Paper online (GRL) on permeability of 3D printed granular media

Wei, D., Wang, Z., Pereira, J.M., Gan, Y. (2021) Permeability of Uniformly Graded 3D Printed Granular Media. Geophysical Research Letters, in press. [DOI]

Permeability of 3D Printed Granular Media

The shape of grains can influence the way how water transports inside a granular material. This study uses 3D printing technique to well control grain shapes. The grain morphology is captured through combinations of relative roughness and fractal dimension, relevant to a wide range of geomaterials. We revisit the classical permeability equation with the prior knowledge of the grain shape. It is found that the model coefficients used in the permeability equation, usually obtained through fitting against experiment measurement, do indeed contain key intrinsic morphological information.

Full paper can be downloaded via Publications page, when available.

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Paper online (Powder Tech) on wet granular packing

Wang, Z., Pereira, J.M., Gan, Y. (2020) Packing of wet monodisperse spheres. Powder Technology, in press. [DOI]

Abstract: We experimentally investigated the packing of wet monodisperse spheres with controlled falling height. The packing fraction are found to decrease with smaller grain size and free fall height. A model describing the effects of interparticle force and falling height on packing fraction is developed by introducing a dimensionless length scale, representing the extent of particle rearrangement towards a denser state due to impacts of falling grains. A universal law is observed for both wet particles where capillary forces dominate and dry powders where van der Waals forces govern the packing behaviour. This study deepens the understanding of packing of cohesive spheres and provide a simple experimental method for generating granular media with tailored packing fraction.

Full paper can be downloaded via Publications page, when available.

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Paper accepted (Int J Hydrogen Energy) on droplet dynamics in PEMFC channels

Bao, Y., Gan, Y. (2020) Roughness effects of gas diffusion layers on droplet dynamics in PEMFC flow channels. International Journal of Hydrogen Energy, in press.

Modes of droplet motion on PEMFC channels

Abstract: Water management remains one of the major challenges in optimising the performance of PEMFCs, in which liquid accumulation and removal in gas diffusion layers (GDLs) and flow channels should be addressed. Here, effects of GDL surface roughness on the water droplet removal inside a PEMFC flow channel have been investigated using the Volume of Fluid method. Rough surfaces are generated according to realistic GDL properties by incorporating RMS roughness and roughness wavelength as the main characteristic parameters. Droplet dynamics including emergence, growth, detachment, and removal in flow channels with various airflow rates are simulated on rough substrates. The influences of airflow rate on droplet dynamics are also discussed by comparing the detachment time and droplet morphology. The liquid removal efficiency subject to different surface roughness parameters is evaluated by droplet detachment time and elongation, and regimes of detachment modes are identified based on the droplet breakup location and detachment ratio. The results suggest that rough surfaces with higher RMS roughness can facilitate the removal of liquid inside flow channel. Whilst surface roughness wavelength is found less significant to the liquid removal efficiency. The results here provide qualitative assessments on identifying the key surface characteristics controlling droplet motion in PEMFC channels.

Full paper can be downloaded via Publications page, when available.

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Paper accepted (Adv. Mat. Interfaces) on droplet motion on wedges

Wang, Z., Owais, A., Neto, C., Pereira, J.M., Gan, Y. (2020) Enhancing Spontaneous Droplet Motion on Structured Surfaces with Tailored Wedge Design. Advanced Materials Interfaces, in press.

Abstract: Spontaneous liquid transport has a wide variety of applications, including fog harvesting, microfluidics, and water-oil separation. Understanding of the droplet movement dynamics on structured surfaces is essential for enhancing the transport performance. In this work, we develop a theoretical model describing the movement process of droplets on surfaces with prescribed wedge shapes. Agreement is observed between the predictions from the model and experimental results. Through theoretical analysis and quantitative comparison between the transport performance of different wedge shapes, we identify the factors affecting the movement process and provide guidelines for wedge shape optimisation for spontaneous droplet transport.

Full paper can be downloaded via Publications page, when available.

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Paper accepted (Langmuir) on wetting transition in porous media

Wang, Z; Pereira, J.M.; Gan, Y. (2020) Effect of wetting transition during multiphase displacement in porous media. Langmuir, in press.

The effect of wetting transition at the pore-scale on the flow pattern inside porous media.

Abstract: The effects of wettability on multiphase displacement in porous media have been studied extensively in the past, and the contact angle is identified as an important factor influencing the displacement patterns. At the same time, it has been found that effective contact angle can vary drastically in a time-dependent manner on rough surfaces due to Cassie-Wenzel wetting transition. In this study, we develop a theoretical model at the pore scale describing the apparent contact angle on rough interface as a function of time. The theory is then incorporated into the Lattice Boltzmann method for simulation of multiphase displacement in disordered porous media. A dimensionless time ratio, Dy, describing the relative speed of wetting transition and pore invasion is defined. We show that the displacement patterns can be significantly influenced by Dy, where more trapped defending ganglia are observed at large Dy, leading to lower displacement efficiency. We investigate the mobilization of trapped ganglia through identifying different mobilization dynamics during displacement, including translation, coalescence, and fragmentation. Agreement is observed between the mobilization statistics and the total pressure gradient across a wide range of Dy. Understanding the effect of wetting transition during multiphase displacement in porous media is of importance for applications such as carbon geosequestration and oil recovery, especially for porous media where solid surface roughness cannot be neglected.

Full paper can be downloaded via Publications page, when available.

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Paper accepted (Chem Eng Sci) on LBM-PNM for immiscible flow

Suo, S., Liu, M. and Gan, Y. (2020) An LBM-PNM framework for immiscible flow: With applications to droplet spreading on porous surfaces. Chemical Engineering Science, in press. [DOI]

Droplet dynamics of spreading on and imbibition into porous surfaces with different tip angles.

Abstract: The behaviour of droplets on porous media, combining the spreading above and imbibition into the medium, is of foundational interests for a series of applications. In this work, we combine the lattice Boltzmann method (LBM) and pore-network method (PNM) to develop an efficient and robust numerical framework in which the coarse-scale multiphase transport in porous media are modelled by the PNM while the fluid dynamics containing multiple components are resolved by the LBM. An effective transport mechanism has been implemented to ensure mass conservation and pressure continuity across the interface. This method is ideally adapted to flow domains, with two distinguishable characteristic length-scales. The numerical framework is validated by comparison with experimental observations of droplets on flat porous surfaces. Moreover, the behaviour of droplets on non-flat porous surfaces has been also investigated, demonstrating the influence of surface curvature on the competing mechanisms of droplet spreading and imbibition.

Full paper can be downloaded via Publications page, when available.

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Paper accepted (Phys Rev Fluids) on flow in hierarchical porous media

Suo, S., Liu, M. and Gan, Y. (2020) Fingering patterns in hierarchical porous media. Physical Review Fluids, in press.

Three main modes demonstrating different flow patterns due to the existence of second order porous structures.

Abstract: Porous media with hierarchical structures are commonly encountered in both natural and synthetic materials, e.g., fractured rock formations, porous electrodes and fibrous materials, which generally consist of two or more distinguishable levels of pore structure with different characteristic lengths. The multiphase flow behaviours in hierarchical porous media have remained elusive. In this study, we investigate the influences of hierarchical structures in porous media on the dynamics of immiscible fingering during fluid-fluid displacement. Divided by the breakthrough, such displacement process includes pre- and post-breakthrough stages during which the fingering evolution is dominated by viscous and capillary effects, respectively. Through conducting a series of numerical simulations, we found that the immiscible fingering can be suppressed due to the existence of secondary porous structures. To characterise the fingering dynamics in hierarchical porous media, a phase diagram, which describes the switch among the three fingering modes (the suppressing, crossover and dendrite mode), is constructed by introducing a scaling parameter, i.e., the ratio of time scales considering the combined effect of characteristic pore sizes and wettability. The findings presented in this work provide a basis for further research on the application of hierarchical porous media for controlling immiscible fingerings.

Full paper can be downloaded via Publications page, when available.

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Paper online (Int J Solids and Structures) on rough sphere contacts

Wei, D., Zhai, C., Hanaor, D., Gan, Y. (2020) Contact behaviour of simulated rough spheres generated with spherical harmonic. International Journal of Solids and Structures, in press. [DOI]

Visualisation of contact areas highlighting the importance of fractal dimension and surface roughness on contact mechanics, deviating from classic Hertz solution (indicated as the dashed red circle).

Abstract: Normal contact behaviour between non-adhesive fractal rough particles is studied using a finite element method (FEM). A series of spherical grain surfaces with distinguished roughness features are generated by means of Spherical Harmonics. These surfaces are described by two roughness descriptors, namely, relative roughness (Rr) and fractal dimension (FD). The contact behaviour of rough spheres with a rigid flat surface is simulated using FEM to quantify the influences of surface structure and sphere morphology by focusing on contact stiffness and true contact area. The dependence of normal contact stiffness (k) on applied normal force (F) is found to follow a power law (k = αFβ) over four orders of magnitude, with both α and β being highly correlated with Rr and FD. With increasing load, the power exponent converges to that of Hertzian contact, e.g., 1/3, independent of Rr. Regions of true contact evolve through the formation of new microcontacts and their progressive merging, meanwhile the area distributions of contact island induced by various forces tend to obey similar Weibull distributions due to fractal nature in their surfaces. Contacts with larger values of Rr are found to produce contact contours with higher fractal dimension as calculated by a 2D box-counting method. Our results suggest that the correlation between radial lengths in a quasi-spherical particle should be considered in studying contact behaviour.

Full paper can be downloaded via Publications page, when available.

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Dr Gan is visiting Oxford Mathematical Institute

From December 2019 to February 2020, Dr Gan will be visiting Mathematical Institute at the University of Oxford, as a part of his sabbatical leave. He will be working with Professor Dominic Vella on bio-inspired structure design for spontaneous directional liquid transport, a research project funded by Endeavour Executive Leadership Awards.

Project background: Harvesting water from atmospheric humidity is a promising solution for addressing freshwater scarcity across the globe. Such systems require functionalities such as condensing, collecting and transporting of water droplets from humid air. Several lessons learned from studying natural systems, such as plants (e.g., pegged rhizoids in liverworts) and insects (e.g., elytra of Namib Dessert beetles), are now being used for designing components for spontaneous directional transport (SDT) of liquid. Recent innovation has strived to bridge mismatch in mechanics between the engineered and natural worlds. Current design concepts focus on wettability patterns and structural features (e.g., corner flow and edge pinning). However, large-scale applications hinder on many aspects, such as lack of consistent theoretical framework and limited system scalability and durability, demanding better engineering solutions. This project aims to provide better engineering solutions for design and employment of functionalised spontaneous directional liquid transport systems.

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Ms Verena Becker is visiting the lab

Ms Verena Becker, a second year PhD student at Karlsruhe Institute of Technology (KIT), Germany, is visiting the lab at The University of Sydney. Verena is working on the numerical modelling lithium-ion battery systems, with special focus on the effective properties and the particle shape. She will stay with us for two months (from May until the end of July) and working on modelling electro-mechanical properties of battery systems.

Group photo in front of The Quadrangle at The University of Sydney, 28/05/2019. 
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Paper accepted (Eng Fracture Mech) on non-spherical particle breakage

Wei, D., Zhao, B., Dias-da-Costa, D., Gan, Y. (2019) An FDEM study of particle breakage under rotational point loading. Engineering Fracture Mechanics. In press. [DOI]

Abstract: The most commonly adopted method to test the strength of single sand particles is based on platen experiments. This setup tends to align the loading direction towards the particle minimum axis and provide an upper limit for the breakage stress. This paper numerically bypasses such limitation by using a combined finite and discrete element method (FDEM). FDEM was first validated via a mesh size analysis of a spherical particle and calibrated by in-situ experimental compressions of the single quartz sand particle, where the particle shape was obtained by X-ray micro-computed tomography (XCT) and then imported into the numerical model. Systematic point loading tests were recreated to explore the role of the curvature at contacting points on the breakage behaviour. The simulations allow to probe the same non-spherical particles, i.e., realistic quartz sand and ellipsoid particles, with multiple measurements highlighting the importance of the loading direction, which was inaccessible experimentally. Results show that FDEM can capture not only the crack initiation but also fracture patterns, while taking into account realistic shapes. It is found that the distance between two contact points and their combined curvedness reflecting the particle morphology are the two major factors governing fracture patterns and stresses. When loading is roughly parallel to the minimum principal dimension of particles, the obtained breakage stress and the number of fragments approach the upper limits.

Full paper can be downloaded via Publications page, when available.

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Endeavour Executive Leadership Awards

Dr Gan is among the recipients of the recently announced Endeavour Executive Leadership Awards, supported by Australian Government Department of Education and Training. This award will enable him to work with Professor Dominic Vella at the Mathematical Institute in the University of Oxford, on project related to spontaneous directional transport (SDT) of liquids. This collaboration will aim to develop engineering solutions towards harvesting water from atmospheric humidity for addressing freshwater scarcity across the globe.

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Paper accepted (Phys Rev Fluids) on topological disorder of porous media

Wang, Z., Chauhan, K., Pereira, J.M., Gan, Y. (2019) Disorder characterization of porous media and its effect on fluid displacement. Physical Review Fluids. In press.

Abstract: We investigate the effects of topological disorder and wettability on fluid displacement in porous media. A modified disorder index Iv is proposed to characterize the disorder of porous media. By changing Iv, different displacement patterns (stable displacement and fingering) under the same flow condition and fluid property are obtained. We analytically demonstrate how increase in disorder promotes fingering due to uneven distribution of local capillary pressure. It is shown that the displacement efficiency for different wettability conditions and disorder well correlates with the distribution of local capillary pressure. A power law relation between fluid-fluid interfacial length and saturation of invading fluid is proposed by taking geometry into account, where the parameters in power law relation can be predicted by the capillary index, Ic, unifying the effects of topological disorder and wettability.

Full paper can be downloaded via Publications page.

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Funded Australia-India Fellowship

The Australian Academy of Science has announced the successful recipients of the Australia–India Strategic Research Fund (AISRF) Early- and Mid-Career Researcher (EMCR) 2019 Fellowships. Dr Yixiang Gan was named among 11 recipients from different universities in Australia. He will be working with colleagues from IIT Madras and IIT Bhubaneswar on “Optimising thermal energy storage with phase change materials: with applications to solar energy storage”.

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Paper online (JCIS) on designing porous media for capillary flow

Liu, M., Suo, S., Wu, J., Gan, Y. Hanaor, D.A.H., Chen, C.Q. (2019) Tailoring porous media for controllable capillary flow. Journal of Colloid and Interface Science. 539:379-387.

Abstract: Control of capillary flow through porous media has broad practical implications. However, achieving accurate and reliable control of such processes by tuning the pore size or by modification of interface wettability remains challenging. Here we propose that the liquid flow by capillary penetration can be accurately adjusted by tuning the geometry of porous media. On the basis of Darcy’s law, a general framework is proposed to facilitate the control of capillary flow in porous systems by tailoring the geometric shape of porous structures. A numerical simulation approach based on finite element method is also employed to validate the theoretical prediction. A basic capillary component with a tunable velocity gradient is designed according to the proposed framework. By using the basic component, two functional capillary elements, namely, (i) flow accelerator and (ii) flow resistor, are demonstrated. Then, multi-functional fluidic devices with controllable capillary flow are realized by assembling the designed capillary elements. All the theoretical designs are validated by numerical simulations. Finally, it is shown that the proposed concept can be extended to three-dimensional design of porous media.

Full paper can be downloaded via Publications page.

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Paper accepted (AMS) on SPH modelling of contact angle dynamics

Bao, Y., Li, L., Shen, L., Lei, C., Gan, Y. (2019) A modified smoothed particle hydrodynamics approach for modelling dynamic contact angle hysteresis. Acta Mechanica Sinica. In press.

Dynamic wetting plays an important role in the physics of multiphase flow, and has significant influence on many industrial and geotechnical applications. In this work, a modified smoothed particle hydrodynamics (SPH) model is employed to simulate surface tension, contact angle, and dynamic wetting effects at meso-scale. The wetting and dewetting phenomena are simulated in a capillary tube, where the liquid particles are raised or withdrawn by a shifting substrate. The SPH model is modified by introducing a newly-developed viscous force formulation at liquid-solid interface to reproduce the rate-dependent behaviour of moving contact line. Dynamic contact angle simulations with interfacial viscous force are conducted to verify the effectiveness and accuracy of this new formulation. In addition, the influence of interfacial viscous force with different magnitude on contact angle dynamics is examined by empirical power law correlations, and the derived constants suggest the dynamic contact angle changes monotonically with interfacial viscous force. The simulation results are consistent with the experimental observations and theoretical predictions, implying that the interfacial viscous force can be associated with slip length of flow and microscopic surface roughness. This work has demonstrated that the modified SPH model can successfully account for the rate-dependent effects of moving contact line, and can be used for realistic multiphase flow simulation under dynamic conditions.

Full paper can be downloaded via Publications page.

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Paper accepted (IJSS) on effective properties of fluid-filled porous media

Liu, M., Wu, J., Gan, Y. Hanaor, D.A.H., Chen, C.Q. (2019) Multiscale modeling of the effective elastic properties of fluid-filled porous materials. International Journal of Solids and Structures. In press. [DOI]

Fluid-filled porous materials are widely encountered in natural and artificial systems. A comprehensive understanding of the elastic behavior of such materials and its dependence on fluid diffusion is therefore of fundamental importance. In this work, a multiscale framework is developed to model the overall elastic response of fluid-filled porous materials. By utilizing a two-dimensional micromechanical model with porosity at two scales, the effects of fluid diffusion and the geometric arrangement of pores on the evolution of effective properties in fluid-filled porous materials are investigated. Initially, for a single-porosity model the effective elastic properties of the dry and fluid-filled porous materials with ordered pores are obtained theoretically by considering a geometrical factor, which is related to the distribution of pores in the matrix. Model predictions are validated by finite element simulations. By employing a double-porosity model, fluid diffusion from macro- to micro-scale pores driven by a pressure gradient is investigated, and the resulting time-dependent effective elastic properties are obtained for both constant pressure and constant injection rate conditions. It is found that the presence and diffusion of pressurized pore fluid significantly affect the elastic response of porous materials, and this must be considered when modeling such materials. It is expected that the proposed theoretical model will advance the understanding of the fluid-governed elastic response of porous materials with implications towards the analysis of geophysical, biological and artificial fluid-filled porous systems.

Full paper can be downloaded via Publications page.

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PostDoc Opening in Clay Modelling

A postdoctoral research associate in modelling of clay behavior has been advertised in the School of Civil Engineering, Faculty of Engineering at the University of Sydney. The official advertisement by The University of Sydney can be found via

About the opportunity

The School of Civil Engineering is seeking a Postdoctoral Research Associate that will help to develop new formulations and computer codes for the modelling of fracturing and healing of clay and will use the codes to simulate the behaviour of clay material under cycles of wetting and drying in geotechnical and geoenvironmental engineering applications.

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Funded Australia-Germany joint research project

Our team will be funded under Australia-Germany Joint Research Cooperation Scheme between Universities Australia and German Academic Exchange Service (DAAD). The project will enable us to work closely with the research team from Karlsruhe Institute of Technology on multi-scale modelling of Li-ion batteries in the coming 2 years (2019-2020). The project title is “A microstructure informed model for effective properties in Li-ion batteries”, including seven exchange trips between two research teams.

Media release by Universities Australia.

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Congratulations to Si Suo for the poster award

Last week, Mr Si Suo won the second prize for Student Research Poster Event 2018 at School of Civil Engineering. Si is a first year PhD student working on multiphase flow in heterogeneous porous media.

Congratulations to Si!

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