Flows inside cells: From natural cytoplasmic streaming to microfluidic systems

Cytoplasmic streaming, the persistent flow of fluid inside a cell, induces intracellular transport. This plays a key role in fundamental biological processes. In this thesis, we discuss fluid flows inside cells, from naturally occurring cytoplasmic streaming to experimental techniques for inducing cytoplasmic flows and controlling micron-sized particles. First, we consider cytoplasmic streaming observed during cell division, specifically in meiosis II mouse oocytes (developing egg cells) awaiting fertilisation. Inside the oocyte, the spindle, which is the protein structure responsible for dividing genetic material in a cell, must maintain its position near the cell cortex (the thin actin network bound to the cell membrane) for many hours. However, the cytoplasmic streaming that accompanies this stable positioning would intuitively appear to destabilise the spindle position. Through a combination of numerical and analytical modelling, we reveal a hydrodynamic mechanism for stable spindle positioning beneath the cortical cap. We show that this stability depends critically on the spindle size and the active driving from the cortex, and demonstrate that stable spindle positioning can result purely from a hydrodynamic suction force exerted on the spindle by the cytoplasmic flow. Our findings show that local fluid dynamic forces can be sufficient to stabilise the spindle, explaining robustness against perturbations not only perpendicular but also parallel to the cortex. Next, we turn our attention to artificially induced cytoplasmic streaming. This work was done in collaboration with the research group of Prof. Moritz Kreysing (previously at the Max Planck Institute of Molecular Cell Biology and Genetics, now at Karlsruhe Institute of Technology), who performed the experiments. We begin by analysing an experimental technique for inducing flows inside cells, known as focused-light-induced cytoplasmic streaming (FLUCS). Recent experimental studies in cell biology have applied this method to probe the impact of intracellular flows in the spatiotemporal organisation of cells and organisms. In these FLUCS experiments, mild heating (a few kelvins) via focused infrared light from a laser leads to long-range, thermoviscous flows of the cytoplasm inside a cell. To extend future use of FLUCS in cell biology, new quantitative models are needed to link the external light forcing to the flows and transport produced. We present a fully analytical, theoretical model describing the thermoviscous fluid flow induced by the laser stimulus at all length scales in two-dimensional confinement. We show that the leading-order instantaneous flow field during one scan of the laser results purely from the thermal expansion of the fluid. In contrast, the average velocity of a tracer particle over a full scan arises from the interplay between thermal expansion and thermal shear viscosity changes, and is a two-dimensional hydrodynamic source dipole in the far field. Our quantitative findings show excellent agreement with experimental results. Having established the theoretical basis of standard FLUCS, we develop a variety of new techniques based on thermoviscous flows in microfluidic systems, for applications in biology at the subcellular scale, microrobotics and beyond. Many existing methods for controlling particles in microfluidic devices are restricted to only two dimensions. In microfluidic settings, geometry has a significant influence on the flow driven by the focused light. Next, complementing our work on FLUCS in two-dimensional confinement, we present a theoretical model for the thermoviscous and thermal expansion flows and net transport induced by a translating, spherically symmetric heat spot in three-dimensional, unbounded fluid. We first numerically solve for the temperature field due to a translating heat source in the experimentally relevant limit. Then, in our flow model, considering the effect of the localised temperature increase on the density, shear viscosity and bulk viscosity of the fluid, we derive analytically the instantaneous flow generated during one scan and compute the net transport of passive tracers due to a full scan. We find that the flow and transport are independent of bulk viscosity and that in the far field, the leading-order average velocity of tracers is a three-dimensional source dipole, whose strength and direction depend on the relative magnitudes of the thermal expansion and thermal shear viscosity coefficients. These theoretical results lay the foundations for three-dimensional net transport and manipulation of particles at the microscale. Both inside a cell and in other applications of thermoviscous flows, nearby boundaries can modify the net transport observed. Achieving high-precision microfluidic manipulation of particles in these contexts therefore requires innovative design of laser scan patterns, along with quantitative theoretical understanding. To model complex environments, we focus on fundamental geometries. In two-dimensional confinement, we first extend our theory for the transport induced by FLUCS, now allowing the laser to trace arbitrary, user-defined scan paths, instead of scanning only along a straight line segment. Our theory quantitatively predicts tunable transport patterns unlocked by arbitrary scan paths, throughout space. We then address how the presence of a rigid wall, an obstacle modelled by a disc or circular confinement affects thermoviscous net flows. We show that a reduced model, using a far-field approximation, analytically captures the altered transport measured in experiments, shedding light on methods to deliver cargo to cells. Next, we consider the question of boundaries in three dimensions. While our theoretical modelling with a spherical heat spot in unconfined fluid predicts three-dimensional net flow, boundaries are ever present in experimental microfluidics. We demonstrate that in chambers of sufficient height, variation of the temperature profile along the direction perpendicular to the plates enables fully three-dimensional, localised thermoviscous net transport, in contrast with our original, essentially two-dimensional FLUCS model. We find that the introduction of walls, compared with our theory for unbounded fluid, creates finite-sized vortices, thus revealing the potential for transport and rotation of microparticles out of the plane of the laser scan path. We show how confinement provided by a spherical model cell modifies the flow due to the FLUCS methodology. Our results for FLUCS-based techniques so far each relate to repeated scanning of the laser along a single designated path. However, when aiming to perturb only the physical transport inside cells using traditional FLUCS, this localised heating by several kelvins can give rise to undesired biological responses. More generally, in the field of micromanipulation, many popular methods for trapping or moving micron-sized particles, such as optical tweezers, rely on the material properties of the particles or are limited to control of only a small number of particles simultaneously. Building on our fundamental understanding of FLUCS and working closely with the experimentalists, we show that sequential laser scanning of multiple, carefully selected basic scan paths enables precise positioning of particles via thermoviscous transport, overcoming these limitations. After validating our theory through comparison with experimental data, we illustrate how scan patterns can be designed for three broad classes of experimental applications. First, for highly temperature-sensitive systems, we develop scan patterns that induce net flow while keeping the time-averaged temperature fields close to homogeneous. These underpin a perturbation technique for cell biology even less invasive than the original FLUCS. Next, we demonstrate how different algorithms, employing the near- and far-field behaviours of the transport due to one scan path, allow simultaneous control of up to 15 microparticles in a thin chamber. Finally, we show that in thicker chambers, rotation of microstructures by fully three-dimensional, spatially extended vortical flows enables higher-resolution imaging in light microscopy.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2025

Author dc:creator

• Liao, Weida

Advisor dc:contributor.advisor

• Lauga, Eric

• fluid dynamics
• cytoplasmic streaming
• biophysics
• cell division
• thermoviscous flow

Licence

• https://www.repository.cam.ac.uk/bitstreams/06f03a9b-8b4e-4330-b9f1-5cb0a1f8a93e/download
• http://purl.org/NET/rdflicense/allrightsreserved

Language dc:language

eng

Author Identifier

0000-0002-0000-228X

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/390523