Oscillatory Experiments Research Articles

Effect of particle stiffness on microgel self-assembly and suspension phase behavior over a broad temperature range

We synthesized thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) colloidal microgel particles of different stiffnesses by controlling the concentration of a polar crosslinker in a precipitation polymerization synthesis method. When suspended in an aqueous medium, the particles collapsed by expelling water as the temperature was raised toward the volume phase transition temperature (VPTT) of ≈ 34 °C. We noted that the sizes of the stiffer particles, synthesized with higher crosslinker concentration, collapsed less abruptly. Using Fourier transform infrared spectroscopy, we observed enhanced particle dehydration with increasing temperature and decreasing particle stiffness. Oscillatory rheology experiments on dense aqueous PNIPAM suspensions, prepared at a fixed particle effective volume fraction ϕeff = 1.5 at 25°C, revealed that suspensions constituted by the stiffest particles are the most elastic over a broad temperature range. Above the VPTT, suspensions of particles of intermediate stiffnesses exhibited two-step yielding, a typical signature of fragile gel formation. Zeta potential measurements showed that PNIPAM particles of lower stiffnesses are rendered electrostatically unstable in aqueous suspension. Combining cryogenic scanning electron microscopy and rheology, we noted a glass–glass transition when the temperature of a dense suspension of stiff PNIPAM particles was raised across the VPTT. In contrast, suspensions of particles of the lowest stiffnesses showed a gel-viscoelastic liquid–gel transition during an identical temperature ramp experiment. Our study reveals that temperature-induced phase transformations in dense PNIPAM suspensions depend sensitively on the stiffness of the constituent particles and can be explained by considering amphiphilicity-driven morphological changes in the suspension microstructures.

Effects of freeze-thaw leaching on physicochemical properties and cadmium transformation in cadmium contaminated soil

This study aims to investigate the effect of the combined method of freeze–thaw and leaching on the removal of cadmium (Cd) in soil and to provide a theoretical basis for the remediation of farmland soil polluted by heavy metals. The removal process and mechanism of Cd were deduced through oscillatory leaching experiments and freeze–thaw leaching simulation experiments, and the influence of the freeze–thaw leaching technology on the soil environment was evaluated. The results of oscillatory leaching showed that a mixture consisting of 0.80 mol/L citric acid and 0.80 mol/L ferric chloride in a 1:19 vol ratio effectively remove 47.75 % of Cd, indicating that the composite leaching agent could effectively remove Cd from the soil. The results of the freeze–thaw leaching simulation experiment showed that although the freeze–thaw leaching treatment increased the total Cd content in the 0–5 cm soil layer, the total Cd content in the 5–10 cm, 10–15 cm, and 15–20 cm soil layers decreased by 5.08 %, 2.39 %, and 5.68 %, respectively. The freeze–thaw leaching increased the content of exchangeable Cd (p<0.05), carbonate bound Cd, but decreased organic bound Cd and residual Cd (p<0.05), thereby increasing the bioavailability of Cd. Freeze–thaw leaching not only increased the competitive adsorption of Cd2+ by decreasing soil pH, cation exchange capacity, and increasing the content of exchangeable calcium and exchangeable magnesium, thus reducing the adsorption of Cd in soil. And the results of XPS and FTIR similarly showed that the freeze–thaw leaching could promote the chelation between Cd2+ and hydroxyl, carboxyl and carbonyl functional groups. Although the freeze–thaw leaching destroyed the large particle structure (0.05–2 mm) and large pores in the soil, and increased the clay content (<0.002 mm) and the proportion of small pores in the soil, the XRD results showed that freeze–thaw leaching had no significant effect on the minerals in the soil. In summary, this study shows that freeze–thaw leaching has a significant effect on the removal of soil heavy metals, suggesting that the synergistic effect of freeze-thaw and leaching should be considered in the process of removing soil pollutants in seasonal freeze–thaw zones, and that this method provides a new insight into the remediation of contaminated soils.

Egg-free mayonnaise-type emulsions stabilized with proteins derived from the larvae of Tenebrio molitor

In view of demanding alternative protein sources to replace the conventional ones, egg-yolk free mayonnaise-type emulsions were made from protein preparations extracted from Tenebrio molitor larvae by applying alkaline extraction (ASP), alkaline extraction followed by isoelectric precipitation (AIP) and salt-assisted treatment followed by dialysis (SSP). Their droplet size, microstructure and rheological properties were compared with egg-yolk mayonnaise (CEY). All emulsions exhibited similar droplet size distribution, with the CEY-mayonnaise having a broader range. SSP showed the smallest mean droplet size acting as a more effective emulsifier. Steady-state flow properties and dynamic oscillatory experiments within the linear viscoelastic region of mayonnaise-type emulsions were performed. The higher value of G′ compared to the value of G″ indicated the elastic character of all samples. Both moduli were no-highly frequency dependent, which was in line with their solid-like nature. The thixotropic behaviour of the emulsions was evaluated by three-interval thixotropy tests revealing higher recovery rate in the case of CEY and ASP. Temperature sweeps were also performed and revealed that AIP and SSP showed less temperature dependence, indicating their lower sensitivity to temperature variations. Overall, the results of this study suggest that the protein preparations extracted from Tenebrio molitor larvae have the potential for producing egg yolk-free mayonnaise-type emulsions.

Effect of PCL chain length on rheological and mechanical properties of PCL‐PDMS‐PCL triblock copolymer films

AbstractThe polycaprolactone‐polydimethylsiloxane‐polycaprolactone (PCL‐PDMS‐PCL) triblock copolymer films of variable PCL chain are analyzed for rheological characteristics in this paper. The dynamics of viscoelastic behavior of photocrosslinked films above their crystal melting temperature (Tcm) is measured using oscillatory shear rheology experiments. The frequency sweep tests are analyzed at 0.1% strain as obtained from linear viscoelastic region. Shear thinning is observed in the study of complex viscosity. It has been noted that viscosity rises with increase in crosslink density and decrease in with PCL chain length. Time‐dependent effect on the copolymer films has been analyzed by creep recovery behavior. The Burgers model is fitted in creep compliance data to analyze its viscoelastic and viscous region. An examination of structure recovery revealed that recovery increased as PCL chain length decreased. Tensile strength and percentage elongation are studied at ambient conditions with a universal testing machine, while storage modulus and tan δ are analyzed by a dynamic mechanical analyzer at varying temperatures.

A Novel Artificial Coronary Plaque to Model Coronary Heart Disease.

Experimental coronary artery interventions are currently being performed on non-diseased blood vessels in healthy animals. To provide a more realistic pathoanatomical scenario for investigations on novel interventional and surgical therapies, we aimed to fabricate a stenotic lesion, mimicking the morphology and structure of a human atherosclerotic plaque. In an interdisciplinary setting, we engineered a casting mold to create an atherosclerotic plaque with the dimensions to fit in a porcine coronary artery. Oscillatory rheology experiments took place along with long-term stability tests assessed by microscopic examination and weight monitoring. For the implantability in future in vivo setups, we performed a cytotoxicity assessment, inserted the plaque in resected pig hearts, and performed diagnostic imaging to visualize the plaque in its final position. The most promising composition consists of gelatin, cholesterol, phospholipids, hydroxyapatite, and fine-grained calcium carbonate. It can be inserted in the coronary artery of human-sized pig hearts, producing a local partial stenosis and interacting like the atherosclerotic plaque by stretching and shrinking with the vessel wall and surrounding tissue. This artificial atherosclerotic plaque model works as a simulating tool for future medical testing and could be crucial for further specified research on coronary artery disease and is going to help to provide information about the optimal interventional and surgical care of the disease.

Evaluation of the printability of agar and hydroxypropyl methylcellulose gels as gummy formulations: Insights from rheological properties

The trial-and-error method currently used to create formulations with excellent printability demands considerable time and resources, primarily due to the increasing number of variables involved. Rheology serves as a relatively rapid and highly beneficial method for assessing materials and evaluating their effectiveness as 3D constructs. However, the data obtained can be overwhelming, especially for users lacking experience in this field. This study examined the rheological properties of formulations of agar, hydroxypropyl methylcellulose, and the model drug caffeine, alongside exploring their printability as gummy formulations. The gels’ rheological properties were characterized using oscillatory and rotational experiments. The correlation between these gels’ rheological properties and their printability was established, and three clusters were formed based on the rheological properties and printability of the samples using principal component analysis. Furthermore, the printability was predicted using the sample’s rheological property that correlated most with printability, the phase angle δ, and the regression models resulted in an accuracy of over 80%. Although these relationships merit confirmation in later studies, this study suggests a quantitative definition of the relationship between printability and one rheological property and can be used for the development of formulations destined for extrusion 3D printing.

The Influence of Oscillatory Frequency on the Structural Breakup and Recovery of Coastal Mud

The structural breakup and recovery of coastal mud are closely related to wave propagation, mud transportation, and coastal morphology evolution. Due to the influence of climate, topography, and other factors, the wave frequency in marine environments is more variable than fixed. To investigate the mud structural breakup and recovery process under oscillatory shear loads with different frequencies, a series of oscillatory rheological experiments of the coastal mud collected from the tidal flats of Zhairuoshan Island, Zhejiang province, China, were carried out. The results revealed that the structural breakup of coastal mud had a two-step transition process. The fluidization occurs more rapidly at higher frequencies, but the influence of frequency on the two yield stresses is limited. In addition, frequency has a complex effect on the structural recovery of coastal mud. The normalized equilibrium storage modulus (G∞′/G0′) does not change monotonically with frequency. Moreover, the viscosity quickly approaches equilibrium when a shear load is applied. After that, when a low-frequency load is applied, G∞′/G0′ is no longer related to the pre-shear duration. However, when a high-frequency load is applied, G∞′/G0′ of the mud sample pre-sheared for 500 s is significantly larger than that of the sample pre-sheared to the minimum viscosity. This study is anticipated to provide reference and supplementary test data for understanding the interaction between waves of different frequencies and muddy seabed.

Decoupling of mechanical properties and ionic conductivity in a low molecular weight polymer nanocomposites with highly connected particle aggregates

HypothesisHigh molecular weight polymer nanocomposites (PNCs) with good thermal stability and elevated mechanical strengths can serve as composite polymer electrolytes (CPEs) and thus have been considered as an alternative to conventional liquid electrolytes. However, resolving low ionic conductivity issues of CPEs arising from the low chain mobility derived from a long polymer chain remains challenging. ExperimentsHere, we introduce PNCs for use in CPEs which show 1.4 × 10−4 S/cm of ionic conductivity at room temperature with a high shear modulus of 107 Pa using a low molecular weight poly(ethylene glycol) (PEG) matrix and dopamine-modified PEG brush polymer grafted-silica nanoparticles. FindingsWe found that densely interconnected supramolecular particle networks can decouple the mechanical strength and ionic conductivity, as the Li-doped interfacial polymer layer supports a direct lithium-ion transport pathway. The extensive structural and rheological studies characterized by small-angle X-ray scattering and oscillatory rheometry experiments revealed that particle connections through interfacial polymer layers play an important role in supporting the elevated mechanical and electrochemical properties with a good thermal stability of PNCs.

Self-aggregation, dilational surface rheology and foaming properties of 1-O-dodecyl diglyceryl ether compared to n-dodecyl-β-D-maltoside and pentaethyleneglycol monododecyl ether

Diglyceryl alkyl esters are good foaming agents but the ester linker can be sensitive to pH, chemicals and temperature unlike the ether function. In this work, the foaming properties of a diglyceryl ether, namely 1-O-dodecyl diglyceryl ether (C12Gly2), are compared to those of n-dodecyl-β-D-maltoside (C12Glc2) and pentaethyleneglycol monododecyl ether (C12E5). The self-aggregation behaviour of C12Gly2 and adsorption at the air–water interface has been first investigated. For the three surfactants, the dynamic response of the interface, measured in oscillatory bubble interfacial rheology experiments below CMC, is compared and put in relation with their foaming properties. In particular, the foamability by air sparging at a concentration of 10 times the CMC, the foam stability over 1 h and the foam density are quantified.It is shown that C12Gly2 forms liquid crystals at low concentration (∼10CMC). C12E5, with a lower elasticity high frequency limit ε0, forms unstable foam with quick drainage and breakdown, whereas higher ε0 surfactants C12Gly2 and C12Glc 2 form much more stable foams, resulting from hydrogen bonds between the polar heads of C12Glc 2 and C12Gly2. Differences in C12Gly2 and C12Glc2 foams lay in initial bubble size (smaller for the C12Glc 2). In C12Gly2 foam, the main destabilization phenomenon is coalescence over drainage, and the foam volume only decreases by 30% in 1 h.

Co‐crystallization behaviors and thermo‐physical properties of SSPCM consisting of an extra low melt viscosity HDPE and paraffin waxes

AbstractShape stabilized phase change materials (SSPCMs) consisting of an extra low melt viscosity high‐density polyethylene (lv‐HDPE) and paraffin waxes (PW) were fabricated using a twin screw extruder. Thermal and mechanical properties of these composites were assessed. The latent heats (ΔH) of the SSPCMs fabricated were up to 78 J g−1. Co‐crystallization between lv‐HDPE and paraffin waxes increased with increasing wax content. Dynamic mechanical thermal analysis confirmed the paraffin wax having a higher melting point (H‐PW, Tm ranging from 56 to 58°C) plasticized the lv‐HDPE, decreasing both its crystalline content and melting temperature. Static mechanical testing, disregarding the form of deformation (tension, flexural, and compression), showed addition of paraffin waxes resulted in a diminution of all mechanical properties of lv‐HDPE, but the properties of the composites with H‐PW were good in comparison to those with a lower melting point wax (L‐PW, Tm ranging from 18 to 23°C). Evidences from Cole–Cole plots obtained from oscillatory rheology experiments showed there was no interaction between lv‐HDPE and H‐PW, inducing a heterogeneous blend in contrast to the more homogeneous lv‐HDPE/L‐PW composites.

The improvement of the oxidative oat (Avena sativa L.) protein based on ultrasound treatment: Study of structural, emulsifying, and rheological properties

Oat protein (OP) inevitably suffers oxidative attack during long-term storage to form oat oxidized aggregates (OOPA), thus causing deterioration of emulsification properties, so modification of OOPA to enhance their emulsification properties is essential to expand the application of emulsions in the food industry. In this experiment, OOPA was sonicated using ultrasound at different lasted times to obtain ultrasonic oxidative oat protein aggregates (UOOPA) in order to investigate the effects of ultrasound on the structure, emulsification properties and interfacial properties of the OOPA, and to establish the structure-activity association between the structure and emulsifying properties of OOPA under ultrasound by correlation analysis. Based on the physical structure analysis, ultrasound treatment at 25 min led to the reduction of particle size and disulfide bond (S–S) content, exposure of the hydrophobic group, an increase of ζ-potential absolute value. The Fourier transform infrared (FTIR) spectroscopy analysis and atomic force microscopy (AFM) observation also suggested ultrasound-induced depolymerization behavior, improvement of flexibility, and reduction of the roughness of OOPA. The emulsifying properties were optimized when ultrasound treatment lasted for 25 min; the emulsifying activity (EAI) and emulsifying stability (ESI) increased by 2 times and 1 time, compared with the OOPA. Then, viscosity curve and frequency sweep oscillatory experiments proved the ultrasound improved the rheological properties of OOPA and the observations of the confocal laser scanning microscope (CLSM) further provided visual evidence. In general, these results further confirmed the role of ultrasound in promoting the emulsifying properties of OOPA.

Rheological Considerations of Pharmaceutical Formulations: Focus on Viscoelasticity.

Controlling rheological properties offers the opportunity to gain insight into the physical characteristics, structure, stability and drug release rate of formulations. To better understand the physical properties of hydrogels, not only rotational but also oscillatory experiments should be performed. Viscoelastic properties, including elastic and viscous properties, are measured using oscillatory rheology. The gel strength and elasticity of hydrogels are of great importance for pharmaceutical development as the application of viscoelastic preparations has considerably expanded in recent decades. Viscosupplementation, ophthalmic surgery and tissue engineering are just a few examples from the wide range of possible applications of viscoelastic hydrogels. Hyaluronic acid, alginate, gellan gum, pectin and chitosan are remarkable representatives of gelling agents that attract great attention applied in biomedical fields. This review provides a brief summary of rheological properties, highlighting the viscoelasticity of hydrogels with great potential in biomedicine.

Extremely early onset of nonlinear viscoelasticity in dynamic shear of ideally monodisperse DNA

AbstractThis work reports that the rheology of ideally monodisperse lambda () phage DNA solutions is extremely sensitive to the applied strain in dynamic oscillatory experiments. DNA exhibits nonlinearity at strains far smaller than what is observed in conventional synthetic polymers. However, it is found that polydisperse calf thymus DNA does not exhibit the extreme strain sensitivity. By mixing samples of monodisperse DNA with the polydisperse calf thymus DNA, we correlate the molecular weight distribution (or polydispersity) to the onset of nonlinear viscoelasticity. We demonstrate that the strain sensitivity weakens as the polydispersity index increases. This is the first work correlating the inception of nonlinear viscoelasticity in entangled polymer system such as lambda DNA solution to the polydispersity of the samples. The experimental data suggests that ideally monodisperse polymer systems are very sensitive to the applied strain and have a very early transition to the nonlinear regime. We conclude that nominally monodisperse synthetic polymers are not sufficiently monodispersed to exhibit the early onset.

The Ideological and Political Design of B-Z Oscillatory Reaction Experiment

The Ideological and Political Design of B-Z Oscillatory Reaction Experiment

The extrusion of EPDM using an external gear pump: experiments and simulations

Abstract External gear pumps are used in fluid transport systems because of their tight clearances and accurate flow control. These tight clearances are a challenge for numerical studies in terms of spatial discretization. In earlier work, the flow of a viscous fluid in an external gear pump is computed using the finite element method (FEM). An element size based on the respective distance between boundaries is proposed. In this study, results based on the earlier work are compared to extrusion experiments of EPDM. The aim of this study is not only to validate the numerical simulations, but also to determine what material characteristics need to be taken into account for an accurate output prediction of the external gear pump. Especially the introduction of shear-thinning behavior results in an improvement of the amplitude of the pressure difference fluctuation. Taking into account compressibility, alters the torque fluctuation in such a way that it mimics the experiments. Unfortunately, the fluctuation in torque still has a too high amplitude. Eventually, simulations are performed including shear-thinning behavior, a temperature- and pressure-dependent viscosity, and compressibility. The effect of measuring the material behavior using oscillatory or shear experiments is shown. Furthermore, the simulations are applied to a second EPDM. Finally, different processing conditions are tested. For the simulations, only qualitative agreement is found, possibly as a result of the no slip boundary condition.

Impact of maturity on mechanical and textural properties and rheological properties modeling of wood apple fruit (Limonia acidissima)

AbstractWood apple (Limonia acidissima) is monsoonal fruit comprising of pulp with an exterior hard shell. The investigation of mechanical and textural properties according to maturity stages (unripe, semi‐ripe, and ripe) and rheological modeling of ripe fruits is beneficial for post‐harvest maturity grader and processing equipment development. The compression mechanical properties of whole fruit at maturity stages along the x‐axis, y‐axis, and z‐axis orientations recorded minimum rupture force (962.73 N) for ripe fruits followed by semi‐ripe and unripe along z‐axis and maximum force (2,256.2 N) for unripe fruits along y‐axis. The pulp puncture test at maturity stages for textural properties recorded reduced hardness value with maturity having 12.84, 4.30, and 3.88 N for unripe, semi‐ripe, and ripe fruits, respectively. The rheological properties for ripe fruits pulp validated cross and Carreau‐Yasuda best‐fit models (R2 = 0.9) for steady shear indicating non‐Newtonian, shear‐thinning behavior as viscosity (η) reduced from 1,508.45 Pa s while dynamic shear for frequency sweep test indicated elastic behavior as storage modulus G′ &gt; loss modulus G″ and tan δ &lt; 1 for power model (R2 = 0.9). The mechanical and textural properties of wood apple exhibited significant variance according to the maturity of the fruit.Practical ApplicationsWood apple, despite being underutilized in the global fruit trade, has vast application in nutraceuticals industries. The steady shear flow properties of wood apple pulp can be used in engineering design applications such as manufacturing, packaging, and storage; they can be utilized to determine flow parameters and predict the degree of structural breakdown with shear. Because of the numerous processing systems in use, it is essential to precisely forecast rheological behavior. Oscillatory experiments of wood apple pulp can be used to study pulp structure, the strength of pulp and dependency on the shear rate of pulp. The findings of this study will substantially assist food processing industries in producing wood apple pulp‐based products.

The effect of sodium polymethacrylate on the rheology of the positive paste and performance of the lead‐acid battery

AbstractDue to its rheological properties, positive lead‐acid battery paste can be difficult to spread on lead current collectors accurately and efficiently under industry machinery and setting. Sodium polymethacrylate dispersant was studied as an effective positive paste additive that could lower the yield stress of the paste without affecting paste density and battery performance. Under a four‐blade vane rheometer setup, stress growth and oscillatory amplitude strain sweep experiments evaluated the rheological properties of positive paste with the addition of varying amounts of sodium polymethacrylate. Further, the electrochemical effects of sodium polymethacrylate were also evaluated in 2V batteries by testing positive active material utilization and cycle life.

Quantitative Elasticity of Flexible Polymer Chains Using Interferometer-Based AFM.

We estimate the elasticity of single polymer chains using atomic force microscope (AFM)-based oscillatory experiments. An accurate estimate of elasticity using AFM is limited by assumptions in describing the dynamics of an oscillating cantilever. Here, we use a home-built fiber-interferometry-based detection system that allows a simple and universal point-mass description of cantilever oscillations. By oscillating the cantilever base and detecting changes in cantilever oscillations with an interferometer, we extracted stiffness versus extension profiles for polymers. For polyethylene glycol (PEG) in a good solvent, stiffness–extension data showed significant deviation from conventional force–extension curves (FECs) measured in constant velocity pulling experiments. Furthermore, modeling stiffness data with an entropic worm-like chain (WLC) model yielded a persistence length of (0.5 ± 0.2 nm) compared to anomaly low value (0.12 nm ± 0.01) in conventional pulling experiments. This value also matched well with equilibrium measurements performed using magnetic tweezers. In contrast, polystyrene (PS) in a poor solvent, like water, showed no deviation between the two experiments. However, the stiffness profile for PS in good solvent (8M Urea) showed significant deviation from conventional force–extension curves. We obtained a persistence length of (0.8 ± 0.2 nm) compared to (0.22 nm ± 0.01) in pulling experiments. Our unambiguous measurements using interferometer yield physically acceptable values of persistence length. It validates the WLC model in good solvents but suggests caution for its use in poor solvents.

Rheological properties for determining the interaction of soluble cress seed mucilage and β-lactoglobulin nanocomplexes under sucrose and lactose treatments

Protein-polysaccharide complexes are commonly applied in different food products. Their interaction and their functional properties that arise as a consequence of interactions are remarkably influenced by the presence of co-solutes in the system. In this study, general rheological properties and the aggregation behavior of cress seed mucilage (CSM)-β-lactoglobulin (Blg) complexes were studied in the presence of sucrose (5–20% w/v) and lactose (5–20% w/v). The highest values of apparent viscosity and stability (zeta potential) in CSM-Blg complexes were measured when the medium contained 5% w/v lactose (10.00 Pa.s at 0.1 s−1, −25 ± 0.8 mV) and 20% w/v sucrose (12.89 Pa.s at 0.1 s−1, −35 ± 0.2 mV). The results of oscillatory experiments indicated that the gel-like feature of the complexes improved, parallel to a decrease in frequency, which highlighted the shear-induced gelation phenomenon. The thermal analysis test demonstrated that the thermal stability of Blg (70.5◦C), with its complexation to CSM, improved through denaturation. Also, the association of CSM-Blg (82◦C) nanocomplexes with lactose (96◦C) can enhance the thermal stability more effectively. Considering the widespread use of protein-polysaccharide complexes in diverse sugar-containing food formulations, the results of this study can contribute to the creation of new compounds with special techno-functional features.

Rheological behaviour of concentrated maltodextrins describes skin formation and morphology development during droplet drying

We have studied the rheological behaviour of concentrated maltodextrins varying in dextrose equivalence value (DE) over a range of temperatures (≤70 °C) and concentrations (≥62% (w/w) solids) using small amplitude oscillatory experiments. Rheological data could be mapped onto master curves for storage and loss modulus as described with a model for transient or entangled polymeric networks. Maltodextrins DE5 and DE12 showed a rubber-to-glass transition, where the rubber-like behaviour is hypothesized to be the consequence of physical entanglements at sufficiently high concentrations of polysaccharides. Maltodextrins DE21 and DE38 mostly consisting of small sugars and oligomers exhibited viscous behaviour over a large range of frequencies and only at high frequencies the onset of a cross-over was visible which could signify the vicinity of the glass transition. An effective diffusion drying model validated with sessile single droplet drying experiments was coupled to rheological models to estimate rheological behaviour during droplet drying. The predicted rheological properties of the skin formed at the surface of the droplet could be related to the occurrence of surface instabilities later during drying, including cavitation for elastic skins and wrinkling for viscous skins. Insight in rheological behaviour at high concentrations may therefore offer help in predicting the morphology of droplets generated with spray drying.