A quantitative framework for the formation of liquid marbles and hollow granules from hydrophobic powders

Liquid marbles are micro reservoirs of fluid surrounded by a powder shell. Their unique properties show promise for high-value technological applications in the medical, biotechnology, chemical and pharmaceutical industries. In this study, liquid marbles were prepared from six mixtures of water and glycerol by releasing droplets from different heights onto a loosely packed powder bed using a 1 mL syringe. Pictures of the liquid marbles were taken with a stereo microscope (SMZ series) with a 3MP Motic camera. The powders used were Aerosil R202, hydrophobic glass ballotini and PTFE spheres. In this paper, our previous proposed qualitative framework for liquid marble formation via solid spreading coefficient mechanism was updated in light of recent new research and the quantitative criteria for each step was established and validated. The last step of the framework was modified to reflect the physical flow mechanism now understood to be responsible for liquid marble formation. The revised framework presents the step by step process and quantitative criteria for liquid marble formation via a preformed droplet template. In addition, new frameworks for the formation of liquid marbles via the mechanical dispersion regime and for the formation of hollow granules were also developed and validated. These frameworks give a better understanding of the mechanisms controlling the granulation of hydrophobic particles to form liquid marbles and provide guidance on how to mass produce liquid marbles and hollow granules

Liquid marble formation: Spreading coefficients or kinetic energy?

A liquid marble is a network of self-assembled hydrophobic powder around a droplet. The mechanism and driving force leading to the formation of liquid marbles have not been investigated. In this study, the solid- liquid spreading coefficient (ëSL) is calculated and the effect of the impact of kinetic energy on liquid marble formation for various fluids and particles is investigated. Single drops of fluid were produced using a syringe and released from different heights onto loosely packed powder bed. The degree of powder coverage over liquid droplet after impact was photographed and analyzed using image analysis. The results show that the spreading coefficients do not predict liquid marble formation, but instead that powder coverage of the drop is proportional to the applied kinetic energy. As the kinetic energy is increased, the percentage of coverage of liquid droplet by powder increases, and as the particle size decreases the percentage of coverage also increases. These results demonstrate that good powder coverage is assisted by increasing the kinetic energy of impact, which increases the size of the initial fluid-powder contact area and causes internal fluid flow within the droplet during impact and rebound, which entrains the particles and forms the powder shell. The knowledge that the level of agitation applied is an important factor in whether liquid marble is successfully produced, is expected to facilitate progress in creating liquid marbles as precursors to a wide range of structured powder-liquid products in cosmetics, pharmaceuticals and other advanced materials

The laminar/turbulent transition in a sludge pipeline

Globally, wastewater treatment plants are under pressure to handle high concentration sludge in a sludge treatment line. Unawareness of the non-Newtonian behaviour of the thickened sludge has the potential to cause unexpected problems when the fluid behaviour changes from turbulent to laminar flow. In this study, sludge apparent viscosity was plotted as a function of total suspended solids concentration (TSS) and shear rate. Then, the transition velocity based on several predictive models in the literature was determined. This analysis provides a practical basis for the prediction of the pipe flow behaviour of thickened sludge in troubleshooting and engineering design

The impact of temperature on the rheological behaviour of anaerobic digested sludge

The rheological properties of municipal anaerobic digested sludge rheology are temperature dependent. In this paper, we show that both solid and liquid characteristics decrease with temperature. We also show that the yield stress and the high shear (Bingham) viscosity are the two key parameters determining the rheological behaviour. By normalising the shear stress with the yield stress and the shear rate with the yield stress divided by the Bingham viscosity, a master curve was obtained, independent of both temperature and concentration. We also show that the rheological behaviour is irreversibly altered by the thermal history. Dissolution of some of the solids may cause a decrease of the yield stress and an increase of the Bingham viscosity. This result suggests that the usual laws used to describe the thermal evolution of the rheological behaviour of fluids are no longer valid with anaerobic digested sludge. Finally, the impact of temperature and thermal history have to be taken into account for the design of engineering hydrodynamic processes such as mixing and pumping

Producing hollow granules from hydrophobic powders in high-shear mixer granulators

The formation of hollow granules from hydrophobic powders in a high-shear mixer granulator has been investigated by changing the binder/powder mass ratio and studying its effects on granule size and structure. In this study, a mixer granulator was filled with 100 g of hydrophobic fumed silica and then varying quantities of 5% Hydroxy Propyl Cellulose solution was slowly sprayed into granulator. A range of liquid to solid mass ratios between from 0.5:1 to 15:1 was used. Granules were then dried at 60°C in a fan forced oven. This paper compares the particle size distributions, scanning electron microscopy (SEM) images and X-ray tomography (XRT) images of hollow granules as a function of the liquid to solid mass ratio. The granule mean size increased and the fraction of un-granulated (fine) particles decreased as the liquid to solid mass ratio increased. Simultaneously, the morphology and structure of the hollow granules changed from a spherical to a deformed structure which indicates the importance of choosing the optimal liquid to solid mass ratio. The optimal liquid to solid mass ratio for Aerosil R202 powder in this study was found to be between 3:1 and 6:1. The final granule shape and size distribution are dependent on the liquid to solid ratio if the liquid marble nucleation process starts with a preformed droplet template

Formation of hollow granules from liquid marbles: Small Scale experiments

Research into formation of hollowgranules fromliquidmarbles is an emerging field in hydrophobic granulation. A liquidmarble is formed by a network of self-assembled hydrophobic powder around a droplet, and this paper investigates the conditions required for forming hollow granules from a liquid marble precursor. Single drops of fluidwere produced using a syringe and placed onto loosely packed powder beds of hydrophobic powders. Liquid marbles formed from several powder/liquid combinations were dried at several conditions to investigate the drying conditions required for formation of a stable hollow granule. The formation of stable hollow granules was found to depend on drying temperature and binder concentration. ForHPMC and PVP binder, formation of hollowgranule is proportional to binder viscosity and forHPC binder, this relationship is constant. Different combinations of powder and binder at both drying temperatures - 60 ºC and 100 ºC - had mixed success rates in forming hollow granules, but generally the success rate was improved by using higher drying temperatures, smaller particles or higher viscosity binder fluids

Rheology of a primary and secondary sewage sludge mixture: Dependency on temperature and solid concentration

The main objective of this study was to investigate the rheology of mixed primary and secondary sludge and its dependency on solid content and temperature. Results of this study showed that the temperature and solid concentration are critical parameters affecting the mixed sludge rheology. It was found that the yield stress increases with an increase in the sludge solid content and decreases with increasing temperature. The rheological behaviour of sludges was modelled using the Herschel-Bulkley model. The results of the model showed a good agreement with experimental data. Depending on the total solid content, the average error varied between 3.25% and 6.22%

An analysis of the thermodynamic conditions for solid powder particles spreading over liquid surface

The spreading of solid powder over a liquid surface is a prevalent phenomenon encountered in many industrial processes such as food and pharmaceutical processes. The driving force for powder spreading over a liquid surface is not clearly understood. The Marangoni effect due to a temperature gradient and the spreading coefficient for solid powder over liquid (lambda S/L) have both been proposed as causes for powder spreading over liquids. The proposed lambda S/L was based on the same form of the spreading coefficient for a liquid over a solid surface (lambda L/S). Whereas lambda L/S has a clear thermodynamic definition, the spreading coefficient of solid powder over liquid, lambda S/L, which was defined by simply interchanging the subscripts of the interfacial energy terms, has not been thoroughly analysed. Our experimental results showed that the spreading behaviour of solid powders over liquids cannot be explained or predicted by lambda S/L. In this study we focus on problems associated with the lambda S/L. Through a thermodynamic analysis we conclude that the existing parameter lambda S/L is unable to predict the spreading behaviour of solid powder on liquid surface, since the interfacial energy approach does not capture the actual physical process of powder spreading over liquid surface. A closer examination of the powder spreading process reveals the fundamental different between liquid spreading over solid surface and solid powder spreading over liquid. This work shows that further research is required to identify and analyse the physical mechanisms which are responsible for powder particles spreading over liquid surfaces

The viscoelastic behaviour of raw and anaerobic digested sludge: Strong similarities with soft-glassy materials

Over the last few decades, municipal and industrial wastewater treatment activities have been confronted with a dramatically increasing flow of sewage sludge. To improve treatment efficiency, process and material parameters are needed but engineers are dealing with vast quantities of fundamentally poorly understood and unpredictable material Thus, accurate prediction of critically important, but analytically elusive process parameters is unattainable and is a matter of grave concern. Because engineers need reliable flow properties to simulate the process, this work is an attempt to approach sludge rheological behaviour with well-known materials which have similar characteristics. Sludge liquid-like behaviour is already well documented so, we have focused mainly on the solid-like behaviour of both raw and digested sludge by performing oscillatory measurements in the linear and non-linear regimes. We have shown that the viscoelastic behaviour of sludge presents strong similarities with soft-glassy materials but differences can be observed between raw and digested sludge. Finally, we confirm that colloidal glasses and emulsions may be used to model the rheological behaviour of raw and anaerobic digested sludge

Phase retrieval tomography in the presence of noise

We describe the use of single-plane phase retrieval tomography using a laboratory-based x-ray source, under conditions where the retrieval is not formally valid, to present images of the internal structure of an Aerosil granule and a hydrated bentonite gel. The technique provides phase images for samples that interact weakly with the x-ray beam. As the method is less affected by noise than an alternative two-plane phase retrieval method that is otherwise formally valid, object structure can be observed that would not otherwise be seen. We demonstrate our results for phase imaging in tomographic measurements