6.3 Experimental results
7.2.1 Experimental setup and material properties
The experimental setup for the 3D2C µPIV-study mainly equals the design of the proof of principle described in App. C.2. Since the 3D2CµPIV measurements demand a higher instrumental complexity than the proof of principle, the experimental design is reconsidered in detail. This serves the representation and discussion of the experimental results.
As in the proof of principle, a modified Zeiss LSM410 microscope (Fig. 7.1) is used. The integrated continuous wave (CW) lasers of the LSM are deactivated and replaced by a pulsed
New Wave Research Solo-PIV III laser, which is guided via a laser arm into the beam-scan-
system. Utilizing the beam-scan-system of the microscope, the laser beam can be precisely orientated via a motorized concave mirror to maximize the excitation light intensity, thus maximizing the fluorescence intensity yield. After passing the beam-scan-system, the laser light enters the inverted microscope (Zeiss Axiovert 100) and is widened by a planoconvex lens. Subsequently, a holographic diffuser equalizes the laser to illuminate the whole sample uniformly. An optical filter set, consisting of a dichroic mirror and lowpass filter, separates the excitation light wavelength from the fluorescence signal. The used microscope objective offers a working distance of 570 µm at a 25x magnification and a depth of field DOF = 1.034 µm. Alternatively to the laser, a mercury-vapor-lamp can be used to align the microfluidic devices when preparing the µPIV measurements.
Inverted microscope
Transmitted light system
Mercury fluorescence lamp
Scanning unit
Detector unit
Internal lasers
External laser coupler
(a)
(b)
Fig. 7.1.: Adapted confocal-laser-scanning microscope Zeiss LSM-410 used for the µPIV measurements. a) schematic drawing of the optical components b) photograph of adapted microscope with peripheral equipment, PIV-camera and external Nd:YAG-laser. Subfigure a) adapted from (Zeiss, 1995)
The PIV double-images are recorded by a PCO sensicam qe 670 LD 3078, that is mounted on an Carl Zeiss TV Adapter 2/32 C with a 0.63 magnification to demagnify the image to the CCD-Chip size of the camera (6.45 µm x 6.45 µm). A possible loss of information is accepted to obtain a wider field of view and to use the entire CCD-chip size of 1376 px x 1040 px. The laser timing and power, as well as the camera timing, are controlled via an ILA
mini PIV-synchronizer. During the measurements, the time between two images is chosen
such, that a particle displacement of 12 px in the flow regions with the highest velocity is obtained. The final interrogation window size (IWS) is 16 px with 50% overlap.
Spherical polystyrene microparticles (PS-FluoRot-Fi277 from microParticles GmbH, properties in Tab. 7.1) are used as velocity tracer. The particles are purchased as dry powder to exclude possible contamination caused by the preparation of a particle emulsion for shipping by the manufacturer. The fluorescent particles show low dye leaching and high chemical stability for alkanes and fulfill the requirements of a small particle diameter, as well as a small density difference to the liquid phase. The latter ensures a low drag between particles and the flow phase and prevents the deposition of the particles due to inertia. The particle’s fluorescence dye is adjusted to the available laser (λex= 532 nm) to ensure a high light absorption and an
intense fluorescence signal (λpeak= 600 nm). The chosen fluorescence dye (FluoRot) offers
an excellent fluorescence signal (560 nm - 850 nm) distinctly shifted from the excitation
wavelength (532 nm). This allows the usage of a dicroic mirror with a cutoff frequency of 550 nm to cover a large wavelength range of the fluorescence signal.
Tab. 7.1.: Properties of used fluorescent tracer particles PS-FluoRot-1.5
Property value unit
batch PS-FluoRot-Fi277 - excitation peak λex 530 nm emission peak λem 607 nm density 1.05 g/cm median diameter 1.61 µm coefficient of variation (CV) 2.3 %
For the PIV-study of the disperse and continuous phase, both phases require a sufficient seeding with fluorescence particles. Preliminary measurements indicate, that the available PS-particles within the double-binary nonpolar phase sediment and deposit at the walls after preparation. Thus, with these specific particles, a sufficient particle concentration inside the non-polar disperse phase could only be reached in one measurement. This behavior results from the surface properties of the used FluoRot-dye coated tracers. Microparticles coated with a different fluorescence-dye (green) do not show this behavior. Unfortunately, the green- labeled particles are not compatible to the µPIV-system (λex! 532 nm), thus n-dodecane
is used as the disperse phase and a one degree of freedom RIM-approach (single-binary mixture) is utilized (Mießner et al., 2008). The refractive index of several water/DMSO mixtures is measured using the method described in Sec. 6.2.2. The measurements indi- cate, that for a mass-fraction of ξDM SO“ 0.58 the refractive index of the binary solution
water/DMSO is matched with the RI of n-dodecane (Fig. 7.2)
refractiv
e index / -
0.50
0.52
0.54
0.56
0.58
0.60
1.425
1.420
1.415
1.410
1.405
massfraction ξ / -
DMSO measurements linear regression n-dodecane RI=1.4216Fig. 7.2.: Measurements of refractive index for water/DMSO-mixture at different mass-fractions ξ. At ξ “ 0.58 the refractive index of the mixture equals the RI of n-dodecane
To further improve the particle suspension stability, 1 µmol/l AOT (Dioctyl sulfosuccinate sodium) is added to the nonpolar phase in consultation with the particle manufacturer. This allows a redispersion of the particles by the flow (visualized in Fig. 7.3) and prevents capillary clogging in the microchannel-feed. The changes of the interfacial tension caused by the addition of AOT is carefully determined.
liquid
phase
g
(a)liquid
phase
g
(b)Fig. 7.3.: PS-fluorescence tracer particles in n-dodecane. Without an addition of the surfactant AOT (a), the particles directly deposit at the walls of the snap-cap vial and cannot be redispersed
by shaking. After addition of AOT (b) the particles redisperse by movement of the vial
Since the second degree of freedom is omitted in this approach, the Ca-number can only be adjusted by tuning the interfacial tension of the water/DMSO-dodecane system. Thus, a mixture of water/DMSO with added sodium dodecyl sulfate (SDS), an anionic surfactant with a HLB-value of 40 (Rowe, 2009, p.651–652), is added to the continuous phase. The author is aware, that the addition of surfactants alters the system drastically, as the surfactant may absorb locally to the interfacial area (Olgac and Muradoglu, 2013) and therefore influence the viscosity of the interfacial area and the film thickness.
The properties of the surfactants are shown in Tab. 7.2. Tab. 7.2.: Properties of used surfactants
Surfactant molar mass / g moĺ1 density / g cḿ3 solubility in water /g Ĺ1
AOT 444.559 1.146 8.17
SDS 288.38 1.1 150
The main influences of surfactants on two-phase flows are often linked to the critical micelle concentration (CMC) (Fuerstman et al., 2007). The CMC defines the concentration, at which the surfactant molecules start to orientate as geometrical defined structures (micelles). At concentrations below the CMC, surfactant molecules mainly occur as single molecules and absorb to the liquid’s interface. With an increasing surfactant concentration, the interfacial tension is decreased rapidly since the probability of surfactant molecules to absorb into the interfacial area rises. When the CMC is reached, the interface can be considered energetically saturated and only minor changes are achieves when increasing the surfactant concentration. At this point, the formation of even complex micellar-structures enables a lower energy level in the system and the surfactant molecules align depending on the bulk solvent. However, it has to be mentioned, that this explanatory approach is a phenomonelogical reduction of the real thermodynamic behavior as already below the CMC singular micelles exist. Since in this work the focus lies on the hydrodynamic influence of the surfactants, the author considers this valid. For a detailed thermodynamic and energetic consideration of the solubilization of surfactants and the formation of micelles, please refer to dedicated literature (Butt et al., 2003, p.251).
The CMC depends on the composition of the substance the surfactants are solved in. For that reason, interfacial tension measurements are performed as stated in the appendix of the previous chapter (App. C.1): Different SDS concentrations are dissolved in water/DMSO (ξ “ 0.58) and the interfacial tension is measured (Fig. 7.4). Additionally, 1 µmol l-1AOT is added like in the later experiments for particle dispersion.
SDS concentration / (mmol l ) -1 in terfacial tensi on γ / (mN m ) -1
γ
γ
γ
Fig. 7.4.: Measurements of the interfacial tension for different SDS concentrations in a mixture of water/DMSO ξ “ 0.58 with 1 µmol l-1AOT in the n-dodecane fraction
The CMC is retrieved from the point, where the sloped cd γ
SDS changes, since for concentrations
above the CMC the interfacial tension changes less. Within the measurements, this equals 35 mmol l-1
Please note, that the addition of SDS not only influences the interfacial tension and surface viscosity, but also alters the bulk viscosity of the water/DMSO-mixture. Thus the viscosity ratio λ is also effected. This occurs when concentrations beyond the CMC are applied due to the formation of a lyotropic micelle phase. The corresponding measurements for the viscosity are shown in App. D.1. While the increase of viscosity for concentrations below the CMC is less than 5 %, the concentration beyond the CMC increases the bulk viscosity about 30%. This interrelation needs to be considered when interpreting the variations of the flow field and the change of the excess velocity.