Northumbria University

Glacier flow is an example of a gravity driven non-linear viscous flow at low Reynolds numbers. As a glacier flows over an undulating bed, the surface topography is modified in response. Some information about bed conditions is therefore contained in the shape of the surface and the surface velocity field. I will present theoretical and numerical work on how basal conditions on glaciers affect ice flow, and how one can obtain information about basal conditions through surface-to-bed inversion. I’ll give an overview over inverse methodology currently used in glaciology, and how satellite data is now routinely used to invert for bed properties of the Greenland and the Antarctic Ice Sheets.

What if one desires to have a World perfectly slippery to water? What are the strategies that can be adopted? And how can smart slippery surfaces be created? In this seminar, I will outline approaches to creating slippery surfaces, which all involve reducing or removing droplet contact with the solid, whilst still supporting the droplet. The first concept is to decorate the droplet surface with particles, thus creating liquid marbles and converting the droplet-solid contact into a solid-solid contact. The second concept is to use the Leidenfrost effect to instantly vaporize a layer of water, thus creating a film of vapor and converting the droplet-solid contact into vapor-solid contact. The third concept is to infuse oil into the surface, thus creating a layer of oil and converting the droplet-solid contact into a lubricant-solid contact. I will also explain how we design such to have smart functionality whilst retaining and using the mobility of contact lines and droplets. I will show how Leidenfrost levitation can lead to new types of heat engines [1], how a microsystems approach to the Leidenfrost effect can reduce energy input and lead to a new type of droplet microfluidics [2] (Fig. 1a) and how liquid diodes can be created [3]. I will explain how lubricant impregnated surfaces lead to apparent contact angles [4] and how the large retained footprint of the droplet allows droplet transport and microfluidics using energy coupled via a surface acoustic wave (SAW) [5]. I will argue that droplets confined between reconfigurable slippery boundaries can be continuously translated in an energy invariant manner [6] (Fig. 1b). I will show that a droplet Cheerios effect induced by the menisci arising from structuring the underlying lubricated surface or by droplets in close proximity to each other can be used to guide and position droplets [7] (Fig. 1c). Finally, I will show that active control of droplet spreading by electric field induced control of droplet spreading, via electrowetting or dielectrowetting, can be achieved with little hysteresis [8] and can be a new method to investigate the dewetting of surfaces [9].

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Figure 1 Transportation and positioning of droplets using slippery surfaces: (a) Localized Leidenfrost effect, (b) Reconfigurable boundaries, and (c) Droplet Cheerio’s effect.

Acknowledgements The financial support of the UK Engineering & Physical Sciences Research Council (EPSRC) and Reece Innovation ltd is gratefully acknowledged. Many collaborators at Durham, Edinburgh, Nottingham Trent and Northumbria Universities were instrumental in the work described.

[1] G.G. Wells, R. Ledesma-Aguilar, G. McHale and K.A. Sefiane, Nature Communications, 2015, 6, 6390.

[2] L.E. Dodd, D. Wood, N.R. Geraldi, G.G. Wells, et al., ACS Applied & Materials Interfaces, 2016, 8, 22658.

[3] J. Li, X. Zhou , J. Li, L. Che, J. Yao, G. McHale, et al., Science Advances, 2017, 3, eaao3530.

[4] C. Semprebon, G. McHale, and H. Kusumaatmaja, Soft Matter, 2017, 13, 101.

[5] J.T. Luo, N.R. Geraldi, J.H. Guan, G. McHale, et al., Physical Review Applied, 2017, 7, 014017.

[6] É. Ruiz-Gutiérrez, J.H. Guan, B.B. Xu, G. McHale, et al., Physical Review Letters, 2017, 118, 218003.

[7] J.H. Guan, É. Ruiz-Gutiérrez, B.B. Xu, D. Wood, G. McHale, et al., Soft Matter, 2017, 13, 3404.

[8] Z. Brabcová, G. McHale, G.G. Wells, et al., Applied Physics Letters, 2017, 110, 121603.

[9] A.M.J. Edwards, R. Ledesma-Aguilar, et al., Science Advances, 2016, 2, e1600183

The contact angle of a liquid droplet on a surface can be controlled by making the droplet part of a capacitive structure where the droplet contact area forms one electrode to create an electrowetting-on-dielectric (EWOD) configuration [1]. EWOD introduces a capacitive energy associated with the charging of the solid-liquid interface, in addition to the surface free energy, to allow the contact angle, and hence effective hydrophilicity of a surface, to be controlled using a voltage. However, the substrate must include an electrode coated with a thin, and typically hydrophobic, solid insulating layer and the liquid must be conducting, typically a salt solution, and have a direct electrical contact. In this seminar I show that reversible voltage programmed control of droplet wetting of a surface can be achieved using non-conducting dielectric liquids and without direct electrical contact. The approach is based on non-uniform electric fields generated via interdigitated electrodes and liquid dielectrophoresis to alter the energy balance of a droplet on a solid surface (Fig. 1a,b). Data is shown for thick droplets demonstrating the change in the cosine of the contact angle is proportional to the square of the applied voltage and it is shown theoretically why this equation, similar to that found for EWOD can be expected [2]. I also show that as the droplet spreads and becomes a film, the dominant change in surface free energy to be expected occurs by a wrinkling/undulation of the liquid-vapor interface (Fig. 1c) [3,4]. This type of wrinkle is shown to be a method to create a voltage programmable phase grating [5]. Finally, I argue that dielectrowetting can be used to modify the dynamic contact angle observed during droplet spreading and that this is described by a modified form of the Hoffman-de Gennes law for the relationship between edge speed and contact angle. In this dynamic situation, three distinct regimes can be predicted theoretically and are observed experimentally. These correspond to an exponential approach to equilibrium, a pure Tanner’s law type power law and a voltage determined superspreading power law behavior [6]. 

Acknowledgements

GM acknowledges the contributions of colleagues Professor Carl Brown, Dr. Mike Newton, Dr. Gary Wells and Mr Naresh Sampara at Nottingham Trent University who were central to the development of this work. EPSRC funding under grant EP/E063489/1 is also gratefully acknowledged.

References

[1]   F. Mugele and J.C. Baret, “Electrowetting: From basics to applications”, J. Phys.: Condens. Matt., 2005, 17, R705-R774.

[2]  G. McHale, C.V. Brown, M.I. Newton, G.G. Wells and N. Sampara, “Dielectrowetting driven spreading of droplets”, Phys. Rev. Lett., 2011, 107, art. 186101.

[3]  C.V. Brown, W. Al-Shabib, G.G. Wells, G. McHale and M.I. Newton, “Amplitude scaling of a static wrinkle at an oil-air interface created by dielectrophoresis forces”, Appl. Phys. Lett., 2010,  97, art. 242904.

[4]  C.V. Brown, G. McHale and N.J. Mottram, “Analysis of a static wrinkle on the surface of a thin dielectric liquid layer formed by dielectrophoresis forces”, J. Appl. Phys. 2011, 110 art. 024107.

[5]  C.V. Brown, G. G. Wells, M.I. Newton and G. McHale, “Voltage-programmable liquid optical interface”, Nature Photonics, 2009, 3, 403-405.

[6]  C.V. Brown, G. McHale and N. Sampara, “Voltage induced superspreading of droplets”, submitted (2012)