Stability and Interfacial Viscoelasticity of Oil-Water Nanoemulsions Stabilized by Soy 2 Lecithin and Tween 20 for the Encapsulation of Bioactive Carvacrol

The rheology of oil-in-water (O/W) droplet interfaces stabilized by food-grade emulsifiers (soy lecithin or Tween 20) under controlled aqueous conditions was investigated to elucidate its contribution in the kinetic stabilization of nanoemulsion-based delivery systems containing carvacrol, a naturally-derived antimicrobial compound. Dilational rheology of surfactant-laden O/W interfaces was measured using axisymmetric drop shape analysis. The kinetic stability of corresponding nanoemulsions (containing mixtures of carvacrol and medium-chain triglyceride (MCT) oil dispersed in water (pH 7)) was characterized using dynamic light scattering. Zwitterionic lecithin molecules adsorbed to the O/W interface for 24 h formed a notably viscoelastic layer, compared to nonionic Tween 20 molecules. The kinetic stability within the first 24 h for each nanoemulsion was strongly dependent upon encapsulated carvacrol concentration, with higher carvacrol concentrations leading to lower kinetic stability. Lecithin-stabilized nanoemulsions (pH 7) were highly stable, yielding monodispersed droplet size distributions and high resistance to increases in droplet size over 30 days. Contrarily, corresponding Tween 20-stabilized nanoemulsions showed large increases in the droplet size and developed a bimodal droplet size distribution over time. The initial size of oil droplets stabilized by lecithin was slightly dependent on pH, yielding smaller droplets at pH 7 and larger droplets at pH 3; however, the extended kinetic stability was not greatly impacted by pH modulation. Determining a positive association between interfacial viscoelasticity and nanoemulsion stability may potentially be very useful for food manufacturers seeking to optimize the encapsulation and delivery of lipophilic antimicrobial molecules using food-grade emulsifiers

Use of Oscillatory Shear to Study the Effect of Limestone Filler on the Rheology of Early-Age Portland Cement

Cement is a material that has been in use since the ancient times and is the most widely manufactured material in industry today. During the production of cement, limestone undergoes a process called calcination which releases CO2. In order to reduce the environmental impact and cost of cement production it has become standard practice to replace a portion of the cement mixture with ground limestone, but this causes a change in the rheological profile of the mixture. This change in rheology affects both the short and long term workability of the material. In this study, small amplitude oscillatory shear (SAOS) was used to characterize the rheology of cement mixtures with a water to cement ratio (w/c) of .42. The tested samples were unaltered cement, cement blended with coarse limestone (10.8 µm), and cement blended with fine limestone (1.3 µm). The evolution of G’ and G” was tracked during the early stages of cement setting. Results of the study show that the storage (G’) and loss (G”) moduli increase as the limestone particle size is made smaller than the cement particle size. Tests also show that cement pastes exhibit greater shrinkage with the finer particles

Interfacial Rheological Mechanics of a Non-ionic, Tri-block Copolymer at Water/Hexadecane Interface

Growing interest on the stability of foams and emulsions has lead to concentrated research of interfacial rheology. The response of an interfacial layer to mechanical deformation in size and shape is dependent on its composition. [Miller 2010] This research analysis focused on the adsorption and rheological mechanics of the non-ionic, tri-block copolymer, Pluronic 17R4 at the water/hexadecane interface. The adsorption and viscoelastic properties of the interface were measured via methods of pendant drop tensiometry and dynamic oscillation with drop shape analysis software. Interfacial tension measurements were taken to study the surface pressures of Pluronic 17R4 solutions with concentrations ranging from 1x10-6 M to 1x10-2 M. Dynamic oscillation experiments were conducted to study the elastic and viscous responses of the Pluronic 17R4 solutions subjected to a series of sinusoidal area deformations at frequencies ranging from 0.01 Hz to 1 Hz. Experimental analysis showed that as concentration and deformation rates increase, the elastic response of the interfacial layer decreases and the viscous response increases. This implies that at higher concentrations and deformation rates, the interfacial layer is behaving more like a viscous film than an elastic solid

Characterization of Superabsorbent Poly(Sodium-Acrylate Acrylamide) Hydrogels and Infuence of Chemical Structure on Internally Cured Mortar

Internal curing of mortar through superabsorbent polymer hydrogels is explored as a solution to self-desiccation. Four different hydrogels of poly(sodium-acrylate acry- lamide) are synthesized and the impact of chemical composition on mortar is assessed with relative humidity and autogenous shrinkage testing. The hydrogels are characterized with swelling tests in different salt solutions and compression tests. Chemical composition af- fected both swelling kinetics and gel network size. Mortar containing these hydrogels had increased relative humidity and markedly reduced autogenous shrinkage. Additionally, the chemical structure of the hydrogels was found to signifcantly impact the mortar’s shrink- age. Hydrogels that quickly released most of their absorbed fuid were able to better reduce autogenous shrinkage compared to hydrogels that retained fuid for longer periods (\u3e 4 hours), although this performance was highly sensitive to total water content. The release of absorbed water in hydrogels is most likely a function of both Laplace pressure of emptying voids and chemically-linked osmotic pressure developing from an ion concentration gradient between the hydrogels and cement pore solution. If the osmotic pressure is strong enough, the hydrogels can disperse most of the absorbed water before the depercolation of capillary porosity occurs, allowing the water to permeate the bulk of the mortar microstructure and most effectively reduce self-desiccation and autogenous shrinkage

Investigation of Fracture and Healing Behavior of Thermoreversible Gels via Oscillation Rheology

Thermoreversible gels have the unique ability to self-heal, or repair themselves, once they are fractured. They are physically cross-linked, thus providing them with the capability to reform their broken bonds as a function of temperature. The objective of this project is to determine the extent of the gels’ recovery. If self-healing does in fact occur, these gels can be applied in various industries, including medicine for drug delivery or paints and coatings. The tri-block polymer poly(methyl methacrylate)-poly(n-butyl acrylate)-poly(methyl methacrylate) (PMMA-PnBA-PMMA) was heated and stirred with 2-ethyl-1-hexanol to create a polymer gel. Through the use of a rheometer, a shear stress was applied to fracture the bonds. The fixture was then oscillated to gently probe the polymer gel at 28°C, 25°C, 23°C, and 21°C. This data was compared to the unfractured gel to determine the degree of recovery. It was found that the bonds did, indeed, reform, as over 100% recovery was presented in the storage modulus for all four temperatures and in the loss modulus at 23°C and 21°C. However, the plane of fracture is in question. The exact location of the fractured gel hasn’t been found, thus to determine exactly how much has been fractured, the applied shear stress time can be extended. Other further experimentation includes using a rheo-PIV (particle image velocimetry) system to properly determine whether the rheometer recorded the fractured or original gel

Substrate temperature effects on the peel behavior of temporary pavement marking tapes

Temporary pavement marking (TPM) tapes are utilized In road construction to delineate temporary traffic lanes and work zones. Adhesive failure of TPM tapes can therefore remove lane and work zone designations, confusing drivers and causing serious accidents, especially in high speed zones. Thus, the adhesion of TPM tapes to pavement surface plays an important role in road construction traffic safety. Pressure sensitive adhesives (PSAs) comprise the adhesive layer of TPM tapes. The adhesion of PSAs depends on their temperature-dependent viscoelastic properties. Since environmental conditions vary during construction, the adhesion of TPM tapes will change over a range of operating temperatures. The viscoelastic properties and peel force of four brands of commercial TPM tapes were characterized via double lap shear dynamic mechanical analysis and 90° angle peel adhesion testing over a range of temperatures (−20°C to 40°C). The interfacial fracture behavior and peel forces were analyzed with respect to the measured viscoelastic properties of TPM tapes. For temperatures below the glass transition temperature of the top layer and the transition temperature into the rubbery plateau of the PSA, the peel force decreased. Through this simple technique, an effective operating temperature range for each TPM tape was determined

Microstructural refinement of cement paste internally cured by polyacrylamide composite hydrogel particles containing silica fume and nanosilica

Supplementary cementitious materials were incorporated into hydrogel-based internal curing agents to improve the hydration, microstructure, and ultimately strength of internally cured high-performance cement paste. Polyacrylamide composite hydrogel particles containing amorphous silica – either silica fume or nanosilica – and two different polymer network crosslink densities were synthesized and incorporated into cement paste. The presence of silica and low crosslink density increased the absorption capacity of the particles in pore solution. Micrographs of internally cured paste indicated a significant improvement in hydrogel-related void-filling ability and an increase in void size for low crosslink density particles containing silica. Compressive strength and electrical resistivity increased at later ages for paste samples containing particles with higher silica dosage. The relationship between extent of hydration, void size, and void-filling activity was found to strongly influence the paste\u27s long-term strength and is thus an important structure-property relationship to consider when selecting hydrogels for internal curing purposes

Synthesis and Characterization of Polymer-Silica Composite Hydrogel Particles and Influence of Hydrogel Composition on Cement Paste Microstructure

The objective of this research is to define the fundamental structure-property relationships of water-swollen polymer hydrogel particles that are employed as internal curing agents in cementitious mixtures, in addition to reporting a novel synthesis procedure for combining pozzolanic materials with hydrogel particles. Solution polymerization was performed to incorporate amorphous nanosilica particles within acrylic-based polymer hydrogel particles of varying chemical compositions (i.e., monomer ratio of acrylic acid (AA) to acrylamide (AM)). Experiments were designed to measure the absorption capacity and kinetics of hydrogel particles immersed in pure water and cementitious pore solution, as well as determine the impact of particles on cement paste microstructure. While majority-AM hydrogel particles displayed relatively stable absorption values during immersion in pore solution, majority-AA hydrogel particles desorbed fluid over time, most likely due to the interactions of multivalent cations in the absorbed solution with the anionic polymer network. Interestingly, the addition of negatively charged nanosilica particles accelerated and enhanced this desorption response. When incorporated into cement paste, majority-AM hydrogel particles encouraged the formation of calcium hydroxide and calcium silicate hydrate within the void space previously occupied by the swollen particles. When nanosilica was added to the hydrogel particles, a 53 % increase in the number of hydrogel voids containing hydrated product was observed for the 17 % AA hydrogel particles, and a 140 % increase was observed for the 83 % AA hydrogel particles. These results suggest that the combination of nanosilica with polymeric hydrogel particles provides a favorable environment for the pozzolanic reaction to proceed and that nanosilica aids in the replenishment of hydrogel void space with hydrated cement phases

Effect of Ionic Crosslinking on The Swelling and Mechanical Response of Model Superabsorbent Polymer Hydrogels for Internally Cured Concrete

The chemical and physical structure-property relationships of model superabsorbent polymer (SAP) hydrogels were characterized with respect to swelling behavior and mechanical properties in different ionic solutions (Na+, Ca2+, and Al3+). The model hydrogels were composed of poly(sodium acrylate-acrylamide) (PANa-PAM) copolymer with varying concentrations of PANa (0, 17, 33, 67, and 83 wt.%) and covalent crosslinking densities of 1, 1.5, and 2 wt.%. By synthesizing the hydrogels in-house, systems with independently tunable amounts of covalent crosslinking and anionic functional groups were created, allowing for the relative effects of covalent and ionic crosslinking on the properties of the hydrogels to be directly quantified. It was found that the presence of Ca2+ and Al3+ in the absorbed fluid significantly decreased the swelling capacity and altered the swelling kinetics of the PANa-PAM hydrogels. The presence of Al3+ in solution resulted in the unexpected formation of a mechanically stiff barrier layer at the hydrogel’s surface, which hindered the release of fluid and caused the overall elastic modulus of the hydrogel to increase from O(10 kPa) for hydrogels immersed in Ca2+ solutions to O(100 kPa) for hydrogels immersed in Al3+ solutions. Tensile tests performed on isolated specimens of the stiff barrier layer yielded elastic moduli in the O(50-100 MPa) range

Controllable Internal Mixing in Coalescing Droplets Induced by The Solutal Marangoni Convection of Surfactants with Distinct Headgroup Architectures

Through several complementary experiments, an investigation of the bulk and interfacial flows that emerged during the coalescence of two water-in-oil droplets with asymmetric compositional properties was performed. By adding surfactant to one of the coalescing droplets and leaving the other surfactant-free, a strong interfacial tension gradient (i.e., solutal Marangoni) driving energy between the merging droplets generated pronounced internal mixing. The contributions of two distinct types of surfactant, anionic ammonium lauryl sulfate (ALS) and cationic cetyltrimethylammonium bromide (CTAB) on the rate of coalescence bridge expansion and on the generation of opposing flows during coalescence were investigated. All coalescence experiments supported the power law relation between the radius of the expanding connective liquid bridge and time, rb ∝ t1/2. However, the presence of surfactant decreased the magnitude of the prefactor in this relationship due to induced interfacial solutal Marangoni convection. Experiments showed that packing efficiency, diffusivity, and bulk concentration of the selected surfactant are vital in solutal Marangoni convection and thus the degree and timescale of internal mixing between merging droplets, which has yet to be adequately discussed within the literature. Denser interfacial packing efficiency and lower diffusivity of CTAB produced stronger opposing bulk and interfacial flow as well as greater bulk mixing. A discussion of how optimized surfactant selection and solutal Marangoni convection can be used for passively inducing convective mixing between coalescing drops in microfluidic channels when viscosity modulation is not feasible is provided