Amol A. Kulkarni

Size-Controlled Flow Synthesis of Gold Nanoparticles Using a Segmented Flow Microfluidic Platform

Segmented flow is often used in the synthesis of nanomaterials to achieve narrow particle size distribution. The narrowness of the distribution is commonly attributed to the reduced dispersion associated with segmented flows. On the basis of the analysis of flow fields and the resulting particle size distribution, we demonstrate that it is the slip velocity between the two fluids and internal mixing in the continuous-phase slugs that govern the nature of the particle size distribution. The reduction in the axial dispersion has less impact on particle growth and hence on the particle size distribution. Synthesis of gold nanoparticles from HAuCl(4) with rapid reduction by NaBH(4) serves as a model system. Rapid reduction yields gold nuclei, which grow by agglomeration, and it is controlled by the interaction of the nuclei with local flow. Thus, the difference in the physical properties of the two phases and the inlet flow rates ultimately control the particle growth. Hence, a careful choice of continuous and dispersed phases is necessary to control the nanoparticle size and size distribution.

Application of multiresolution analysis for simultaneous measurement of gas and liquid velocities and fractional gas hold-up in bubble column using LDA

A new strategy using wavelet transforms for multiresolution analysis of bubble column hydrodynamics from laser Doppler anemometer measurements of the axial velocity has been formulated. The advantage of the methodology is that we may infer intrinsic features of the bubble column hydrodynamics by accurately detecting the passage of bubbles at local positions in the column. The detection of bubbles was possible by denoising local time series data and analyzing for its intermittent behavior. The methodology allows for calculating the changes in the local average liquid velocities and local gas hold-up and was related to the intermittent nature of turbulence in the system. The variation in these properties and flow behavior as a function of radial distance have been studied.

Simultaneous measurement of hold-up profiles and interfacial area using LDA in bubble columns: predictions by multiresolution analysis and comparison with experiments

A wavelet-based technique was used for the identification of bubble events from the velocity-time data obtained using LDA in bubble column reactors. Bubble rise velocity distribution was also measured from the velocity-time series, which enabled the estimation of bubble size and shape distributions. From this information, the values of local fractional gas hold-up (e G ) and effective interfacial area (a) were calculated. The estimated values of a have been compared with those obtained from the chemical method of air-oxidation of sodium sulfite. For this purpose, LDA measurements were carried out during the reaction of sulfite oxidation.

Wavelet transform of velocity-time data for the analysis of turbulent structures in a bubble column

Hydrodynamic and turbulent structures in bubble column reactors have been characterized from velocity-time series data obtained by laser Doppler anemometer. Measurements were made in two bubble columns of 100 and 150 mm inner diameter under various operating conditions. The superficial gas velocity was varied in the range of 13-36.5 mm/s and in order to investigate the role of sparger design, different types namely sieve plate, single hole and sintered plate were employed. Wavelet transform analysis was used for denoising the measured data and also for evaluating quantifiers of turbulence, viz., local intermittency measure, intermittency index and flatness factor. These quantifiers help in detecting the passage of bubbles and reveal the hidden structures and patterns in the data. The low-frequency structures that were detected for a wide range of experimental conditions showed the likely existence of circulation cells. Also, the wavelet scalewise analysis of the turbulence at various locations in the column and under different operating conditions could be used to build a direct relationship between fractional gas hold-up and flatness factor values. The methodology is therefore suitable for online evaluation of bubble column performance and shows promise for developing strategies for improving process performance.

Insights in the Diffusion Controlled Interfacial Flow Synthesis of Au Nanostructures in a Microfluidic System

Continuous segmented flow interfacial synthesis of Au nanostructures is demonstrated in a microchannel reactor. This study brings new insights into the growth of nanostructures at continuous interfaces. The size as well as the shape of the nanostructures showed significant dependence on the reactant concentrations, reaction time, temperature, and surface tension, which actually controlled the interfacial mass transfer. The microchannel reactor assisted in achieving a high interfacial area, as well as uniformity in mass transfer effects. Hexagonal nanostructures were seen to be formed in synthesis times as short as 10 min. The wettability of the channel showed significant effect on the particle size as well as the actual shape. The hydrophobic channel yielded hexagonal structures of relatively smaller size than the hydrophilic microchannel, which yielded sharp hexagonal bipyramidal particles (diagonal distance of 30 nm). The evolution of particle size and shape for the case of hydrophilic microchannel is also shown as a function of the residence time. The interfacial synthesis approach based on a stable segmented flow promoted an excellent control on the reaction extent, reduction in axial dispersion as well as the particle size distribution.

Assessing the possibilities of designing a unified multistep continuous flow synthesis platform

The multistep flow synthesis of complex molecules has gained momentum over the last few years. A wide range of reaction types and conditions have been integrated seamlessly on a single platform including in-line separation as well as monitoring. Beyond merely getting considered as ‘flow version’ of conventional ‘one-pot synthesis’, multistep flow synthesis has become the next generation tool for creating libraries of new molecules. Here we give a more ‘engineering’ look at the possibility of developing a ‘unified multistep flow synthesis platform’. A detailed analysis of various scenarios is presented considering 4 different classes of drugs already reported in the literature. The possible complexities that an automated and controlled platform needs to handle are also discussed in detail. Three different design approaches are proposed: (i) one molecule at a time, (ii) many molecules at a time and (iii) cybernetic approach. Each approach would lead to the effortless integration of different synthesis stages and also at different synthesis scales. While one may expect such a platform to operate like a ‘driverless car’ or a ‘robo chemist’ or a ‘transformer’, in reality, such an envisaged system would be much more complex than these examples.