Dr Ian Williams

Research Fellow

Academic and research departments

Soft Matter Group, School of Mathematics and Physics.


Areas of specialism

Soft Matter; Microscopy; Optical Tweezers; Colloids; Diffusiophoresis; Interfacial Rheology


Research interests


Ian Williams, Sangyoon Lee, Azzurra Apriceno, Richard P. Sear & Giuseppe Battaglia (2020) Diffusioosmotic and convective flows induced by a nonelectrolyte concentration gradient. PNAS 117 (41) 25263-25271

Glucose is an important energy source in our bodies, and its consumption results in gradients over length scales ranging from the subcellular to entire organs. Concentration gradients can drive material transport through both diffusioosmosis and convection. Convection arises because concentration gradients are mass density gradients. Diffusioosmosis is fluid flow induced by the interaction between a solute and a solid surface. A concentration gradient parallel to a surface creates an osmotic pressure gradient near the surface, resulting in flow. Diffusioosmosis is well understood for electrolyte solutes, but is more poorly characterized for nonelectrolytes such as glucose. We measure fluid flow in glucose gradients formed in a millimeter-long thin channel and find that increasing the gradient causes a crossover from diffusioosmosis-dominated to convection-dominated flow. We cannot explain this with established theories of these phenomena which predict that both scale linearly. In our system, the convection speed is linear in the gradient, but the diffusioosmotic speed has a much weaker concentration dependence and is large even for dilute solutions. We develop existing models and show that a strong surface–solute interaction, a heterogeneous surface, and accounting for a concentration-dependent solution viscosity can explain our data. This demonstrates how sensitive nonelectrolyte diffusioosmosis is to surface and solution properties and to surface–solute interactions. A comprehensive understanding of this sensitivity is required to understand transport in biological systems on length scales from micrometers to millimeters where surfaces are invariably complex and heterogeneous.

Ian Williams, Joseph A. Zasadzinski & Todd M. Squires (2019) Interfacial rheology and direct imaging reveal domain-templated network formation in phospholipid monolayers penetrated by fibrinogen. Soft Matter, 15, 9076-9084

Phospholipids are found throughout the natural world, including the lung surfactant (LS) layer that reduces pulmonary surface tension and enables breathing. Fibrinogen, a protein involved in the blood clotting process, is implicated in LS inactivation and the progression of disorders such as acute respiratory distress syndrome. However, the interaction between fibrinogen and LS at the air–water interface is poorly understood. Through a combined microrheological, confocal and epifluorescence microscopy approach we quantify the interfacial shear response and directly image the morphological evolution when a model LS monolayer is penetrated by fibrinogen. When injected into the subphase beneath a monolayer of the phospholipid dipalmitoylphosphatidylcholine (DPPC, the majority component of LS), fibrinogen preferentially penetrates disordered liquid expanded (LE) regions and accumulates on the boundaries between LE DPPC and liquid condensed (LC) DPPC domains. Thus, fibrinogen is line active. Aggregates grow from the LC domain boundaries, ultimately forming a percolating network. This network stiffens the interface compared to pure DPPC and imparts the penetrated monolayer with a viscoelastic character reminiscent of a weak gel. When the DPPC monolayer is initially compressed beyond LE–LC coexistence, stiffening is significantly more modest and the penetrated monolayer retains a viscous-dominated, DPPC-like character.

Chih-Cheng Chang, Ian Williams, Arash Nowbahar, Vincent Mansard, Jodi Mecca, Kathryn A. Whitaker, Adam K. Schmitt, Christopher J. Tucker, Tom H. Kalantar, Tzu-Chi Kuo, & Todd M. Squires (2019) Effect of Ethylcellulose on the Rheology and Mechanical Heterogeneity of Asphaltene Films at the Oil–Water Interface. Langmuir, 35, 29, 9374–9381

Asphaltenes are surface-active molecules that exist naturally in crude oil. They adsorb at the water–oil interface and form viscoelastic interfacial films that stabilize emulsion droplets, making water–oil separation extremely challenging. There is, thus, a need for chemical demulsifiers to disrupt the interfacial asphaltene films, and, thereby, facilitate water–oil separation. Here, we examine ethylcellulose (EC) as a model demulsifier and measure its impact on the interfacial properties of asphaltene films using interfacial shear microrheology. When EC is mixed with an oil and asphaltene solution, it retards the interfacial stiffening that occurs between the oil phase in contact with a water phase. Moreover, EC introduces relatively weak regions within the film. When EC is introduced to a pre-existing asphaltene film, the stiffness of the films decreases abruptly and significantly. Direct visualization of interfacial dynamics further reveals that EC acts inhomogeneously, and that relatively soft regions in the initial film are seen to expand. This mechanism likely impacts emulsion destabilization and provides new insight to the process of demulsification.

Chih-Cheng Chang, Arash Nowbahar, Vincent Mansard, Ian Williams, Jodi Mecca, Adam K. Schmitt, Tom H. Kalantar, Tzu-Chi Kuo, & Todd M. Squires (2018) Interfacial Rheology and Heterogeneity of Aging Asphaltene Layers at the Water–Oil Interface. Langmuir, 34, 19, 5409–5415

Surface-active asphaltene molecules are naturally found in crude oil, causing serious problems in the petroleum industry by stabilizing emulsion drops, thus hindering the separation of water and oil. Asphaltenes can adsorb at water–oil interfaces to form viscoelastic interfacial films that retard or prevent coalescence. Here, we measure the evolving interfacial shear rheology of water–oil interfaces as asphaltenes adsorb. Generally, interfaces stiffen with time, and the response crosses over from viscous-dominated to elastic-dominated. However, significant variations in the stiffness evolution are observed in putatively identical experiments. Direct visualization of the interfacial strain field reveals significant heterogeneities within each evolving film, which appear to be an inherent feature of the asphaltene interfaces. Our results reveal the adsorption process and aged interfacial structure to be more complex than that previously described. The complexities likely impact the coalescence of asphaltene-stabilized droplets, and suggest new challenges in destabilizing crude oil emulsions.

Ian Williams & Todd M. Squires (2018) Evolution and mechanics of mixed phospholipid fibrinogen monolayers. Journal of the Royal Society Interface, 15, 20170895

All mammals depend on lung surfactant (LS) to reduce surface tension at the alveolar interface and facilitate respiration. The inactivation of LS in acute respiratory distress syndrome (ARDS) is generally accompanied by elevated levels of fibrinogen and other blood plasma proteins in the alveolar space. Motivated by the mechanical role fibrinogen may play in LS inactivation, we measure the interfacial rheology of mixed monolayers of fibrinogen and dipalmitoylphosphatidylcholine (DPPC), the main constituent of LS, and compare these to the single species monolayers. We find DPPC to be ineffective at displacing preadsorbed fibrinogen, which gives the resulting mixed monolayer a strongly elastic shear response. By contrast, how effectively a pre-existing DPPC monolayer prevents fibrinogen adsorption depends upon its surface pressure. At low DPPC surface pressures, fibrinogen penetrates DPPC monolayers, imparting a mixed viscoelastic shear response. At higher initial DPPC surface pressures, this response becomes increasingly viscous-dominated, and the monolayer retains a more fluid, DPPC-like character. Fluorescence microscopy reveals that the mixed monolayers exhibit qualitatively different morphologies. Fibrinogen has a strong, albeit preparation-dependent, mechanical effect on phospholipid monolayers, which may contribute to LS inactivation and disorders such as ARDS.

Ian Williams, Francesco Turci, James E. Hallett, Peter Crowther, Chiara Cammarota, Giulio Biroli & C. Patrick Royall (2018) Experimental determination of configurational entropy in a two-dimensional liquid under random pinning. J. Phys.: Condens. Matter 30 094003

A quasi two-dimensional colloidal suspension is studied under the influence of immobilisation (pinning) of a random fraction of its particles. We introduce a novel experimental method to perform random pinning and, with the support of numerical simulation, we find that increasing the pinning concentration smoothly arrests the system, with a cross-over from a regime of high mobility and high entropy to a regime of low mobility and low entropy. At the local level, we study fluctuations in area fraction and concentration of pins and map them to entropic structural signatures and local mobility, obtaining a measure for the local entropic fluctuations of the experimental system.

Anirudha Banerjee, Ian Williams, Rodrigo Nery Azevedo, Matthew E. Helgeson, & Todd M. Squires (2016) Soluto-inertial phenomena: Designing long-range, long-lasting, surface-specific interactions in suspensions. PNAS, 113 (31) 8612-8617

Liquid suspensions of micron-scale particles and drops play a ubiquitous role in a broad spectrum of materials of central importance to modern life. A suite of interactions has long been known and exploited to formulate such suspensions; however, all such interactions act over less than a micron in water—and often much less. Here we present a concept to design and engineer nonequilibrium interactions in suspensions, which are particle surface-dependent, may last for hundreds of seconds, and extend hundreds of times farther than is currently possible. The conceptual versatility of the results presented here suggests new capabilities for manipulating suspensions, sorting particles, and synthesizing novel materials and particles.

Ian Williams, Erdal C. Oğuz, Thomas Speck, Hartmut Löwen & C. Patrick Royall (2016) Transmission of Torque at the Nanoscale. Nature Physics volume 12, 98–103

In macroscopic mechanical devices, torque is transmitted through gearwheels and clutches. In the construction of devices at the nanoscale, torque and its transmission through soft materials will be a key component. However, this regime is dominated by thermal fluctuations leading to dissipation. Here we demonstrate the principle of torque transmission for a disc-like colloidal assembly exhibiting clutch-like behaviour, driven by 27 particles in optical traps. These are translated on a circular path to form a rotating boundary that transmits torque to additional particles confined to the interior. We investigate this transmission and find that it is determined by solid-like or fluid-like behaviour of the device and a stick–slip mechanism reminiscent of macroscopic gearwheels slipping. The transmission behaviour is predominantly governed by the rotation rate of the boundary and the density of the confined system. We determine the efficiency of our device and thus optimize conditions to maximize power output.

Ian Williams, Erdal C. Oğuz, Paul Bartlett, Hartmut Löwen & C. Patrick Royall (2015) Flexible confinement leads to multiple relaxation regimes in glassy colloidal liquids. J. Chem. Phys. 142, 024505

Understanding relaxation of supercooled fluids is a major challenge and confining such systems can lead to bewildering behaviour. Here, we exploit an optically confined colloidal model system in which we use reduced pressure as a control parameter. The dynamics of the system are “Arrhenius” at low and moderate pressure, but at higher pressures relaxation is faster than expected. We associate this faster relaxation with a decrease in density adjacent to the confining boundary due to local ordering in the system enabled by the flexible wall.

Andrew T. Gray, Elizabeth Mould, C. Patrick Royall & Ian Williams (2015) Structural characterisation of polycrystalline colloidal monolayers in the presence of aspherical impurities. J. Phys.: Condens. Matter 27 194108

Impurities in crystalline materials introduce disorder into an otherwise ordered structure due to the formation of lattice defects and grain boundaries. The properties of the resulting polycrystal can differ remarkably from those of the ideal single crystal. Here we investigate a quasi-two-dimensional system of colloidal spheres containing a small fraction of aspherical impurities and characterise the resulting polycrystalline monolayer. We find that, in the vicinity of an impurity, the underlying hexagonal lattice is deformed due to a preference for five-fold co-ordinated particles adjacent to impurities. This results in a reduction in local hexagonal ordering around an impurity. Increasing the concentration of impurities leads to an increase in the number of these defects and consequently a reduction in system-wide hexagonal ordering and a corresponding increase in entropy as measured from the distribution of Voronoi cell areas. Furthermore, through both considering orientational correlations and directly identifying crystalline domains we observe a decrease in the average polycrystalline grain size on increasing the concentration of impurities. Our data show that, for the concentrations considered, local structural modifications due to the presence of impurities are independent of their concentration, while structure on longer lengthscales (i.e. the size of polycrystalline grains) is determined by the impurity concentration.

Ian Williams, Erdal C. Oğuz, Robert L. Jack, Paul Bartlett, Hartmut Löwen & C. Patrick Royall (2014) The effect of boundary adaptivity on hexagonal ordering and bistability in circularly confined quasi hard discs. J. Chem. Phys. 140, 104907

The behaviour of materials under spatial confinement is sensitively dependent on the nature of the confining boundaries. In two dimensions, confinement within a hard circular boundary inhibits the hexagonal ordering observed in bulk systems at high density. Using colloidal experiments and Monte Carlo simulations, we investigate two model systems of quasi hard discs under circularly symmetric confinement. The first system employs an adaptive circular boundary, defined experimentally using holographic optical tweezers. We show that deformation of this boundary allows, and indeed is required for, hexagonal ordering in the confined system. The second system employs a circularly symmetric optical potential to confine particles without a physical boundary. We show that, in the absence of a curved wall, near perfect hexagonal ordering is possible. We propose that the degree to which hexagonal ordering is suppressed by a curved boundary is determined by the “strictness” of that wall.

Ian Williams, Erdal C. Oğuz, Paul Bartlett, Hartmut Löwen & C. Patrick Royall (2013) Direct measurement of osmotic pressure via adaptive confinement of quasi hard disc colloids. Nature Communications volume 4, Article number: 2555

Confining a system in a small volume profoundly alters its behaviour. Hitherto, attention has focused on static confinement where the confining wall is fixed such as in porous media. However, adaptive confinement where the wall responds to the interior has clear relevance in biological systems. Here we investigate this phenomenon with a colloidal system of quasi hard discs confined by a ring of particles trapped in holographic optical tweezers, which form a flexible elastic wall. This elasticity leads to quasi-isobaric conditions within the confined region. By measuring the displacement of the tweezed particles, we obtain the radial osmotic pressure. We further find a novel bistable state of a hexagonal structure and concentrically layered fluid mimicking the shape of the confinement. The hexagonal configurations are found at lower pressure than those of the fluid, thus the bistability is driven by the higher entropy of disordered arrangements, unlike bulk hard systems.