Imperial College London

DrLiShen

Faculty of EngineeringDepartment of Mechanical Engineering

EPSRC Research Fellow
 
 
 
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l.shen14 CV

 
 
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Location

 

City and Guilds BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

6 results found

Yu M, Shen L, Mutasa T, Dou P, Wu T, Reddyhoff Tet al., 2020, Exact analytical solution to ultrasonic interfacial reflection enabling optimal oil film thickness measurement, Tribology International, Vol: 151, Pages: 1-10, ISSN: 0301-679X

The ultrasonic reflection from a lubricated interface has been widely analyzed to measure fluid film thickness, with different algorithms being applied to overcome measurement accuracy and resolution issues. Existing algorithms use either the amplitude or the phase angle of the ultrasonic interfacial reflection. In this paper, a new algorithm (named the “exact model – complex”) that simultaneously utilizes both the amplitude and the phase of the complex ultrasonic reflection coefficient is proposed and mathematically derived. General procedures for theoretical analysis in terms of measurement accuracy and uncertainty are proposed and applied to the new algorithm, the beneficial features of which (as compared to other existing algorithms) can be summarized as: 1) a direct calculation, instead of an iterative approximation, 2) guaranteed maximum measurement accuracy, and 3) acceptable measurement uncertainty. None of the existing methods have showed this combination of benefits. Moreover, two groups of raw data from previous experimental studies are utilized to further validate the practical feasibility of the new algorithm. Overall, the proposed “exact model – complex” algorithm fully exploits the potential of ultrasonic reflection for oil film thickness measurement, with an accurate and a convenient calculation suited to practical implementation.

Journal article

Shen L, Denner F, Morgan N, Van Wachem B, Dini Det al., 2020, Transient structures in rupturing thin-films: Marangoni-induced symmetry-breaking pattern formation in viscous fluids, Science Advances, Vol: 6, ISSN: 2375-2548

In the minutes immediately preceeding the rupture of a soap bubble,distinctive and repeatable patterns can be observed. These quasi-stabletransient structures are associated with the instabilities of the complexMarangoni flows on the curved thin film in the presence of a surfactantsolution. Here, we report a generalised Cahn-Hilliard-Swift-Hohenberg modelderived using asymptotic theory which describes the quasi-elastic wrinklingpattern formation and the consequent coarsening dynamics in a curvedsurfactant-laden thin film. By testing the theory against experiments on soapbubbles, we find quantitative agreement with the analytical predictions of thenucleation and the early coarsening phases associated with the patterns. Ourfindings provide fundamental physical understanding that can be used to(de-)stabilise thin films in the presence of surfactants and have importantimplications for both natural and industrial contexts, such as the productionof thin coating films, foams, emulsions and sprays.

Journal article

Shen L, Denner F, Morgan N, Wachem BV, Dini Det al., 2019, Transient structures in rupturing thin-films: Marangoni-induced symmetry-breaking pattern formation in viscous fluids

In the minutes immediately preceeding the rupture of a soap bubble,distinctive and repeatable patterns can be observed. These quasi-stabletransient structures are associated with the instabilities of the complexMarangoni flows on the curved thin film in the presence of a surfactantsolution. Here, we report a generalised Cahn-Hilliard-Swift-Hohenberg modelderived using asymptotic theory which describes the quasi-elastic wrinklingpattern formation and the consequent coarsening dynamics in a curvedsurfactant-laden thin film. By testing the theory against experiments on soapbubbles, we find quantitative agreement with the analytical predictions of thenucleation and the early coarsening phases associated with the patterns. Ourfindings provide fundamental physical understanding that can be used to(de-)stabilise thin films in the presence of surfactants and have importantimplications for both natural and industrial contexts, such as the productionof thin coating films, foams, emulsions and sprays.

Working paper

Shen L, Denner F, Morgan N, Van Wachem B, Dini Det al., 2018, Capillary waves with surface viscosity, Journal of Fluid Mechanics, Vol: 847, Pages: 644-663, ISSN: 0022-1120

Experiments over the last 50 years have suggested a tentative correlation between the surface (shear) viscosity and the stability of a foam or emulsion. We examine this link theoretically using small-amplitude capillary waves in the presence of a surfactant solution of dilute concentrations where the associated Marangoni and surface viscosity effects are modelled via the Boussinesq-Scriven formulation. The resulting integro-differential initial value problem is solved analyticallyand surface viscosity is found to contribute an overall damping effect on the amplitude of the capillary wave with varying degrees depending on the lengthscale of the system.Numerically, we find the critical damping wavelength to increase for increasing surface concentration but the rate of increase remains different for both the surface viscosity and the Marangoni effect.

Journal article

Shen L, Denner F, Morgan N, van Wachem B, Dini Det al., 2017, Marangoni effect on small-amplitude capillary waves in viscous fluids, Physical Review E, Vol: 96, Pages: 053110-053110, ISSN: 1539-3755

We derive a general integro-differential equation for the transient behavior of small-amplitude capillary waves on the planar surface of a viscous fluid in the presence of the Marangoni effect. The equation is solved for an insoluble surfactant solution in concentration below the critical micelle concentration undergoing convective-diffusive surface transport. The special case of a diffusion-driven surfactant is considered near the the critical damping wavelength. The Marangoni effect is shown to contribute to the overall damping mechanism, and a first-order term correction to the critical wavelength with respect to the surfactant concentration difference and the Schmidt number is proposed.

Journal article

Shen L, Denner F, Morgan N, Van Wachem B, Dini Det al., 2017, Before the bubble ruptures, Physical Review Fluids, Vol: 2, Pages: 090505-090505, ISSN: 2469-990X

This paper is associated with a video winner of a 2016 APS/DFD Gallery of Fluid Motion Award. The original video is available from the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2016.GFM.V0092

Journal article

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