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Joshua McGraw's research interests focus on soft condensed matter physics, nanofluidics, microfluidics, interfacial dynamics, and pattern formation. This involves polymer physics and fluid dynamics in confined environments, where the topics range from the dependence of thin film properties on preparation conditions to the effects of interfaces on polymer entanglement networks. Investigations also include the dependence of the slip boundary condition on the physical character of the solid-liquid interface, the effects of slip on micro- and nano-fluidic flows, and the effects of geometry and confinement on nanofluidic flows.

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Contact Dependence and Velocity Crossover in Friction between Microscopic Solid/Solid Contacts

McGraw and al., Nano. Lett. 2017


Friction at the nanoscale differs markedly from that between surfaces of macroscopic extent.

Characteristically, the velocity dependence of friction between apparent solid/solid contacts can strongly deviate from the classically assumed velocity independence. Here, we show that a nondestructive friction between solid tips with radius on the scale of hundreds of nanometers and solid hydrophobic self-assembled monolayers has a strong velocity dependence. Specifically, using laterally oscillating quartz tuning forks, we observe a linear scaling in the velocity at the lowest accessed velocities, typically hundreds of micrometers per second, crossing over into a logarithmic velocity dependence.

This crossover is consistent with a general multicontact friction model that includes thermally activated breaking of the contacts at subnanometric elongation. We find as well a strong dependence of the friction on the dimensions of the frictional probe.

Beyond the Navier-de Gennes Paradigm: Slip Inhibition on Ideal Substrates

Ilton and al., Journal of Fluid Mechanics. 2017


Hydrodynamic slip of a liquid at a solid surface represents a fundamental phenomenon in fluid dynamics that governs liquid transport at small scales. For polymeric liquids, de Gennes predicted that the Navier boundary condition together with the theory of polymer dynamics imply extraordinarily

large interfacial slip for entangled polymer melts on ideal surfaces; this prediction was confirmed using dewetting experiments on ultra-smooth, low-energy substrates. Here, we use capillary leveling – surface tension driven flow of films with initially non-uniform thickness – of polymeric films on these same substrates. Measurement of the slip length from a robust one-parameter fit to a lubrication model is achieved. We show that at the lower shear rates involved in leveling experiments

as compared to dewetting ones, the employed substrates can no longer be considered ideal. The data is instead consistent with physical adsorption of polymer chains at the solid/liquid interface.

We extend the Navier-de Gennes description using one additional parameter, namely the density of physically adsorbed chains per unit surface. The resulting model is found to be in excellent agreement with the experimental observations

Morphological evolution of microscopic dewetting droplets with slip

Chan and al., Journal of Fluid Mechanics. 2017


We investigate the dewetting of a droplet on a smooth horizontal solid surface for different slip lengths and equilibrium contact angles. Specifically, we solve for the axisymmetric Stokes flow using the boundary element method with (i) the Navier-slip boundary condition at the solid/liquid boundary and (ii) a time-independent equilibrium contact angle at the contact line. When decreasing the rescaled slip length with respect to the initial central height of the droplet, the typical non-sphericity of a droplet first increases, reaches a maximum at a characteristic rescaled slip length and then decreases. Regarding different equilibrium contact angles, two universal rescalings are proposed to describe the behaviour of the non-sphericity for rescaled slip lengths larger or smaller than . Around , the early time evolution of the profiles at the rim can be described by similarity solutions. The results are explained in terms of the structure of the flow field governed by different dissipation channels: elongational flows for , friction at the substrate for and shear flows for . Following the changes between these dominant dissipation mechanisms, our study indicates a crossover to the quasistatic regime when is many orders of magnitude smaller than  .

Nucleated dewetting in supported ultra-thin liquid films with hydrodynamic slip

Matthias and al., Soft Matter. 2017


This study reveals the influence of the surface energy and solid/liquid boundary condition on the breakup mechanism of dewetting ultra-thin polymer films. Using silane self-assembled monolayers, SiO2 substrates are rendered hydrophobic and provide a strong slip rather than a no-slip solid/liquid boundary condition.

On undergoing these changes, the thin-film breakup morphology changes dramatically – from a spinodal mechanism to a breakup which is governed by nucleation and growth. The experiments reveal a dependence of the hole density on film thickness and temperature. The combination of lowered surface energy and hydrodynamic slip brings the studied system closer to the conditions encountered in bursting unsupported films. As for unsupported polymer films, a critical nucleus size is inferred from a free energy model.

This critical nucleus size is supported by the film breakup observed in the experiments using high speed in situ atomic force microscopy.

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