# research

I work in the field of soft matter theory, on problems ranging from the topological properties of ordered media and colloids in liquid crystals to low Reynolds number motility and collective behaviour in soft active matter. Running through all of these things is my particular interest in the application, and development, of topological, or geometrical, techniques to problems in physics.

#### Group

**Wednesday 27th September 2017: Congratulations to Joel Nicholls for successfully defending his thesis!! ***(and thanks to Sebastian Ahnert and Robin Ball as examiners)*

**Monday 26th June 2017: Congratulations to Diana Khoromskaia for successfully defending her thesis!! ***(and thanks to Rhoda Hawkins and Matthew Turner as examiners)*

**Thursday 23rd March 2017: Congratulations to Jack Binysh on being awarded a David Crighton Fellowship! ***Jack will hold the fellowship in DAMTP, hosted by Ray Goldstein*

**Thursday 22nd September 2016: Congratulations to Dario Papavassiliou for successfully defending his thesis!! ***(and thanks to Julia Yeomans and Robin Ball as examiners)*

**Tuesday 31st May 2016: Congratulations to Tom Machon for winning a Faculty of Science PhD Thesis Prize!**

**Monday 8th February 2016: Welcome to Carl Whitfield, who joins us as a postdoc.**

**Thursday 10th December 2015: Congratulations to Tom Machon for successfully defending his thesis!!** *(and thanks to Mark Dennis and Matthew Turner as examiners)*

I am currently working with Jack Binysh (start 2015) on Knots in the FitzHugh-Nagumo Model, Alex Houston (start 2017) on Active Cholesterics and Joe Pollard (start 2017) on Liquid Crystal Sensors, and other problems of Geometry and Topology in Soft Matter.

Past members: Davide Michieletto (PhD 2015, supervisor Matthew Turner), now postdoc at Edinburgh

Tom Machon (PhD 2016, *Aspects of Geometry and Topology in Liquid Crystalline Phases*), now postdoc with Randy Kamien at UPenn

Anja Humpert (PhD 2016, supervisor Mike Allen)

Dario Papavassiliou (PhD 2016, *The Reciprocal Theorem and Swimmer Interactions*)

Carl Whitfield (Postdoc 2016), now postdoc at Manchester

Diana Khoromskaia (PhD 2017, *Active Liquid Crystals in Confined Geometries*), now postdoc at the Francis Crick Institute

Joel Nicholls (PhD 2017, *Polyomino Models of Molecular Monolayers*)

We have weekly group meetings in soft matter physics with Matthew Turner, Marco Polin, Vasily Kantsler and Robin Ball -- the current schedule is available here.

URSS Projects: I am usually happy to offer a URSS project for a capable student (in the summer before their fourth year) wishing to get an insight into modern theoretical physics or applied mathematics. Last summer I worked with Andrea Giudici on Geometry and Spin Transport in Skyrmion Magnets.

#### Latest publications

Straight Round the Twist: Frustration and Chirality in Smectics-A

Elisabetta A. Matsumoto, Randall D. Kamien, and Gareth P. Alexander, Interface Focus **7**: 20160118 (2017).

Frustration is a powerful mechanism in condensed matter systems, driving both order and co plexity. In smectics, the frustration between macroscopic chirality and equally spaced layers generates textures characterised by a proliferation of defects. In this article, we study several different ground states of the chiral Landau-de Gennes free energy for a smectic liquid crystal. The standard theory finds the twist grain boundary (TGB) phase to be the ground state for chiral type II smectics. However, for very highly chiral systems, the hierarchical helical nanofilament (HN) phase can form and is stable over the TGB.

Preprint: arXiv:1611.03830 **[cond-mat.soft]**.

Hydrodynamic instabilities in active cholesteric liquid crystals

Carl A. Whitfield, Tapan Chandra Adhyapak, Adriano Tiribocchi, Gareth P. Alexander, Davide Marenduzzo, and Sriram Ramaswamy, Eur. Phys. J. E **40**: 50 (2017). *Open Access*

We describe the basic properties and consequences of introducing active stresses, with principal direction along the local director, in cholesteric liquid crystals. The helical ground state is found to be linearly unstable to extensile stresses, without threshold in the limit of infinite system size, whereas contractile stresses are hydrodynamically screened by the cholesteric elasticity to give a finite threshold. This is confirmed numerically and the non-linear consequences of instability, in both extensile and contractile cases, are studied. We also consider the stresses associated to defects in the cholesteric pitch (λ lines) and show how the geometry near to the defect generates threshold-less flows reminiscent of those for defects in active nematics. At large extensile activity λ lines are spontaneously created and can form steady state patterns sustained by constant active flows.

Preprint: arXiv:1701.05022 **[cond-mat.soft]**.

Exact solutions for hydrodynamic interactions of two squirming spheres

Dario Papavassiliou and Gareth P. Alexander, J. Fluid Mech. **813**, 618-646 (2017). *Open Access*

We provide exact solutions of the Stokes equations for a squirming sphere close to a no-slip surface, both planar and spherical, and for the interactions between two squirmers, in three dimensions. These allow the hydrodynamic interactions of swimming microscopic organisms with confining boundaries, or each other, to be determined for arbitrary separation and, in particular, in the close proximity regime where approximate methods based on point singularity descriptions cease to be valid. We give a detailed description of the circular motion of an arbitrary squirmer moving parallel to a no-slip spherical boundary or flat free surface at close separation, finding that the circling generically has opposite sense at free surfaces and at solid boundaries. While the asymptotic interaction is symmetric under head-tail reversal of the swimmer, in the near field microscopic structure can result in significant asymmetry. We also find the translational velocity towards the surface for a simple model with only the lowest two squirming modes. By comparing these to asymptotic approximations of the interaction we find that

the transition from near- to far-field behaviour occurs at a separation of about two swimmer diameters. These solutions are for the rotational velocity about the wall normal, or common diameter of two spheres, and the translational speed along that same direction, and are obtained using the Lorentz reciprocal theorem for Stokes flows in conjunction with known solutions for the conjugate Stokes drag problems, the derivations of which are demonstrated here for completeness. The analogous motions in the perpendicular directions, i.e. parallel to the wall, currently cannot be calculated exactly since the relevant Stokes drag solutions needed for the reciprocal theorem are not available.

Preprint: arXiv:1602.06912 **[physics.flu-dyn]**.

Instabilities and Solitons in Minimal Strips

Thomas Machon, Gareth P. Alexander, Raymond E. Goldstein, and Adriana I. Pesci, *Phys. Rev. Lett. ***117**, 017801 (2016). *Open Access*

We show that highly twisted minimal strips can undergo a non-singular transition, unlike the singular transitions seen in the Möbius strip and the catenoid. If the strip is non-orientable this transition is topologically frustrated, and the resulting surface contains a helical defect. Through a controlled analytic approximation the system can be mapped onto a scalar ϕ^{4} theory on a non-orientable line bundle over the circle, where the defect becomes a topologically protected kink soliton or domain wall, thus establishing their existence in minimal surfaces. Experimental studies of soap films confirm these results and demonstrate how the position of the defect can be controlled through boundary deformation.

Preprint: arXiv:1602.02652 **[cond-mat.soft]**.

Global Theory of Defects in Nematic Liquid Crystals

Thomas Machon and Gareth P. Alexander,* Proc. R. Soc. A ***472**, 20160265 (2016). *Open Access*

We give the global homotopy classification of nematic textures for a general domain with weak anchoring boundary conditions and arbitrary defect set in terms of twisted cohomology, and give an explicit computation for the case of knotted and linked defects in R^{3}, showing that the distinct homotopy classes have a 1-1 correspondence with the first homology group of the branched double cover, branched over the disclination loops. We show further that the subset of those classes corresponding to elements of order 2 in this group have representatives that are planar and characterise the obstruction for other classes in terms of merons. The planar textures are a feature of the global defect topology that is not reflected in any local characterisation. Finally, we describe how the global classification relates to recent experiments on nematic droplets and how elements of order 4 relate to the presence of τ lines in cholesterics.

Preprint: arXiv:1604.03429 **[cond-mat.soft]**.

Umbilic Lines in Orientational Order

Thomas Machon and Gareth P. Alexander, *Phys. Rev. X* **6**, 011033 (2016). *Open Access*

Three-dimensional orientational order in systems whose ground states possess non-zero, chiral gradients typically exhibits line-like structures or defects: λ lines in cholesterics or Skyrmion tubes in ferromagnets for example. Here we show that such lines can be identified as a set of natural geometric singularities in a unit vector field, the generalisation of the umbilic points of a surface. We characterise these lines in terms of the natural vector bundles that the order defines and show that they give a way to localise and identify Skyrmion distortions in chiral materials -- in particular that they supply a natural representative of the Poincaré dual of the cocycle describing the topology. Their global structure leads to the definition of a self-linking number and helicity integral which relates the linking of umbilic lines to the Hopf invariant of the texture.

Preprint: arXiv:1507.08875 **[cond-mat.soft]**.

Motility of Active Fluid Drops on Surfaces

Diana Khoromskaia and Gareth P. Alexander, *Phys. Rev. E* **92**, 062311 (2015). *Open Access*

Drops of active liquid crystal have recently shown the ability to self-propel, which was associated with topological defects in the orientation of active filaments [Sanchez et al., Nature 491, 431 (2013)]. Here, we study the onset and different aspects of motility of a three-dimensional drop of active fluid on a planar surface. We analyse theoretically how motility is affected by orientation profiles with defects of various types and locations, by the shape of the drop, and by surface friction at the substrate. In the scope of a thin drop approximation, we derive exact expressions for the flow in the drop that is generated by a given orientation profile. The flow has a natural decomposition into terms that depend entirely on the geometrical properties of the orientation profile, i.e. its bend and splay, and a term coupling the orientation to the shape of the drop. We find that asymmetric splay or bend generates a directed bulk flow and enables the drop to move, with maximal speeds achieved when the splay or bend is induced by a topological defect in the interior of the drop. In motile drops the direction and speed of self-propulsion is controlled by friction at the substrate.

Preprint: arXiv:1508.05242 **[cond-mat.soft]**.

The Geometry of the Cholesteric Phase

Daniel A. Beller, Thomas Machon, Simon Čopar, Daniel M. Sussman, Gareth P. Alexander, Randall D. Kamien, and Ricardo A. Mosna, *Phys. Rev. X* **4**, 031050 (2014). *Open Access*

We propose a construction of a cholesteric pitch axis for an arbitrary nematic director field as an eigenvalue problem. Our definition leads to a Frenet-Serret description of an orthonormal triad determined by this axis, the director, and the mutually perpendicular direction. With this tool we are able to compare defect structures in cholesterics, biaxial nematics, and smectics. Though they all have similar ground state manifolds, the defect structures are different and cannot be, in general, translated from one phase to the other.

Preprint: arXiv:1406.3304 **[cond-mat.soft]**.

Knotted Defects in Nematic Liquid Crystals

Thomas Machon and Gareth P. Alexander, *Phys. Rev. Lett.* **113**, 027801 (2014).

We show that the number of distinct topological states associated to a given knotted defect, L, in a nematic liquid crystal is equal to the determinant of the link L. We give an interpretation of these states, demonstrate how they may be identified in experiments and describe the consequences for material behaviour and interactions between multiple knots. We show that stable knots can be created in a bulk cholesteric and illustrate the topology by classifying a simulated Hopf link. In addition we give a topological heuristic for the resolution of strand crossings in defect coarsening processes which allows us to distinguish topological classes of a given link and to make predictions about defect crossings in nematic liquid crystals.

Freely available at arXiv:1402.6898 **[cond-mat.soft]**.

Threading Dynamics of Ring Polymers in a Gel

Davide Michieletto, Davide Marenduzzo, Enzo Orlandini, Gareth P. Alexander, and Matthew S. Turner, *ACS Macro Letters* **3**, 255-259 (2014). *Open Access*

We perform large scale three-dimensional molecular dynamics simulations of unlinked and unknotted ring polymers diffusing through a background gel, here a three-dimensional cubic lattice. Taking advantage of this architecture, we propose a new method to unambiguously identify and quantify inter-ring threadings (penetrations) and to relate these to the dynamics of the ring polymers. We find that both the number and the persistence time of the threadings increase with the length of the chains, ultimately leading to a percolating network of inter-ring penetrations. We discuss the implications of these findings for the possible emergence of a topological jammed state of very long rings.

Freely available at arXiv:1306.4965 **[cond-mat.soft]**.

Knots and Non-Orientable Surfaces in Chiral Nematics

Thomas Machon and Gareth P. Alexander, *Proc. Natl. Acad. Sci. USA ***110**, 14174-14179 (2013). *Open Access*

Knots and knotted fields enrich physical phenomena ranging from DNA and molecular chemistry to the vortices of fluid flows and textures of ordered media. Liquid crystals provide an ideal setting for exploring such topological phenomena through control of their characteristic defects. The use of colloids in generating defects and knotted configurations in liquid crystals has been demonstrated for spherical and toroidal particles and shows promise for the development of novel photonic devices. Extending this existing work, we describe the full topological implications of colloids representing non-orientable surfaces and use it to construct torus knots and links of type (*p*,2) around multiply-twisted Möbius strips.

Freely available at arXiv:1305.0143 **[cond-mat.soft]**.

*This work has been featured in a University of Warwick press release (prepared by Anna Blackaby) and also reported in several online science and **technology websites. Now also highlighted in Liquid Crystals Today.*

#### Preprints

Vortex formation and defect dynamics in shells of active nematics

Diana Khoromskaia and Gareth P. Alexander, arXiv:1608.02813 **[cond-mat.soft]**.

We present a hydrodynamic model for a thin spherical shell of active nematic liquid crystal with an arbitrary configuration of defects. The active flows generated by defects in the director lead to the formation of stable vortices, analogous to those seen in confined systems in flat geometries, which generate an effective dynamics for four +1/2 defects that reproduces the tetrahedral to planar oscillations observed in experiments. As the activity is increased and the vortices become stronger, the defects are drawn more tightly into pairs, rotating about antipodal points. We extend this situation to also describe the dynamics of other configurations of defects. For example, two +1 defects are found to attract or repel according to the local geometric character of the director field around them, while additional pairs of opposite charge defects can give rise to flow states containing more than two vortices. Finally, we describe the generic relationship between defects in the orientation and singular points of the flow, and suggest implications for the three-dimensional nature of the flow and deformation in the shape of the shell.

###### Collaborators

I am lucky to have friends in many sunny and pleasant places around the world; Miha Ravnik (Ljubljana), Randy Kamien (Philadelphia), Mark Dennis (Bristol), Ray Goldstein and Adriana Pesci (Cambridge), Timon Idema (Delft), Andrea Liu (Philadelphia), Davide Marenduzzo (Edinburgh), Julia Yeomans (Oxford), Halim Kusumaatmaja (Durham).