Applied Mathematics · University of Southampton

Toby L. Kirk

Lecturer in Mathematical Modelling, working on the mathematics of transport — how heat, mass, charge and momentum move through fluids, batteries and engineered surfaces.

About

Portrait of Dr Toby Kirk
Dr Toby Kirk · School of Mathematical Sciences

I am a Lecturer (Assistant Professor) in Mathematical Modelling in the School of Mathematical Sciences at the University of Southampton, where my research focuses on industrial modelling for sustainability.

I develop analytical and asymptotic methods for transport phenomena — coupled problems of fluid flow, heat and mass transfer, and electrochemistry — arising in engineering and energy applications. Much of my work turns problems that would ordinarily demand large numerical simulation into explicit formulae: results that expose the physics, and that engineers can actually use.

Before joining Southampton in October 2024, I was a Chapman Fellow in the Department of Mathematics at Imperial College London (2022–24). From 2018 to 2022 I was a postdoctoral researcher in the Mathematical Institute at the University of Oxford, working with Professors Jon Chapman and Colin Please on multi-scale modelling of lithium-ion batteries within the Faraday Institution's Multi-scale Modelling project. I hold a PhD in Applied Mathematics from Imperial College London (2017), supervised by Professor Demetrios Papageorgiou, for which I was awarded an EPSRC Doctoral Prize Fellowship, and a BA in Mathematics from Trinity Hall, University of Cambridge.

Together with several of my long-standing collaborators, I am a member of the Red Lotus Project, an international consortium on surface engineering for heat transfer, sustainability and energy harvesting.

I am currently accepting applications from prospective PhD students.

26journal publications
441citations
12h-index

Research

Three threads run through my work, tied together by a common toolkit: matched asymptotic expansions, complex analysis, and mixed boundary-value problems — mathematics in service of physical insight.

Analytical & asymptotic transport phenomena

Exact and asymptotic solutions for the coupled transport of momentum, heat and mass. Slip flows over superhydrophobic and structured surfaces, thermocapillary (Marangoni) stresses on curved menisci, conjugate heat transfer, and electrohydrodynamics — multiscale mixed boundary-value problems solved with matched asymptotics and complex-variable methods.

Energy & sustainability

Physics-based modelling of lithium-ion batteries: asymptotic reduction of electrochemical models of porous electrodes, electrode heterogeneity and particle-size distributions, and nonlinear impedance spectroscopy for parameter identification. Current work extends to thermal ratings of power cables and energy harvesting from waste heat.

Electronics cooling

The thermal management of high-power electronics: microchannel and superhydrophobic-surface cooling, longitudinal-fin heat sinks, grooved heat pipes, and thermal spreading and contact resistance. Explicit formulae for Nusselt numbers and flow resistance that replace expensive conjugate simulations in design.

Selected work

Graphical abstract: asymptotic structure of flow through a longitudinal-fin heat sink with tip clearance
Graphical abstract, Journal of Fluid Mechanics

2025 · Journal of Fluid Mechanics

Explicit formulae for convection in longitudinal-fin heat sinks with tip clearance

T. L. Kirk & M. Hodes

Longitudinal-fin heat sinks are a workhorse of electronics cooling, but predicting their performance normally requires numerical simulation of the conjugate heat transfer between fin and fluid. Using matched asymptotic expansions in the limit of small fin spacing, this paper derives explicit formulae for the velocity and temperature fields and the Nusselt number — including the effect of clearance above the fin tips — accurate enough to replace numerical solution across much of the design space.

Graphical abstract: electrode architecture and streamlines of a leaky-dielectric fluid pump
Graphical abstract, Journal of Fluid Mechanics

2025 · Journal of Fluid Mechanics

A leaky-dielectric fluid pump

M. Mayer, T. L. Kirk & D. G. Crowdy

A new architecture for pumping weakly conducting liquids with no moving parts. A periodic array of asymmetrically spaced electrode pairs generates Maxwell stresses on the interface between two layered fluids; the asymmetry produces a net flow whose direction is controlled purely through the applied potentials. A small-period asymptotic analysis yields closed-form solutions for the electric field, the flow and the pumping speed, together with estimates of the power required.

Log-likelihood contour maps showing how nonlinear impedance data resolves battery parameter degeneracies
Likelihood landscapes: nonlinear data (right) breaks the degeneracy in linear EIS (left)

2023 · Journal of The Electrochemical Society

Nonlinear electrochemical impedance spectroscopy for lithium-ion battery model parameterization

T. L. Kirk, A. Lewis-Douglas, D. Howey, C. P. Please & S. J. Chapman

Standard (linear) impedance spectroscopy cannot uniquely identify the parameters of physics-based battery models — different parameter sets produce identical spectra. By modelling the response to moderately large sinusoidal currents, this work derives explicit formulae for the second-harmonic impedance of a full cell and shows that the nonlinear content breaks these degeneracies, validated against time-series simulations to ~11 mV accuracy. Nonlinear EIS becomes a practical, fast diagnostic for battery parameterisation.

Model versus experiment: terminal voltage during discharge and relaxation for a lithium-ion cell
Many-particle model (dashed) against experimental discharge–relaxation data

2021 · Journal of The Electrochemical Society

Physical modelling of the slow voltage relaxation phenomenon in lithium-ion batteries

T. L. Kirk, C. P. Please & S. J. Chapman

After a lithium-ion battery is discharged and left at rest, its voltage keeps drifting for hours — far longer than standard models predict. This paper shows the phenomenon is a signature of heterogeneity: a distribution of particle sizes in the electrodes continues to exchange lithium internally long after the current stops. A many-particle extension of the Doyle–Fuller–Newman model reproduces experimental relaxation quantitatively across discharge rates, with implications for how models should be validated.

Computed streamwise velocity fields over menisci of varying protrusion in a superhydrophobic channel
Velocity fields over menisci of varying protrusion angle

2020 · Journal of Fluid Mechanics

Thermocapillary stress and meniscus curvature effects on slip lengths in ridged microchannels

T. L. Kirk, G. Karamanis, D. G. Crowdy & M. Hodes

Superhydrophobic surfaces reduce drag by letting liquid glide over gas-filled grooves — but in cooling applications the heat itself deforms the menisci and drives thermocapillary stresses that can destroy the benefit. Solving the coupled three-dimensional problem with asymptotic and conformal-mapping techniques, this paper derives slip lengths capturing both effects simultaneously, showing that heating always degrades slip and by how much — essential accounting for any thermal application of these surfaces.

Asymptotic velocity contours in channels with strongly protruding menisci
Asymptotic velocity fields for protruding menisci

2018 · Journal of Fluid Mechanics (Rapids)

Asymptotic formulae for flow in superhydrophobic channels with longitudinal ridges and protruding menisci

T. L. Kirk

A single-author Rapids paper presenting remarkably simple asymptotic formulae for flow in channels patterned with longitudinal ridges and arbitrarily protruding menisci — with no restriction on how far the gas–liquid interface bulges into or out of the liquid. The slip-length formula stays accurate over a very wide range of geometries, giving a design tool for drag-reducing surfaces that previously required full numerical computation.

Schematic of liquid in the Cassie state over ridges with curved menisci in a microchannel
The flow configuration: liquid suspended over ridges with curved menisci

2017 · Journal of Fluid Mechanics

Nusselt numbers for Poiseuille flow over isoflux parallel ridges accounting for meniscus curvature

T. L. Kirk, M. Hodes & D. T. Papageorgiou

My first JFM paper, and a foundational analysis of convective heat transfer in microchannels with ridged superhydrophobic walls. Perturbing about a flat interface, the coupled hydrodynamic and thermal problems are solved analytically via eigenfunction expansions and dual-series equations, yielding Nusselt numbers as explicit functions of the geometry — and revealing that the effect of meniscus curvature is captured, to excellent accuracy, by just two terms of the expansion.

Collaborators

Collaborations span applied mathematics, mechanical engineering and electrochemistry, across three continents. Selected collaborators below. Many of these collaborations run through the Red Lotus Project, a consortium of academic, industry and government partners working on surface engineering for heat transfer and energy harvesting.

Publications

Journal publications, organised by research theme. See also Google Scholar and ORCID. ★ marks the highlighted papers above.

Superhydrophobic & structured-surface transport

  1. 2025Mayer M, Kirk TL, Hodes M, Crowdy DGMechanical power from thermocapillarity on superhydrophobic surfaces. Journal of Fluid Mechanics 1009, A33.
  2. 2023Hodes M, Kane D, Bazant MZ, Kirk TLAsymptotic Nusselt numbers for internal flow in the Cassie state. Journal of Fluid Mechanics 977, A18.
  3. 2023Tomlinson S, Mayer M, Kirk TL, Hodes M, Papageorgiou DTThe effects of a curved liquid–gas interface and thermocapillary stress on superhydrophobic channel flow. ASME Journal of Heat and Mass Transfer 146, 021601.
  4. 2021Yariv E, Kirk TLLongitudinal thermocapillary slip about a dilute periodic mattress of protruding bubbles. IMA Journal of Applied Mathematics 86, 490–501.
  5. 2020Kirk TL, Karamanis G, Crowdy DG, Hodes MEffects of meniscus curvature and thermocapillary stress on slip length for a channel textured with parallel ridges. Journal of Fluid Mechanics 894, A15.
  6. 2018Kirk TLAsymptotic formulae for flow in a channel with ridged walls and curved menisci. Journal of Fluid Mechanics 839, R3.
  7. 2018Karamanis G, Hodes M, Kirk TL, Papageorgiou DTExtended Graetz–Nusselt problem for liquid flow in Cassie state over isothermal parallel ridges. ASME Journal of Heat Transfer 140(6), 061703.
  8. 2018Game S, Hodes M, Kirk TL, Papageorgiou DTNusselt numbers for Poiseuille flow over isoflux parallel ridges for arbitrary meniscus curvature. ASME Journal of Heat Transfer 140(8), 081701.
  9. 2017Kadoko J, Karamanis G, Kirk TL, Hodes MOne-dimensional analysis of gas diffusion-induced Cassie to Wenzel state transition. ASME Journal of Heat Transfer.
  10. 2017Karamanis G, Hodes M, Kirk TL, Papageorgiou DTSolution of the Graetz–Nusselt problem for liquid flow over isothermal parallel ridges. ASME Journal of Heat Transfer 139(9), 091702.
  11. 2017Kirk TL, Hodes M, Papageorgiou DTNusselt numbers for Poiseuille flow over isoflux parallel ridges accounting for meniscus curvature. Journal of Fluid Mechanics 811, 315–349.
  12. 2017Hodes M, Kirk TL, Karamanis G, MacLachlan SEffect of thermocapillary stress on slip length for a channel textured with parallel ridges. Journal of Fluid Mechanics 814, 301–324.
  13. 2016Lam LS, Hodes M, Karamanis G, Kirk TL, MacLachlan SEffect of meniscus curvature on apparent thermal slip. ASME Journal of Heat Transfer 138(12), 122004.

Lithium-ion batteries & electrochemistry

  1. 2025Drummond R, Tredenick EC, Kirk TL, Forghani M, Grant PS, Duncan SRGraded lithium-ion battery pouch cells to homogenise current distributions and reduce lithium plating. Journal of The Electrochemical Society 172(1), 010518.
  2. 2023Kirk TL, Lewis-Douglas A, Howey D, Please CP, Chapman SJNonlinear electrochemical impedance spectroscopy for lithium-ion battery model parameterization. Journal of The Electrochemical Society 170, 010514.
  3. 2022Tomlin R, Roy T, Kirk TL, Marinescu M, Gillespie DImpedance response of ionic liquids in long slit pores. Journal of The Electrochemical Society 169, 120513.
  4. 2022Kirk TL, Evans J, Please CP, Chapman SJModelling electrode heterogeneity in lithium-ion batteries: unimodal and bimodal particle-size distributions. SIAM Journal on Applied Mathematics 82(2).
  5. 2021Kirk TL, Please CP, Chapman SJPhysical modelling of the slow voltage relaxation phenomenon in lithium-ion batteries. Journal of The Electrochemical Society 168, 060554.

Electronics cooling & thermal management

  1. 2025Kirk TL, Hodes MExplicit formulae for convection in longitudinal-fin heat sinks with tip clearance. Journal of Fluid Mechanics 1016, A32.
  2. 2025Miyoshi H, Kirk TL, Hodes MOn spreading resistance for an isothermal source on a compound flux channel. ASME Journal of Heat and Mass Transfer 147(9), 091403.
  3. 2024Miyoshi H, Kirk TL, Hodes M, Crowdy DGFully developed flow through shrouded-fin arrays: exact and asymptotic solutions. Journal of Fluid Mechanics 991, A2.
  4. 2018Mayer M, Hodes M, Kirk TL, Crowdy DGEffect of surface curvature on contact resistance between cylinders. ASME Journal of Heat Transfer 141(3), 032002.
  5. 2018Hodes M, Kirk TL, Crowdy DGThermal spreading and contact resistance formulae capturing boundary curvature and contact distribution effects. ASME Journal of Heat Transfer.

Interfacial dynamics & electrohydrodynamics

  1. 2025Mayer M, Kirk TL, Crowdy DGA leaky-dielectric fluid pump. Journal of Fluid Mechanics 1018, A14.
  2. 2024Mayer M, Kirk TL, Papageorgiou DTStability of a photosurfactant-laden viscous liquid thread under illumination. Journal of Fluid Mechanics 983, A10.
  3. 2020Alexander JP, Kirk TL, Papageorgiou DTStability of falling liquid films on flexible substrates. Journal of Fluid Mechanics 900, A40.

Teaching & supervision

Current — University of Southampton

  • Modelling with Differential Equations — 4th-year course built around three extended modelling projects (Spring 2025, 2026).
  • Mathematical Methods for Scientists — 2nd-year course (Autumn 2025).
  • 3rd-year research project supervision.

Previously

  • Fluid Dynamics II, Imperial College London — 3rd/4th-year lecture course (2023).
  • M1R Undergraduate Research Project, Imperial — led Vortex motion and interaction in fluids for 50 students (2024).
  • Intercollegiate class tutor, University of Oxford — Applied Complex Variables; Waves and Compressible Flow (2019–21).

Student supervision

I have co-supervised ten PhD, Masters and undergraduate research students across Imperial College London, the University of Oxford and Tufts University, in applied mathematics and mechanical engineering. I am currently accepting PhD applications in mathematical modelling of transport phenomena and energy systems.

Contact

For research enquiries, collaboration, or PhD supervision:

School of Mathematical Sciences · University of Southampton · Southampton SO17 1BJ, UK