The effect of magnetic particles covering the droplets on the heating rate of Pickering emulsions in the AC magnetic field

Optimization of ultrasound heating with Pickering droplets using core-shell scattering theory

Nanoparticles find widespread application in various medical contexts, including targeted nanomedicine and enhancing therapeutic efficacy. Moreover, they are employed to stabilize emulsions, giving rise to stabilized droplets known as Pickering droplets. Among the various methods to improve anti-cancer treatment, ultrasound hyperthermia stands out as an efficient approach. This research proposes Pickering droplets as promising sonosensitizer candidates, to enhance the attenuation of ultrasound with simultaneous potential to act as drug carriers. The enhanced ultrasound energy dissipation could be, therefore, optimized by changing the parameters of Pickering droplets. The ultrasound scattering theory, based on the core-shell model, was employed to calculate theoretical ultrasound properties such as attenuation and velocity. Additionally, computer simulations, based on a bioheat transfer model, were utilized to compute heat generation in agar-based phantoms of tissues under different ultrasound wave frequencies. Two types of phantoms were simulated: a pure agar phantom and an agar phantom incorporating spherical inclusions. The spherical inclusions, with a diameter of 10 mm, were doped with various sizes of Pickering droplets, considering their core radius and shell thickness. Computer simulation of these spherical inclusions incorporated within agar phantom resulted in different enhancement of achieved temperature elevation, which depending on the core radius, shell thickness, and the material properties of the system. Notably, spherical inclusions doped with Pickering droplets stabilized by magnetite nanoparticles exhibited a higher temperature rise compared to droplets stabilized by silica nanoparticles. Moreover, nanodroplets with a core radius below 400 nm demonstrated better heating performance compared to microdroplets. Furthermore, Pickering droplets incorporated into agar phantom could allow obtaining a similar effect of local heating as sophisticated focused ultrasound devices.

Ultrasound Control of Pickering Emulsion-Based Capsule Preparation

Capsules with microparticle shells became of great interest due to their potential in many fields. Those capsules can be fabricated at high temperatures from particle-stabilized emulsions (Pickering emulsions) by sintering together particles that cover droplets. One of the problems with such an approach is accurately controlling whether particles are already sintered and creating the rigid capsule shell of a capsule. Here, we propose using a non-destructive ultrasound method for monitoring Pickering emulsion-based capsules prepared using heating under an alternating magnetic field. The polyethylene microparticles that were responsive to temperatures higher than 112 °C were used as droplet stabilizers together with iron oxide nanoparticles. During the coalescence of the droplets, facilitated by an external electric field, the ultrasonic attenuation increased, giving evidence that the ultrasound method detects structural changes in Pickering emulsions. The main change was the difference in the droplets' size, which was also observed via optical microscopy. The attenuation of ultrasound increased even more when measured after magnetic heating for the same concentration of particle stabilizers. Simultaneously, the values of ultrasonic velocity did not exhibit similar variety. The results show that the values of the attenuation coefficient can be used for a quantitative evaluation of the capsule formation process.

Ultrasound Study of Magnetic and Non-Magnetic Nanoparticle Agglomeration in High Viscous Media

Ultrasound attenuation spectroscopy has found wide application in the study of colloidal dispersions such as emulsions or suspensions. The main advantage of this technique is that it can be applied to relatively high concentration systems without sample preparation. In particular, the use of Epstein-Carhart-Allegra-Hawley's (ECAH) ultrasound scattering theory, along with experimental data of ultrasound velocity or attenuation, provide the method of estimation for the particle or droplet size from nanometers to millimeters. In this study, suspensions of magnetite and silica nanoparticles in high viscous media (i.e., castor oil) were characterized by ultrasound spectroscopy. Both theoretical and experimental results showed a significant difference in ultrasound attenuation coefficients between the suspensions of magnetite and silica nanoparticles. The fitting of theoretical model to experimental ultrasound spectra was used to determine the real size of objects suspended in a high viscous medium that differed from the size distributions provided by electron microscopy imaging. The ultrasound spectroscopy technique demonstrated a greater tendency of magnetic particles toward agglomeration when compared with silica particles whose sizes were obtained from the combination of experimental and theoretical ultrasonic data and were more consistent with the electron microscopy images.

Inter-droplet force between magnetically polarizable Pickering oil-in-water nanoemulsions stabilized with γ-Al2O3 nanoparticles: Role of electrostatic and electric dipolar interactions

HYPOTHESIS

The presence of nanoparticles at oil-water interface influences the interaction forces between Pickering emulsions. When charged nanoparticles are at the oil-water interface of an electrostatically stabilized emulsion, in addition to the screened Coulombic interaction, electric dipolar force also influences the total inter-droplet force profiles. An in-depth understanding of the effects of such electric dipolar forces is essential for designing colloidally stable Pickering nanoemulsions for various applications.

EXPERIMENTS

Inter-droplet forces between γ-Al₂O₃ nanoparticle stabilized oil-in-water nanoemulsion, containing superparamagnetic nanoparticles (magnetically polarizable) in the oil phase, are measured using the magnetic-chaining technique at different pH and salt concentrations. The role of mono-, di- and tri-valent salts on the inter-droplet force profiles are assessed.

FINDINGS

Force measurement studies reveal a lowering of inter-droplet spacing, within the linear chains, for higher salt concentrations due to an increased screening. Strong interfacial attachment of the charged nanoparticles results in the formation of an asymmetric charge cloud leading to an electric dipolar interaction. Incorporating the contributions of electric dipolar and screened Coulombic interactions, the theoretically estimated total repulsive force magnitudes are in good agreement with the experimental data. The obtained results offer better insights into the nature of colloidal force between charged particle stabilized nanoemulsions.

Interfacial structures of particle-stabilized emulsions examined by ultrasonic scattering analysis with a core-shell model

Pickering emulsions comprising liquid droplets stabilized by solid microparticles have gained much attention in the field of cosmetics, inks, and drug delivery systems. To ensure that microparticles in Pickering emulsions are localized at the surface of liquid droplets, ultrasonic spectroscopy analysis combined with scattering function theory was conducted in this study. Two specific cases were investigated: (1) silica particles and liquid droplets independently dispersed in liquid and (2) silica particles effectively localized at the surface of the droplets. It was found that the core-shell model was effective for analyzing nanoparticles anchored at the surface of oil droplets. Conversely, it was found that an effective shell comprised of solid particles was no longer observed as the particle size or the distance between solid particles increased. When a large solid particle was applied, the ultrasonic spectra resembled those of conventional surfactant-stabilized emulsions without solid stabilizers.

The Effect of Particle Shell on Cooling Rates in Oil-in-Oil Magnetic Pickering Emulsions

Pickering emulsions (particle-stabilized emulsions) are usually considered because of their unique properties compared to surfactant-stabilized emulsions including better stability against emulsion aging. However, the interesting feature of particle-stabilized emulsions could be revealed during their magnetic heating. When magnetic particles constitute a shell around droplets and the sample is placed in an alternating magnetic field, a temperature increase appears due to energy dissipation from magnetic relaxation and hysteresis within magnetic particles. We hypothesize that the solidity of the magnetic particle shell around droplets can influence the process of heat transfer from inside the droplet to the surrounding medium. In this way, particle-stabilized emulsions can be considered as materials with changeable heat transfer. We investigated macroscopically heating and cooling of oil-in-oil magnetic Pickering emulsions with merely packed particle layers and these with a stable particle shell. The change in stability of the shell was obtained here by using the coalescence of droplets under the electric field. The results from calorimetric measurements show that the presence of a stable particle shell caused a slower temperature decrease in samples, especially for lower intensities of the magnetic field. The retarded heat transfer from magnetic Pickering droplets can be utilized in further potential applications where delayed heat transfer is desirable.