Steven Armstrong

University of Edinburgh

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Gary G. Wells

University of Edinburgh

Glen McHale

University of Edinburgh

Rodrigo Ledesma‐Aguilar

University of Edinburgh

Halim Kusumaatmaja

Durham University

Shruti Mandhani

Sheffield Hallam University

ÉR

Élfego Ruiz-Gutiérrez

Newcastle University

Emma Carter

Sheffield Hallam University

Kislon Voı̈tchovsky

Durham University

Andrew Alderson

Sheffield Hallam University

Mahya Meyari

University of Edinburgh

Cooperative Institutions

University of Edinburgh

Northumbria University

Newcastle University

Durham University

Centre National de la Recherche Scientifique

Newcastle upon Tyne Hospital

Sheffield Hallam University

Universitat de Barcelona

University of Oxford

Kyushu University

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Papers

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Contact-Angle Hysteresis and Contact-Line Friction on Slippery Liquid-like Surfaces

Langmuir (2020)

Hernán Barrio-Zhang Élfego Ruiz-Gutiérrez Steven Armstrong Glen McHale Gary G. Wells

Contact-line pinning and dynamic friction are fundamental forces that oppose the motion of droplets on solid surfaces. Everyday experience suggests that if a solid surface offers low contact-line pinning, it will also impart a relatively low dynamic friction to a moving droplet. Examples of such surfaces are superhydrophobic, slippery porous liquid-infused, and lubricant-impregnated surfaces. Here, however, we show that slippery omniphobic covalently attached liquid-like (SOCAL) surfaces have a remarkable combination of contact-angle hysteresis and contact-line friction properties, which lead to very low droplet pinning but high dynamic friction against the motion of droplets. We present experiments of the response of water droplets to changes in volume at controlled temperature and humidity conditions, which we separately compare to the predictions of a hydrodynamic model and a contact-line model based on molecular kinetic theory. Our results show that SOCAL surfaces offer very low contact-angle hysteresis, between 1 and 3°, but an unexpectedly high dynamic friction controlled by the contact line, where the typical relaxation time scale is on the order of seconds, 4 orders of magnitude larger than the prediction of the classical hydrodynamic model. Our results highlight the remarkable wettability of SOCAL surfaces and their potential application as low-pinning, slow droplet shedding surfaces.

Hysteresis

10.1021/acs.langmuir.0c02668

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Citations (60)

Tuning contact line dynamics on slippery silicone oil grafted surfaces for sessile droplet evaporation

Scientific Reports (2024)

Astrid Raynard Anam Abbas Steven Armstrong Gary G. Wells Glen McHale

Abstract Controlling the dynamics of droplet evaporation is critical to numerous fundamental and industrial applications. The three main modes of evaporation so far reported on smooth surfaces are the constant contact radius (CCR), constant contact angle (CCA), and mixed mode. Previously reported methods for controlling droplet evaporation include chemical or physical modifications of the surfaces via surface coating. These often require complex multiple stage processing, which eventually enables similar droplet-surface interactions. By leveraging the change in the physicochemical properties of the outermost surface by different silicone oil grafting fabrication parameters, the evaporation dynamics and the duration of the different evaporation modes can be controlled. After grafting one layer of oil, the intrinsic hydrophilic silicon surface (contact angle (CA) ≈ 60°) is transformed into a hydrophobic surface (CA ≈ 108°) with low contact angle hysteresis (CAH). The CAH can be tuned between 1° and 20° depending on the fabrication parameters such as oil viscosity, volume, deposition method as well as the number of layers, which in turn control the duration of the different evaporation modes. In addition, the occurrence and strength of stick–slip behaviour during evaporation can be additionally controlled by the silicone oil grafting procedure adopted. These findings provide guidelines for controlling the droplet-surface interactions by either minimizing or maximising contact line initial pinning, stick–slip and/or constant contact angle modes of evaporation. We conclude that the simple and scalable silicone oil grafted coatings reported here provide similar functionalities to slippery liquid infused porous surfaces (SLIPSs), quasi-liquid surfaces (QLS), and/or slippery omniphobic covalently attached liquid (SOCAL) surfaces, by empowering pinning-free surfaces, and have great potential for use in self-cleaning surfaces or uniform particle deposition.

Silicone oil

Surface Modification

10.1038/s41598-023-50579-2

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Citations (0)

Slippery Liquid-Like Solid Surfaces with Promising Antibiofilm Performance under Both Static and Flow Conditions

ACS Applied Materials & Interfaces (2022)

Yufeng Zhu Glen McHale Jack Dawson Steven Armstrong Gary G. Wells

Biofilms are central to some of the most urgent global challenges across diverse fields of application, from medicine to industries to the environment, and exert considerable economic and social impact. A fundamental assumption in anti-biofilms has been that the coating on a substrate surface is solid. The invention of slippery liquid-infused porous surfaces─a continuously wet lubricating coating retained on a solid surface by capillary forces─has led to this being challenged. However, in situations where flow occurs, shear stress may deplete the lubricant and affect the anti-biofilm performance. Here, we report on the use of slippery omniphobic covalently attached liquid (SOCAL) surfaces, which provide a surface coating with short (ca. 4 nm) non-cross-linked polydimethylsiloxane (PDMS) chains retaining liquid-surface properties, as an antibiofilm strategy stable under shear stress from flow. This surface reduced biofilm formation of the key biofilm-forming pathogens Staphylococcus epidermidis and Pseudomonas aeruginosa by three-four orders of magnitude compared to the widely used medical implant material PDMS after 7 days under static and dynamic culture conditions. Throughout the entire dynamic culture period of P. aeruginosa, SOCAL significantly outperformed a typical antibiofilm slippery surface [i.e., swollen PDMS in silicone oil (S-PDMS)]. We have revealed that significant oil loss occurred after 2-7 day flow for S-PDMS, which correlated to increased contact angle hysteresis (CAH), indicating a degradation of the slippery surface properties, and biofilm formation, while SOCAL has stable CAH and sustainable antibiofilm performance after 7 day flow. The significance of this correlation is to provide a useful easy-to-measure physical parameter as an indicator for long-term antibiofilm performance. This biofilm-resistant liquid-like solid surface offers a new antibiofilm strategy for applications in medical devices and other areas where biofilm development is problematic.

Polydimethylsiloxane

Silicone oil

Biofouling

10.1021/acsami.1c14533

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Citations (54)

Effect of Ageing on the Structure and Properties of Model Liquid-Infused Surfaces

Langmuir (2020)

Sarah J. Goodband Steven Armstrong Halim Kusumaatmaja Kislon Voı̈tchovsky

Liquid-infused surfaces (LISs) exhibit unique properties that make them ideal candidates for a wide range of applications, from antifouling and anti-icing coatings to self-healing surfaces and controlled wetting. However, when exposed to realistic environmental conditions, LISs tend to age and progressively lose their desirable properties, potentially compromising their application. The associated ageing mechanisms are still poorly understood, and results reflecting real-life applications are scarce. Here, we track the ageing of a model LIS composed of glass surfaces functionalized with hydrophobic nanoparticles and infused with silicone oil. The LISs are fully submerged in aqueous solutions and exposed to acoustic pressure waves for set time intervals. The ageing is monitored by periodic measurements of the LIS's wetting properties. We also track the changes to the LIS's nanoscale structure. We find that the LISs rapidly lose their slippery properties because of a combination of oil loss, smoothing of the nanoporous functional layer, and substrate degradation when directly exposed to the solution. The oil loss is consistent with water microdroplets entering the oil layer and displacing oil away from the surface. These mechanisms are general and could play a role in the ageing of most LISs.

Nanoporous

Silicone oil

Biofouling

10.1021/acs.langmuir.0c00059

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Citations (25)

The Effect of Ageing on the Structure and Properties of Model Liquid Infused Surfaces

arXiv (Cornell University) (2020)

Sarah J. Goodband Steven Armstrong Halim Kusumaatmaja Kislon Voı̈tchovsky

Liquid infused surfaces (LIS) exhibit unique properties that make them ideal candidates for a wide range of applications, from anti-fouling and anti-icing coatings to self-healing surfaces and controlled wetting. However, when exposed to realistic environmental conditions, LIS tend to age and progressively lose their desirable properties, potentially compromising their application. The associated ageing mechanisms are still poorly understood, and results reflecting real-life applications are scarce. Here we track the ageing of model LIS composed of glass surfaces functionalized with hydrophobic nanoparticles and infused with silicone oil. The LIS are fully submerged in aqueous solutions and exposed to acoustic pressure waves for set time intervals. The ageing is monitored by periodic measurements of the LIS' wetting properties. We also track changes to the LIS' nanoscale structure. We find that the LIS rapidly lose their slippery properties due to a combination of oil loss, smoothing of the nanoporous functional layer and substrate degradation when directly exposed to the solution. The oil loss is consistent with water microdroplets entering the oil layer and displacing oil away from the surface. These mechanisms are general and could play a role in the ageing of most LIS.

Silicone oil

Nanoporous

Source

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Citations (0)

Suppression of crystallization in saline drop evaporation on pinning-free surfaces

The Journal of Chemical Physics (2023)

A McL Jenkins Gary G. Wells Rodrigo Ledesma‐Aguilar Daniel Orejon Steven Armstrong

For sessile droplets of pure liquid on a surface, evaporation depends on surface wettability, the surrounding environment, contact angle hysteresis, and surface roughness. For non-pure liquids, the evaporation characteristics are further complicated by the constituents and impurities within the droplet. For saline solutions, this complication takes the form of a modified partial vapor pressure/water activity caused by the increasing salt concentration as the aqueous solvent evaporates. It is generally thought that droplets on surfaces will crystallize when the saturation concentration is reached, i.e., 26.3% for NaCl in water. This crystallization is initiated by contact with the surface and is thus due to surface roughness and heterogeneities. Recently, smooth, low contact angle hysteresis surfaces have been created by molecular grafting of polymer chains. In this work, we hypothesize that by using these very smooth surfaces to evaporate saline droplets, we can suppress the crystallization caused by the surface interactions and thus achieve constant volume droplets above the saturation concentration. In our experiments, we used several different surfaces to examine the possibility of crystallization suppression. We show that on polymer grafted surfaces, i.e., Slippery Omniphobic Covalently Attached Liquid-like (SOCAL) and polyethyleneglycol(PEGylated) surfaces, we can achieve stable droplets as low as 55% relative humidity at 25 °C with high reproducibility using NaCl in water solutions. We also show that it is possible to achieve stable droplets above the saturation concentration on other surfaces, including superhydrophobic surfaces. We present an analytical model, based on water activity, which accurately describes the final stable volume as a function of the initial salt concentration. These findings are important for heat and mass transfer in relatively low humidity environments.

Saturation (graph theory)

10.1063/5.0139448

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Citations (6)

Wetting transitions on superhydrophobic auxetic metamaterials

Applied Physics Letters (2023)

Steven Armstrong Glen McHale Andrew Alderson Shruti Mandhani Mahya Meyari

Superhydrophobicity plays a pivotal role in numerous applications. Recently, we have demonstrated the potential of auxetic metamaterials in creating superhydrophobic materials with unique wetting properties. However, the superhydrophobic properties are lost when the liquid penetrates into the surface structure. Understanding the conditions for droplet penetration is crucial for advancing wetting control. Here, we experimentally identify the transition from droplet suspension to full-penetration on an auxetic bowtie/honeycomb lattice membrane. We develop a comprehensive physical model surface representing different states of strain, ranging from auxetic to conventional lattice membranes, and consider the wetting as the liquid surface tension is varied using water/ethanol mixtures. By examining the interplay between contact angle and lattice structure, we gain valuable insights into the conditions for droplet suspension and full-penetration. Additionally, we develop a simple touch test to discern whether a droplet has effectively fully penetrated the structure, providing a practical and efficient means of distinguishing the different wetting states (suspended or partially penetrating vs fully penetrating).

Auxetics

Wetting transition

Penetration (warfare)

Honeycomb structure

Lattice (music)

10.1063/5.0173464

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Citations (2)

Transforming Auxetic Metamaterials into Superhydrophobic Surfaces

Small Structures (2024)

Glen McHale Andrew Alderson Steven Armstrong Shruti Mandhani Mahya Meyari

Superhydrophobic materials are often inspired by nature, whereas metamaterials are engineered to have properties not usually occurring naturally. In both, the key to their unique properties is structure. Here, it is shown that a negative Poisson's ratio (auxetic) mechanical metamaterial can transform into a unique superhydrophobic material. When stretched, its surface has the counterintuitive property that it also expands in the orthogonal lateral direction. The change in the solid surface fraction as strain is applied is modeled, and it is shown that it decreases as the space between solid elements of the auxetic lattice expands. This results in a unique dependence of the superhydrophobicity on strain. Experimental models are constructed to illustrate the relationship between different states of strain and superhydrophobicity as the lattice transitions from an auxetic to a conventional structure. The findings offer a new approach to designing superhydrophobic materials for self‐cleaning surfaces, droplet transportation, droplet encapsulation, and oil–water separation.

Auxetics

Poisson's ratio

Lattice (music)

10.1002/sstr.202300458

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Citations (3)

Evaporation and Electrowetting of Sessile Droplets on Slippery Liquid-Like Surfaces and Slippery Liquid-Infused Porous Surfaces (SLIPS)

Langmuir (2020)

Steven Armstrong Glen McHale Rodrigo Ledesma‐Aguilar Gary G. Wells

Sessile droplet evaporation underpins a wide range of applications from inkjet printing to coating. However, drying times can be variable and contact-line pinning often leads to undesirable effects, such as ring stain formation. Here, we show voltage programmable control of contact angles during evaporation on two pinning-free surfaces. We use an electrowetting-on-dielectric approach and Slippery Liquid-Infused Porous (SLIP) and Slippery Omniphobic Covalently Attached Liquid-Like (SOCAL) surfaces to achieve a constant contact angle mode of evaporation. We report evaporation sequences and droplet lifetimes across a broad range of contact angles from 105°–67°. The values of the contact angles during evaporation are consistent with expectations from electrowetting and the Young-Lippman equation. The droplet contact areas reduce linearly in time, and this provides estimates of diffusion coefficients close to the expected literature value. We further find that the total time of evaporation over the broad contact angle range studied is only weakly dependent on the value of the contact angle. We conclude that on these types of slippery surfaces, droplet lifetimes can be predicted and controlled by the droplet's volume and physical properties (density, diffusion coefficient, and vapor concentration difference to the vapor phase) largely independent of the precise value of contact angle. These results are relevant to applications, such as printing, spraying, coating, and other processes, where controlling droplet evaporation and drying is important.

10.1021/acs.langmuir.0c02020

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Citations (31)

The long cross-over dynamics of capillary imbibition

Journal of Fluid Mechanics (2022)

Élfego Ruiz-Gutiérrez Steven Armstrong Simon Lévêque Célestin Michel Ignacio Pagonabarraga

Spontaneous capillary imbibition is a classical problem in interfacial fluid dynamics with a broad range of applications, from microfluidics to agriculture. Here we study the duration of the cross-over between an initial linear growth of the imbibition front to the diffusive-like growth limit of Washburn's law. We show that local-resistance sources, such as the inertial resistance and the friction caused by the advancing meniscus, always limit the motion of an imbibing front. Both effects give rise to a cross-over of the growth exponent between the linear and the diffusive-like regimes. We show how this cross-over is much longer than previously thought – even longer than the time it takes the liquid to fill the porous medium. Such slowly slowing-down dynamics is likely to cause similar long cross-over phenomena in processes governed by wetting.

Imbibition

Dynamics

Linear growth

10.1017/jfm.2022.248

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Citations (12)