Deposits formed from the evaporation of sessile droplets and the interaction of particles with a liquid
Abstract
The deposition of particles onto a surface from an evaporating sessile droplet was investigated using the Galerkin Finite element method. In the computer code that was developed to simulate fluid, vapor and particle phases simultaneously, a convective-diffusive equation along the drop/substrate interface was successfully added into the simulation for more accurate deposition profile calculations as well as calculating the profile when there is desorption. The effect of different dimensionless groups on final deposition profiles was investigated: Pe number, Da number and Da-1 number. Simulation results indicate that, higher Pe and lower Da generated an edge deposition pattern. The deposition rate decreased as Da number is decreased. With desorption, an edged deposition pattern is formed at higher Da-1 numbers. The rate of deposition decreases as the desorption increases. An edge deposition pattern was observed in an experiment with water droplets sitting on a glass substrate suspended with silica particles. The analysis of the interaction potential between silica particles and a glass substrate shows that the deposition rate constant is very small for such case. A solid phase is considered in this case when particle concentration reaches maximum-packing density and the movement of the solid/fluid interface was captured. Simulation results show that the under this case, the edge pattern does not disappear even the droplet is evaporating under very high humidity. The second part of our research is to study the interaction between water and pharmaceutical powders. The real (ϵ') and imaginary (ϵ") components of the relative permittivity of anhydrous lactose and microcrystalline cellulose (MCC) under different bulk densities, moisture contents and times of hydration (for anhydrous lactose) were measured using a microwave resonator sensor operating in the range of 700-800MHz. Measurements of sensor resonant frequency and conductance allow, through calibration, determination of the complex dielectric properties ϵ' (relative permittivity) and ϵ"(relative dielectric loss) of the test material. These measurements were done in-situ and non destructively. Characteristic graphs of ϵ" versus ϵ' – 1 curves for each powder were generated as a function of bulk density and moisture content, and as expected, were found to be linear. Such data can be used to develop empirical models for the simultaneous in-situ measurement of the bulk density and moisture content of the powders. Unlike MCC, anhydrous lactose is converted to its hydrate form in the presence of moisture, which causes a reduction in the amount of free water and a subsequent change in the dielectric properties. Raman spectroscopy was used to determine the percentage of α-lactose monohydrate in the wetted anhydrous lactose sample at different conversion times. The amount of the physisorbed + "free" water was measured from the thermogravimetric analysis (TGA) and is in qualitative agreement with the downwards shift in the e" versus e'-1 curves for the wetted anhydrous lactose samples. The data also showed that the microwave resonator sensor was not as sensitive for water incorporated into the lactose crystal lattice (crystallographic water) as compared to physisorbed or free water. For powders such as anhydrous lactose that can form a crystal hydrate in the presence of moisture, a combination of techniques such as vibrational spectroscopy together with microwave resonator measurements are appropriate to characterize, in situ, the physical and chemical properties of the powder.
Degree
Ph.D.
Advisors
Harris, Purdue University.
Subject Area
Chemical engineering
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