Visualization, design, and scaling of drop generation in coflow processes

Manuela Duxenneuner (2009). Visualization, design, and scaling of drop generation in coflow processes PhD Thesis, Chemical Engineering, The University of Queensland.

       
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Author Manuela Duxenneuner
Thesis Title Visualization, design, and scaling of drop generation in coflow processes
School, Centre or Institute Chemical Engineering
Institution The University of Queensland
Publication date 2009
Thesis type PhD Thesis
Subjects 03 Chemical Sciences
Formatted abstract The controlled production of especially monodisperse emulsions is highly soughtafter
in food, cosmetics, pharmaceutical, and chemical industries and is required to
produce well-structured multiphase systems (e.g. double emulsions, microcapsules,
tailor-made structures with functional properties) as well as for lab-on-the-chip
analysis. Narrow droplet size distribution is difficult to achieve and therefore major
effort has been put into development and optimization of various dispersing machines,
such as high pressure homogenizers, rotor-stator toothed disk turbines, static
and dynamic membrane systems, or opposed jet micro fluidizer in order to produce
monodisperse emulsions, however, without satisfying results. The drop size and size
distribution, interfacial properties, and structuring elements in the bulk phase of
emulsions determine quality characteristics of the final product. Monodisperse
emulsions are also very useful for fundamental studies of emulsion processes and
properties, because the interpretation of experimental results is simplified. In order
to design dispersing devices and droplet based reactors, a thorough understanding of
the dispersing process mechanism and precise investigation of drop generation dynamics
are required. In laboratory research, microchannels are often utilized to analyze
the droplet formation kinetics in coflow, flow-focusing, or T-flow channel configurations.
Precise control of drop size and formation dynamics, constant laminar
flow field, high energy efficiency, significant reduction of the sample material, and
the simple manufacturing of the devices are some of the great advantages of this
rather new technology.The present work focuses on single w/o droplet formation and breakup at a capillary
tip in coflow devices, where the disperse phase is injected parallel to a laminar
coflowing stream. The main objective was to investigate the effect of process and
material parameters on the drop formation dynamics at different processing length
scales, as we varied the capillary size (0.03 - 0.11 mm), device material (glass,
PDMS, stainless steel), cell diameter (0.1 - 20 mm) and length (3 - 100 cm), and the
flow rate of both phases (0.001 ≤ Qc ≤ 1900 ml/min, 0.0002 ≤ Qd ≤ 0.9 ml/min). A
homological row of food-grade Tween-surfactants and aqueous solutions containing
surface active biopolymer (HPGG; hydroxypropylether guar gum) were used for the
disperse phase illustrating in particular the influence of interfacial tension forces and
elasticity on the drop formation into sunflower oil. We visualized and analyzed the
droplet dynamics systematically with high-speed imaging systems applying framerates from 1000 to 90’000 fps. Using microchannel, streak imaging (StrIm) and micro
particle image velocimetry (μPIV) were employed to map and visualize the entire
flow situation in and around the forming droplet, which is influenced by the
flow geometry, processing conditions, and material parameters.
The dripping droplet formation mechanism can be categorized into three stages: (i)
start of the droplet forming called filling stage, (ii) necking stage in which the droplet
is still continuously provided with liquid from the capillary, and (iii) pinching or
breaking-off after that the droplet starts to flow along the channel following the
stream of the ambient fluid. The kinetics at the capillary tip in the coflow system
strongly depends on the stresses acting on the droplet during generation. We found
that at high velocity difference (or at lower velocity ratio, vd/vc) between both
phases, the drag forces imposed by the ambient fluid dominate interfacial tension
forces in macro as well as in mini scaled coflow devices. The presence of surfactant
has little influence under such flow conditions but the droplets are highly uniform.
At lower velocity difference (or at higher vd/vc) in the same channel scales the interfacial
forces strongly affect the drop filling, breakup and satellite droplet formation.
In microchannel, however, we observed that the droplet production shows most pronounced
dependency on the viscous drag forces due to the smaller geometrical
lengths scale and lower velocity differences in general. With the addition of HPGG
polymer into the disperse phase, additional differences in the droplet dynamics are
clearly detected. As a result of the elastic extra stresses developed within the viscoelastic
fluid, elongated drop creation and the generation of a thread, while pinching
off, were visualized.
The drop breakup experiments, including all available solutions and process parameters,
were finally characterized in terms of Weber, Reynolds, Capillary, Ohnesorge,
and S-numbers (surface tension number). The resulting Ca-We-phase diagram covers
a wide range of parameters (2.96·10-09 ≤ We ≤ 5.9·103, 2.56·10-05 ≤ Ca ≤ 11.5,
10-4 ≤ Re ≤ 2.5·104, 1.3·10-03 ≤ Oh ≤ 3.53, and 1.7·10-4 ≤ S ≤ 3.62·1004). The graph
represents a significant contribution to predict the flow behavior of drop breakup
dynamics in geometrically similar drop dispersing devices. Additionally, it is a generalized
illustration of the main forces acting in coflow channels at three different
length scales under approximate similar flow conditions in the vicinity of the forming
droplet at the capillary tip. A preliminary force balance based on an analytical
model is discussed for the approximate prediction of the primary droplet size.

Keyword drop formation, coflow, monodispersity, scaling, dimensionless numbers, microfluidics, microPIV, hydroxypropyl-ether guar gum, Tween, sunflower oil

 
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