The research conducted for this thesis has found that the forced spreading mechanism can be linked to the change in contact angle over an oscillation cycle. Over an oscillation period the temporal contact angle will spend more time in the advancing or receding segment depending on the direction of spreading. Consequently, this single oscillation period effect can result in droplet spreading over hundreds or thousands of oscillation cycles. The connection between the amplitude and the degree of spreading is via the oscillation mode. The behaviour of spreading changes when the droplet reaches larger accelerations. Distinct regions are defined each having different degrees of spreading. Moreover, the work conducted for this thesis has found that a droplet experiencing extremely large oscillations can spread so much that the drop can exceed its hysteretic limits. This mechanism can compel multiple droplets to evenly spread over a desired area and, subsequently, aid imaging of those drops. This thesis aims to highlight the prospective advantages that audible frequency actuation has over ultrasonic methods. These benefits include simpler instrumentation, higher transmission of particles, negligible temperature variation and synchronised manipulation of multiple samples.