M. J. Adams

University of Birmingham · School of Chemical Engineering

Introduction: During the COVID-19 pandemic healthcare workers (HCWs) have used respiratory protective equipment for prolonged periods, which has been associated with detrimental effects on the underlying skin. The present study aims to evaluate changes in the main cells (corneocytes) of the stratum corneum (SC) following prolonged and consecutive...

Friction reduction using ultrasonic longitudinal surface vibration can modify the user perception of the touched surface and induce the perception of textured materials. In the current paper, the mechanisms of friction reduction using longitudinal vibration are analyzed at different finger exploration velocities and directions over a plate. The dev...

Background: Skin is the interface between an organism and the external environment, and hence the stratum corneum (SC) is the first to withstand mechanical insults that, in certain conditions, may lead to integrity loss and the development of pressure ulcers. The SC comprises corneocytes, which are vital elements to its barrier function. These cel...

Significance Why have primates evolved epidermal ridges on the volar regions of the hands and feet and with a much greater density of sweat glands than flat skin, which respond to anxiety rather than act as a thermoregulation mechanism? During contact with solid objects, the ridges are important for grip and precision manipulation by regulating moi...

Roller compaction is the main technique employed in dry granulation. Ribbon sticking and splitting are among the major factors that can hinder the use of this process for some formulations. Ribbon splitting can occur either transversally (through the ribbon thickness) or longitudinally (through the ribbon width). It was observed that transverse spl...

A hybrid molecular mechanics–molecular dynamics simulation method has been performed to study the effects of moisture content on the mechanical properties of microcrystalline cellulose (MCC) and the mobility of the water molecules. The specific volume and diffusion coefficient of the water increase with increasing moisture content in the range stud...

A hybrid molecular mechanics-molecular dynamics (MM-MD) method is proposed to calculate the Young's modulus of polymers at various temperature. It overcomes the limitation that MD is restricted to extremely high strain rates. A case study based on poly-methyl-methacrylate demonstrates that, contrary to previous MD studies, the method is able to acc...

The effects of the configuration and temperature on the Young’s modulus of poly (methyl methacrylate) (PMMA) have been studied using molecular dynamics simulations. For the DREIDING force field under ambient temperatures, increasing the number of monomers significantly increases the modulus of isotactic and syndiotactic PMMA while the isotactic for...

The progress is described in developing a parallel computer code to study the dynamics of wet granular systems based on the Fast Multi-pole Boundary Element Method (FMBEM). Here, three examples are considered that have closed-form or numerical solutions and thus able to act as benchmarks. They involved capillary interactions, the formation of a sol...

We describe an instrument intended to study finger contacts under tangential dynamic loading. This type of loading is relevant to the natural conditions when touch is used to discriminate and identify the properties of the surfaces of objects --- it is also crucial during object manipulation. The system comprises a high performance tribometer able...

The coefficient of friction of most solid objects is independent of the applied normal force because of surface roughness. This behaviour is observed for a finger pad except at long contact times (greater than 10 s) against smooth impermeable surfaces such as glass when the coefficient increases with decreasing normal force by about a factor of fiv...

Contact electrification and electrostatic interactions often occur in the fluidization process, which can significantly influence the dynamic behaviour of particles and the fluidization performance. In this study, a discrete element method coupled with computational fluid dynamics (DEM-CFD) is developed by implementing contact electrification and e...

An experimental study of the tribological properties of hair fibres is reported, and the effect of surface treatment on the evolution of friction and wear during sliding. Specifically, orthogonally crossed fibre/fibre contacts under a compressive normal load over a series of 10,000 cycle studies are investigated. Reciprocating sliding at a velocity...

Numerical modelling using computational fluid mechanics (CFD) and discrete element method (DEM) becomes increasingly prevalent for the exploration of agglomeration and deagglomeration in dry powder inhalers (DPIs). These techniques provide detailed information of air flow and particle-particle/wall interaction, respectively. Coupling of CFD and DEM...

Contact electrification is generally referred to as the charge transfer process between particles during collisions. The transferred charge can be accumulated on the surface of the particles especially for insulating materials with irregular shapes, which can lead to a non-uniform charge distribution and eventually affects the charge accumulation p...

Particle interactions play a significant role in controlling the performance of dry powder inhalers (DPIs), which mainly arise through van der Waals potentials, electrostatic interactions, and capillary forces. Our aim is to investigate the influence of electrostatic charge on the performance of DPIs as a basis for improving the formulation of the...

The electrostatic charge can be transferred between particles during collisions. The particle shape plays an important role and, in the current study, the charge accumulation and distribution on elongated particles in a vibrating container are investigated using a discrete element method, in which a contact electrification model is implemented. The...

To investigate dynamic behaviours of mono-charged particle systems, a direct truncation (DT) method and a hybrid particle-cell (HPC) method are implemented into the discrete element method coupled with computational fluid dynamics (DEM-CFD) with defined cut-off distances. The DT method only considers electrostatic interactions between particles wit...

This paper describes an investigation of the effect of fill factor; on the compaction behaviour of the granules during tabletting and hence mechanical properties of tablets formed. The fill factor; which is the ratio of volume of wet powder material to vessel volume of the granulator, was used as an indicator of batch size. It has been established...

Air flow and particle–particle/wall impacts are considered as two primary dispersion mechanisms for dry powder inhalers (DPIs). Hence, an understanding of these mechanisms is critical for the development of DPIs. In this study, a coupled DEM–CFD (discrete element method–computational fluid dynamics) is employed to investigate the influence of air f...

Understanding the adhesive interactions between active pharmaceutical ingredient (API) particles and carrier particles in dry powder inhalers (DPIs) is critical for the development of formulations and process design. In the current study, a discrete element method, which accounts for particle adhesion, is employed to investigate the attachment proc...

NanoBioTouch is an FP7 funded project that has an overall aim of developing NEMS tactile sensors for integration in an articulated robotic finger. The design of the sensors and signal processing are based on a multidisciplinary approach to improving the current understanding of the human mechano-transduction system. A range of NEMS arrays and bio-N...

Hydrogels have been widely used in biomedical applications such as scaffolds for tissue engineering, biosensing, drug delivery and cell immobilization. Due to their 3D structure, tunable mechanical properties, and their ability to mimic the cellular environment, these materials have been extensively studied and analysed in order to investigate thei...

Contact electrification may occur during particle-particle and particle-wall collisions due to the contact potential difference. The aim of the current work is to examine the effect of particle shape on contact electrification. A discrete element method was developed to model irregular shaped particles with a multi-sphere representation and to anal...

The coefficients of friction of concentrated soft-particle suspensions (tomato paste and a microgel suspension) were measured as a function of the slip velocity for a number of substrates. The data are interpreted using a micro-elastohydrodynamic model that is consistent with significant bulk frictional dissipation and an increase in the number of...

The attachment of autoadhesive fine particles onto a large carrier particle in a vibrating container is investigated using a discrete element method (DEM) to determine the influence of the particle properties and vibration conditions. The contact number (number of fine particles contacting the carrier), coverage ratio (ratio of the contact number t...

Lubricants are commonly used in the pharmaceutical industry to prevent adhesion and improve the efficiency of roll compaction and tabletting. The aim of the current work is to develop an improved understanding of the mechanisms involved. Two commonly used pharmaceutical excipients, microcrystalline cellulose (MCC) and di-calcium phosphate dihydrate...

Discrete Element Methods (DEM) is a numerical technique for analysing the mechanics and physics of particulate systems. Originated in the late seventies for analysing geotechnical problems, it has seen significant development and is now employed extensively across disciplines. Produced in celebration of the 70th Birthday of Colin Thornton, this boo...

In pharmaceutical industries, roll compaction is a preferred dry granulation process for moisture and heat sensitive formulations. The roll compacted ribbons are milled to granules for subsequent tablet manufacturing. Thus, milling is also one of the critical processes that determines the properties of the granules and hence the tablets. However, t...

Wind tunnel experiments were performed in order to investigate the effect of mixing on the aeolian transport of sands with different grain sizes. Two types of sand with different grain size distributions and an equal-mass binary mixture of these sands were used. Comparing the gradients of their measured mass flux profiles and some published profile...

Hair shampoos and conditioners commonly include lubricating ingredients that facilitate detangling, reduce combing damage and enhance the perception of smoothness. The quality of the lubrication imparted is routinely assessed using sensory tests as part of product development. The performance can also be investigated using instrumental measurements...

The adhesive characteristics of thin films (0.2-2 μm) of linear poly(dimethylsiloxane) (PDMS) liquids with a wide range of molecular weights have been measured using an atomic force microscope with a colloid probe (diameters 5 and 12 μm) for different separation velocities. The data were consistent with a residual film in the contact region having...

This paper presents a silicon MEMS based capacitive sensing array, which has the ability to resolve forces in the sub mN range, provides directional response to applied loading and has the ability to differentiate between surface textures. Texture recognition is achieved by scanning surfaces over the sensing array and assessing the frequency spectr...

The adhesive characteristics of thin films (0.2 - 2 µm) of linear PDMS liquids with a wide range of molecular weights have been measured using an AFM with a colloid probe (diameters 5 and 12 µm) for different separation velocities. The data were consistent with a residual film in the contact region having a thickness of ~ 6 nm following an extended...

In this paper, the analysis and design of a piezoelectrically actuated micro squeeze flow rheometer (MSFR) is presented. The fabrication of a simple prototype is described and initial experiments show the validity of the theory presented. The rheometer requires small volumes of liquid of the order of 1–10 nL and extends the frequency range an order...

This paper presents the development of a MEMS based capacitive tactile sensor intended to be incorporated into a tactile array as the core element of a biomimetic fingerpad. The use of standard microfabrication technologies in realising the device allowed a cost efficient fabrication involving only a few process steps. A low noise readout electroni...

The flow patterns in a high-shear granulator depend on the fill volume. For example, DEM simulations reported by Terashita et al.[1] show that it affects the velocities and kinetic energies of the particles. It also influences the granule size distribution [2]. Here the effects on the properties of the granule are described. The total mass of the g...

Impeller speed is one of the most crucial process variables that affect the properties of the granules produced in a high-shear granulator. Several reports can be found in literature that discuss the influence of impeller speed on the granules size. For instance some researchers like Knight report an increase of granule size with impeller speed [1...

A micromanipulation technique was used for the mechanical characterisation of two types of agarose microspheres with different material properties. Narrow-size distributions having a mean diameter in the range of 15-22 mu m were prepared using (a) conventional emulsification followed by filtration and (b) membrane emulsification. Single microsphere...

The rheological properties of plastic and viscoplastic complex fluids have been investigated using constant velocity squeeze flow rheometry at small gaps in order to examine the influence of the resulting flow confinement. The fluids investigated were aqueous carbopol suspensions and three commercial products (Tesco English mustard, Tesco value lem...

This paper reports experimental studies illustrating the effects of partially wetting liquids on the mechanical strength of model granules composed of autoadhesive polystyrene particles. The liquids were prepared by mixing different amounts of isopropanol with pure water and their wetting behaviour was characterised by contact angle measurements on...

This chapter focuses on the mechanistic description of granule deformation and breakage. The chapter attempts to reiterate that granule strength stems from the interparticle adhesion and friction forces, which is central to the control of granule strength. The term adhesion is used to include any type of interaction that binds the primary particles...

The in vivo friction of human skin has been measured in the dry, wet and damp states using smooth glass and polypropylene spherically tipped probes. They were selected to be representative of hydrophilic and hydrophobic countersurfaces. The data are interpreted using the adhesion model of friction, which provides an explanation for the influence of...

Gas fluidised beds have many applications in a wide range of industrial sectors and it is important to be able to predict their performance. This requires, for example, a deeper appreciation of the flow of the particles in such systems using both empirical and numerical methods. The coefficient of restitution is an important collisional parameter t...

In this paper, the applicability of elasto-plastic contact mechanics to predict the coefficient of restitution of a dry binderless granule impacting on a flat surface is assessed. The granules were prepared using polystyrene particles to avoid complications arising from energy dissipation by solid and liquid binders since only van der Waals attract...

A granulation procedure is described for preparing model binderless granules from spherical polystyrene colloids. The deformation and breakage behaviour of the granules was also studied. Impact and slow diametrical compression experiments were used to simulate the mechanical response of the granules at high and low strain rates, respectively. They...

An analytical model of orifice extrusion has been developed that allows the material parameters to be derived from experimental data for materials such as pastes that may be described as viscoplastic. The geometry is based on a cylindrical square-ended barrel with variable diameter orifices. A rigid-viscoplastic constitutive relationship was employ...

Based on observations of single impact studies of spherical coherent and agglomerate particles made from a wide range of materials, a classification of failure modes is presented involving low, intermedite. It is based on low, intermediate, and high velocity regimes. The work allows a point of reference for the expected failure modes under impact c...

For wet high-shear granulation, there can be a considerable variability in product quality in terms of the size, binder content, porosity, and appearance. Using the same equipment and feed material, it has been shown that such variability can be reduced by optimising the operating protocol. The associated narrowing of the range of mechanical proper...

The paper reports discrete element simulations of the diametrical compression test applied to two spherical agglomerates: one a dense agglomerate and the other a loosely packed agglomerate. The results obtained for the dense agglomerate show that the agglomerate fractures along a slightly inclined, approximately diametrical plane. Outwardly, the ag...

Growth and breakdown mechanisms in granulation processes may involve collisions between soft plastically deforming agglomerates. It has been established previously that the flow stress in such collisions increases with the strain rate, which is dependent on the impact velocity and the size of the agglomerates. In the current paper, a scaling model...

The impact behaviour of wet granules was studied by measuring their restitution coefficient and maximum contact area. A flat semi-infinite rigid target was used for this purpose and the impact and rebound speeds were recorded by using two high-speed cameras. Different granules were produced by varying the liquid to solid ratio, primary particle siz...

A finite element analysis was described for the upsetting of a cylindrical specimen between two parallel rigid dies. The benefits of radial and longitudinal ultrasonic oscillation of the die in compressive deformation of a model soft solid were studied. The influence of oscillation direction on the interfacial friction boundary condition and therma...

Thin coatings of isotactic poly(methyl methacrylate) were cast on glass from solvents of different quality, which was expected to result in different molecular morphologies. The elastic moduli and hardness of the coatings were measured using a nanoindenter. The aim was to examine the viability of this technique for assessing the influence of the mo...

This paper describes a fluidised bed model developed from the DEM-based Aston granular dynamics code in which the solid–solid interaction rules are based on theoretical contact mechanics thereby enabling particles to be directly specified by material properties such as friction, elasticity, elasto-plasticity and auto-adhesion. For the gas phase whi...

This paper describes an experimental study designed to quantify the elastic depth recovery behaviour of indentations and grooves made in the surface of a poly(methyl methacrylate) sheet with a probe of Berkovich geometry. Significantly greater depth recovery was found for the grooves, and it is shown that this cannot be ascribed simply to differenc...

Experimental data are described for the indentation of highly porous particulate coatings on the submicron length scale using a Berkovich probe; they were composed of colloidal silica (20-24 nm in diameter) with a solvent-deposited isotactic poly(methyl methacrylate) as a binder. A wide variation in the data was observed at different surface locati...

A wedge indentation test has been carried out, in which an ultrasonic vibration was superimposed at a frequency of 20 kHz to investigate the effects of ultrasonic vibration on the indentation mechanics of Plasticine. A finite element simulation was employed as a basis for interpreting the experimental data. The model incorporated material and geome...

This paper describes an experimental and analytical study of contact compliance data for deriving material properties of the model elastoviscoplastic material, plasticine, from spherical and conical indentation measurements. An analytical model is employed to determine the material characteristics such as elastic modulus and yield stress from the e...

An analysis of the rupture behaviour of liquid bridges in simple granular systems is reported. Wet granules present a complex structure in which primary particles are held together at a microscopic scale by means of capillary forces mediated by a liquid binder. These capillary interactions control the mechanical properties of the particle assemblie...

This paper describes the application of dynamic contact stiffness measurements for conveniently monitoring the contact area between an elastomeric hemisphere and a glass flat. This novel method has been validated using direct observations of the contact. The static contact radii measured during compression were well-described by the adhesion model...

The adhesion and friction between two orthogonally arranged carbon fibers has been measured in undersaturated vapor pressures of decane, n-propanol, and water. An analysis, which is described, of the frictional data allowed the normal adhesive force under sliding conditions to be deduced. Contact angle measurements and adsorption studies showed tha...

The lubrication solution for the squeeze flow of a power-law fluid between two rigid spherical particles has been investigated. It is shown that the radial pressure distribution converges to zero within the gap between the particles for any value of the flow index, n, provided that the gap separation distance is sufficiently small. However, in the...

This paper reports experimental data obtained on an instrumented screw extruder. The material used was a starch powder that was independently characterised in a uniaxial compaction experiment. The data have been carefully analysed at some length and are compared with a simple model based on an enhancement of the well-known model by Darnell and Mol...

An experimental study of boundary lubricants is described which is based on a surface force apparatus that had been modified to allow an oscillatory shear stress to be imposed. Phospholipid monolayers having different acyl chain lengths and a phospholipid bilayer were deposited onto the mica surfaces using the LB method at relatively high surface p...

The role of liquids in the non-hydrodynamic lubrication of polymers has been investigated using a model system comprised of polyethylene-terephthalate monofilaments in orthogonal contact. In a liquid environment a number of mechanisms are possible, viz plasticisation, wetting, electrical double-layer repulsion and boundary lubrication. It is shown...

The frictional characteristics of keratin fibres are anisotropic. The most obvious characteristic feature is that the frictional work required to slide one fibre over another is greater in the direction tip-to-root than the converse; this is the so-called differential friction effect (DFE). The authors describe a series of experiments where fine po...

A ram extrusion process with a conical die entry is analysed using a physically based analysis and a numerical modelling procedure involving the finite element method. The aim of the study is to test the applicability of the elasto-viscoplastic constitutive model combined with an established boundary condition criterion for describing this forming...

Theconfigurations and interactions in a system of particles connected by liquid bridges are investigated. The basic phenomena have been elucidated using two-dimensional models with equal sized particles. With this restriction all possible contact configurations can be obtained by considering three particles. Different configurations of the liquid d...

We report a preliminary investigation of the consequences for boundary lubricant performance of incorporating alkyl alcohols into aqueous solutions of quaternary ammonium chloride surfactants. Frictional data taken under single and multi-pass linear sliding conditions are presented for glass sphere-glass flat contacts lubricated by quat/alcohol mix...

The paper reports granular dynamics simulations of a dense spherical agglomerate consisting of a random polydisperse system of autoadhesive particles impacting orthogonally with a target wall. A range of impact velocities has been examined which resulted in rebound, fracture or shattering depending on the magnitude of the impact velocity specified....

A knowledge of the formation and rupture mechanisms for agglomerates is essential when seeking to model equipment designed to produce and process such materials. In the work described here, nanoindentation of “two-dimensional” agglomerate films, basically particulate coatings, was carried out to establish a means of identifying the generic breakage...

Capillary rheometry has been used to examine materials that exhibit wall slip1. This paper presents comprehensive measurements of viscometric data for starch-based pastes using a capillary rheometer. A robust analytical scheme has been applied in order to obtain accurate values of the wall shear rate and wall slip velocity as a function of the bulk...

Computer simulations of agglomerates impacting a wall have been carried out using the distinct element method. The agglomerates comprised 1000 primary particles in a two-dimensional array. Autoadhesive and frictional interaction laws between the primary particles were employed. Results are presented for a range of impact velocities and surface free...

The paper describes various analytical procedures that account for the influence of the geometry of a rigid indenter upon the measured contact compliance of a smooth perfectly elastic half space. The analytical solutions provide a means of interrelating the reaction force, P, and the displacement, h, characteristics in terms of the contact geometry...