Surface tension is one of the most important physical properties that determines
the behaviour of bubbles. While the effect of nanoparticles and surfactants on the
surface tension of bubbles have been widely studied, the role of microparticles has
not received too much attention. In this work, the effect of silica microparticles on
the surface tension and the behaviour of bubbles was explored, both for bubbles at
rest and during dynamic processes such as pinch-off, oscillation and coalescence.
The influence of microparticles on the shape and surface tension of static bub-
bles grown from a submerged nozzle was investigated first using a modified sessile
bubble method. The results show that particles were in static equilibrium at the
static bubble, without any change in bubble surface tension. In addition, it was
found that the presence of particles at the bubble interface led to a change in the
bubble contour, similar to the effect of the changes in surface tension for uncoated
bubbles, which explains why classical sessile bubble tensiometry overestimated the
surface tension of particle-laden bubbles.
Secondly, the influence of microparticles on the surface tension and shape defor-
mation during and immediately after pinch-off was investigated. It was found that
microparticles at the deforming air-liquid interfaces generated surface pressure and
reduced surface tension of the interfaces, leading to a hindered shape deformation
compared to that of uncoated bubbles. Moreover, it was found that changing bubble
surface coverage has noticeable effect on the surface pressure after pinch-off, while
not during pinch-off. In addition, increasing particle contact angle and particle size
both led to an increase in the surface pressure during pinch-off. Experiments on bub-
ble oscillations were conducted, harmonic analysis shows the presence of particles
at bubble surfaces decreased the damping rate of the dominant harmonic signifi-
cantly while having a minor influence on the oscillation frequency during the shape
oscillation of the bubbles.
Lastly, the influence of microparticles on the surface tension and the neck growth
dynamics during the early stage of bubble coalescence was studied. Results show
that the particles at the bubble surface hindered the neck growth dynamics due to
the generation of the surface pressure by particle interactions. The existence of the
surface pressure lead to a lower surface tension compared to that of the uncoated
bubbles, with coarser particles generating higher surface pressure and correspond-
ing to lower surface tension.This thesis provided a systematic study of the behaviour of particle-laden bub-
bles using high-speed photography, macrophotography and image analysis tech-
niques. This work contributes to fundamental understanding of the surface tension
of particle-laden interfaces both on and off equilibrium.