Research
Below listed some of my past and current research. Click on the title to read more about my work.
Many traditional scientific methods codify physical principles, such as conservation of mass and momentum, into partial differential equations (PDEs), which are then solved numerically. Although these numerical solutions can be extremely detailed, they are not necessarily accurate. Their accuracy requires good models and accurate model parameters. This is why experimental testing remains a crucial component of the engineering design process. CFD simulations and experimental results are usually discarded once a design has been chosen, however, despite producing gigabytes of information that could be useful in the future. This project is partly motivated by a desire to use this data effectively and sustainably.
Active Brownian Particle (ABP) is a class of models for particles whose trajectories depend on their noise orientation. It is a particularly popular model for swimming microorganisms such as bacterial, motile and sinking phytoplanktons. In a dilute suspension where these particles only interact hydrodynamically through their disturbance to the fluid bulk, they can be modelled as a continuum phase through the Fokker-Planck equation, which governs the probability distribution of the particles in both the orientational and physical space. However, the two-way coupling between the Fokker-Planck equation and the momentum equation governing the flow, i.e. the Navier-Stokes equation, remains a difficult problem, largely owing to the high number of dimensions in the Fokker-Planck equation.
Many microorganisms are motile, that is, they swim. Some motile micro-organisms (swimmers), such as the micro-algae Chlamydomonas augustae (née nivalis), navigate towards the upper surface by being bottom-heavy. In a downflowing pipe, the balance between the gravitational and viscous torque causes the micro-organism to swim towards the centre, forming a focused beam-like structure. This sideway drift of bottom-heavy swimmers is known as gyrotaxis. Gyrotaxis is responsible for many naturally occurring phenomena, such as bioconvection, micro-patchiness and the formation of thin phytoplankton layers in some parts of the ocean.
Sponges are one of the earliest ancestors of the animal kingdom, and choanoflagellates are a close cousin of sponges. Studying their biophysics may help us unravel the mystery of the origin of multicellularity. For example, are there particular reasons why some choanoflagellates remain singular, but some form colonies? How do these cells coordinate without a nervous system? Why do some choanoflagellates form colonies in the shape of a sphere, like a Choanocytes chamber? While not all questions can be answered purely by physics, looking into the mechanics of their movement and fluid flow around them surely helps biologists better understand the evolutionary reason behind their formation and behaviour.
Phytoplankton plays a vital role in the removal of CO$_2$ from the atmosphere, as they perform photosynthesis on the ocean surface and then sink to the bottom. This process drives almost half of the carbon sequestration (the process of removing carbon from the atmosphere) on Earth. It is known that their sizes and shapes affect their average sinking speed and how they are mixed by the flow in the ocean, but little is known about the underlying mechanism from a fluid dynamics perspective. Furthermore, depending on the particle shape, it is known that some species may tend to aggregate, which in turn can speed up their collective sinking.
G. K. Batchelor pioneered the field of microhydrodynamics in 1970s, leading to significant advancements in this domain in the following years. Analytical techniques like renormalization and computational methods such as Stokesian Dynamics have contributed to a comprehensive understanding of hydrodynamic interactions among particles in low Reynolds number environments. Over the past few decades, these insights have found applications in various fields, including microfluidics, suspension dynamics, and microscale biophysics, such as cell motility and active matter systems. Despite these achievements, it is important to note that most progress has been limited to low Reynolds numbers. There remains a notable absence of theoretical development regarding the application of similar techniques in high Reynolds number flows. Addressing unresolved questions, such as the streamer width in suspensions of sinking fibers, may necessitate exploring regimes beyond the low Reynolds number context. My ongoing work revolve around extending existing techniques in Stokesian flow to intermediate Reynolds number, in applications such as the design of bioreactors and the modelling of phytoplanktons in the ocean.
