The aim of this work is to provide a simple tool for estimating the effective free radical polymerization rate constant and average chain length for multiphase systems when initiation occurs only at the phase boundary. Three cases are considered - polymerization of monomer layer, droplet and jet. The radical distribution profile was found for each geometry by approximating the solution of the appropriate diffusion-reaction problem in a limit of low and high Damköhler number. Averaging of corresponding profile over the system volume allows us to estimate expressions for effective rate constant and average chain length in terms of a Damköhler number. The expressions obtained in this way were shown to be in a good agreement with numerical calculations.

Рассмотрен процесс свободно-радикальной полимеризации микрокапли мономера при течении в несовместимой с ней жидкости, содержащей раствор инициатора. Поскольку генерация первичных радикалов происходит во внешней среде, то инициирование полимеризации возможно только за счет взаимодействия между молекулами мономера на поверхности капли с первичными радикалами в окружающей среде.

The influence of cavitation-generated free radicals on the suspension polymerization of n-butyl methacrylate under ultrasonic irradiation was investigated experimentally and by kinetic modeling. A kinetic model of radical polymerization was proposed, considering the interaction of free radicals generated in cavitation bubbles with both monomer molecules and free radicals formed within the monomer droplets. It was found that the primary contribution to the polymerization acceleration come from cavitation radicals generated inside the monomer droplets, whereas the impact of cavitation radicals formed in the dispersion medium decreases as the fraction of monomer in the mixture decreases. Increasing the concentration of cavitation radicals was shown to lead to a reduction in the average chain length and the dispersity of terminated chains. It was demonstrated that the computational results are in good agreement with experimental data.Highlights Kinetic model for suspension polymerization in ultrasonic field is proposed. Impact of cavitation radicals on suspension polymerization kinetics is studied. Chain length and dispersity decrease with increasing amount of cavitation radicals.

The deformation behavior and hydrodynamic stability of a three-dimensional Newtonian single-core compound droplet during flow in a channel with sudden contraction were studied by numerical modeling. This research was motivated by the quest for conditions of the steady transition of a compound droplet into a composite microfiber, whose core is stretched as much as the shell. With this aim, the dynamics and morphology evolution of the compound droplet were analyzed in detail as functions of capillary number, core-to-shell relative viscosities, interfacial tensions, and the relative initial core radius. It was found that the effective elongation of the core occurs either with a significant increase in the shell viscosity relative to the ambient fluid or with a decrease in the core viscosity with respect to the shell. In this case, as the composite droplet advances into the narrowing zone of the canal, it continues to stretch, becoming a bullet-shaped composite microfiber. A new mechanism of disintegration of the compound droplet was revealed, which is caused by the core destabilizing effect and manifests itself either with an increase in the relative core/shell interfacial tension or the relative core viscosity.

The free-radical polymerization inside monomer droplets surrounded by an immiscible liquid of the initiator solution is studied before the gel effect by means of the generalized method of moments taking into account diffusion of radicals and polymer chains. The influence of the droplet radius and initiator concentration in the surrounding phase on the polymerization kinetics and molecular mass characteristics of the synthesized polymer is analyzed. It is found that the polymerization kinetics matches the bulk polymerization when the droplet is small enough (kinetic regime). However, increasing the droplet radius leads to a diffusion-kinetic regime, in which the polymerization rate becomes significantly lower. As a result, the average length and the dispersity of the produced polymer chains depend significantly on the monomer droplet size. The dependence of the effective polymerization rate coefficient on the droplet radius and the initiator concentration is discussed.

The aim of this work was to study the relationship between hydrodynamic resistance and flow instability of shear-thinning fluids in channels with sudden contraction and expansion. To this end, the critical conditions of bifurcation transitions from symmetric to asymmetric vortex flow depending on the channel geometry and rheological properties of Carreau model fluid were investigated. It was found that increasing the contraction ratio leads to decreasing the critical Reynold number as a power function, while variation in the length of the channel narrow part has almost no effect on the position of the bifurcation transition point, but changes the corner vortex lengths. It was found that the considered bifurcation transitions in shear-thinning fluids lead to a non-monotonic dependence of the hydrodynamic resistance of channels with sudden contraction and expansion on Reynolds number with a minimum located around the critical Reynolds number.

The effect of external fluid on the mixing efficiency of miscible Newtonian liquids inside a two-dimensional microdroplet upon co-flow in a rectilinear microchannel is investigated numerically. With this aim, two different types of continuous media, Newtonian and Carreau–Yasuda shear-thinning fluids, are considered. The mixing time within a microdroplet is estimated from standard deviation of concentration from the average value. The dependencies of the mixing time on Péclet number, viscosity ratio of the fluid components and confinement parameter (ratio of the droplet initial diameter to microchannel cross-section size) are explored. It is shown that for both Newtonian and Carreau–Yasuda continuous phases, the corresponding mixing times in a microdroplet are the power-law functions of Péclet number. It was found that values of the corresponding exponents are determined by rheological properties of the fluids. Three distinct liquid mixing mechanisms inside microdroplet at different values of the confinement parameter are revealed.

The study of shear-thinning non-Newtonian fluids in T-junction parallelized microchannels provides valuable insights into the emulsification process and droplet generation synchronicity. Silicone oil is used as the continuous phase, and sodium carboxymethyl cellulose (CMC) aqueous solutions are used as the dispersed phase. Based on the evolution of the dispersed phase head and neck characteristic size, the droplet generation is classified into four stages: the waiting stage, filling stage, necking stage, and pinch-off stage. The effects of the two-phase flow rates and rheological characteristics of shear-thinning fluids on the four stages are investigated. Additionally, the coupling of interface evolution between parallelized microchannels is studied, revealing periodic changes between synchronous and asynchronous droplet generation types. The impact of this phenomenon on droplet monodispersity is analyzed, providing crucial information for the preparation of uniform droplets.

Experimental and numerical simulation methods have been used to study deformation behavior of viscous droplets flowing with a Newtonian dispersion medium in a channel with an abrupt contraction. The characteristic features of the evolution of droplet shapes have been revealed depending on their current position relative to the entrance into the contraction region. Droplet elongation has been investigated as a function of the confinement parameter, i.e., the ratio of the initial droplet diameter to the cross-sectional size of the narrow part of the channel. The mechanisms of microfiber formation in a convergent flow have been analyzed.

Free-radical polymerization with periodic photoinitiation profile at low conversion degree has been modeled. The method of moments used for the calculations has been able to adequately reproduce the asymptotic solutions at low and high initiation frequency, which has confirmed the validity of the obtained results. The effect of the initiation frequency over its wide range on the characteristics of the obtained polymer and depth of polymerization has been studied. The evolution of the number-average degree of polymerization and the polydispersity index has been investigated. Duration of the induction period, during which the amplitude of the molecular mass oscillation is changed over time has been found as a function of the initiation frequency. It has been stated that the polydispersity index is significantly increased if the photoinitiation frequency is below certain threshold, pointing at the polymodality of the molecular mass distribution. The optimal range of the frequency to determine the propagation rate constant taking advantage of the periodic photoinitiation has been determined.

This pilot study aimed at investigating an alternative to irradiation-crosslinking to increase the structural stability of ethylene tetrafluoroethylene (ETFE), by mixing this polymer matrix with polyoxides. The latter consisted of aluminum polyphosphate (AP) having a flow temperature near to that of ETFE to facilitate melt-mixing by extrusion, and rigid fillers of metakaolin (MK). It was found that the ETFE/AP/MK composite with the formulation 60/20/20 (wt %) exhibited the most relevant properties. Indeed, when comparing this composite with neat ETFE, the structural stability was improved until 120 °C, the onset temperature of degradation passed from 381.5 to 459.4 °C, the elastic modulus evolved from 0.4 GPa to 1.6 GPa, and the tensile strength increased from 23 to 27 MPa. The results were briefly discussed based on a potential interaction between the polyoxides and the polymer matrix and synergistic effect between the two polyoxides.

Using micro-particle image velocimetry (μPIV), the convective flow inside a silicone oil droplet was investigated in detail during its formation in coaxial capillaries under co-flow in a water/glycerol mixture continuous phase. The analysis of μPIV measured flow field revealed that two characteristic flow areas exist in the droplet in formation: an inflow zone and a circulation zone. The intensity of vortex flow in these zones was estimated by calculating the average angular velocity of these vortices under the condition of no shear for different dispersed phase and continuous phase flow rates and for different viscosity ratios between the two phases. The evolution of the vortex flow pattern inside the droplet was investigated thoroughly all the way from the step of their formation to the step of the free-moving droplet. The results of this study are important for understanding the mixing processes inside the droplet at different stages of its formation.

This work is focused on the mechanisms of the dripping and jetting flow modes of viscoelastic semi-dilute polyacrylamide aqueous solutions co-flowing with silicone oil in co-axial capillaries. A phase diagram of the dripping, jetting, and intermediate flow modes is established. It was found that in the dripping mode, the elongation velocity of the filament between the terminal droplet and the inner capillary is controlled solely by the continuous phase rate. At the same time, the decrease in the filament diameter is due to both stretching and outflow of the polymer solution into the terminal droplet. In the jetting mode, the thread diameter was found to evolve in three stages. In the first stage, the average jet velocity increases, whereas in the second and third stages, it becomes constant and corresponds to the velocity of the continuous phase. The transition from the second to the third stage is defined by the appearance of capillary waves resulting in the formation of the beads-on-string structure. In the third stage, the filament diameter between the neighbor beads decreases exponentially and is governed by the relaxation time, which strongly depends on polymer concentration, but does not depend on the continuous phase flow rate. A simple physical model was proposed for describing the evolution of dimensions of filaments and beads during development of jet capillary instability. The universal character of the evolution of filaments and beads sizes, which is independent of concentration of semi-diluted polymer solutions and flow rates of the continuous phase, is revealed.

The possibility of synthesizing and processing incombustible materials based on oligomer and polymer oxides and their hybrids with organic and organoelement compounds at temperatures of 20–200°C is explored. Inorganic thermosetting plastics with a wide range of pour points (100–700°C) are synthesized. A more intense heat treatment can provide the formation of fireproof materials. Cast polymer–polymer blends of inorganic/organic polymers prepared by melt blending are studied. Possible areas of application of reinforced materials based on inorganic and hybrid impregnation composites are determined.