An experimental investigation of the drying mechanisms of single droplets
above boiling point
The aim of this study is to investigate the effects driven by boiling on the drying behaviour
and morphology evolution. Three skin forming materials were chosen for this study namely,
sucrose, sodium silicate and Hydroxypropyl Methylcellulose (HPMC). The drying behaviour
and drying kinetics have been investigated across a range of air temperatures and initial solid
concentrations using a filament single droplet drying rig. The three materials showed three
different drying mechanisms, boiling without any inflation/deflation cycles, inflation and
deflation, inflation and puffing. Drying kinetics and morphology time-series at different drying
conditions collected from the single droplet drying rig have been extensively investigated to
further understand the differences in drying behaviour and morphology evolution route
encountered above boiling. Several drying metrics including relative size at locking, relative
final particle size, moisture content at locking were collated at different drying conditions.
The differences in these drying metrics have been related to key material properties.
Structures obtained from the single droplet drying experiments showed high similarities to
structures collected from a lab scaled spray dryer.
BUBBLE DYNAMICS INSIDE A DROPLET
The effects of mass transfer on the oscillation dynamics of a single bubble centrally located
within a droplet, which is termed as ‘bubble-droplet system’ in this report, are investigated.
The governing equations for the oscillations dynamics of the bubble and the droplet are
derived from the mass and the momentum balances and the Navier-Stokes equation. Both
spherically symmetric 1-D equations were derived and solved and a 2-D model was also
constructed and solved to allow for asymmetry to be investigated. The 2-D model, of the
dynamics of a initially over pressured bubble, behaved as anticipated showing bubble
oscillations which decayed with time. The rate of decay increasing with increasing viscosity.
An off-centre bubble was also investigated and a self-centering behaviour was observed due
to the asymmetric pressure and velocity field. An initial assessment of the influence of mass
transfer on the expansion of a single bubble in an infinite liquid media was conducted and an
expression for the expansion of a single centralized bubble within a liquid droplet was derived.
The solution of this model is currently underway.