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On the visualization of droplet deformation and breakup during high-pressure homogenization

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Abstract

The properties of emulsions are strongly influenced by the size distribution of the droplets. In order to achieve droplets on a microscale, high-pressure homogenization is used to transfer stresses to the droplet surface in the flow field upstream, in and downstream the disruption unit of the homogenizer. The droplets are deformed and eventually break up when exceeding critical values. Inline measurement techniques are still very challenging, due to highly complex flow conditions on microscales, high process pressures and large velocities. In this work, the optical flow measurement technique micro particle image velocimetry (μPIV) is used to quantify the flow field, the local stresses as well as droplet deformation and breakup. A special homogenization orifice which is optical accessible enabled the visualization in the whole area of interest before, in and after the restriction up to 80 bars homogenization pressure. The study of the single-phase flow with particular focus on the local stresses showed laminar and transitional conditions at Re number ranging from 285 to 1280. Droplets of two different viscosities are then examined at these conditions while passing the orifice. At the inlet, their size, deformation and position are investigated by an automated image processing algorithm and correlated with the local velocity gradients. At the outlet and downstream, deformation and breakup of droplets are shown within the possibilities of the μPIV and discussed in relation to known droplet breakup mechanisms. Finally, the droplet size distributions offline obtained by static light scattering are compared with observed phenomena of the individual drops in order to gain insights into droplet disruption in high-pressure homogenization.

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Abbreviations

Ca :

Capillary number (–)

Ca crit :

Critical Capillary number (–)

d :

Diameter of the orifice (mm)

d h :

Hydrodynamic diameter (mm)

d out :

Diameter of the inlet and outlet of the orifice unit (mm)

D :

Deformation of droplet (–)

G :

Velocity gradient (1/s)

l :

Length of the orifice (mm)

l E :

Entrance length (mm)

L :

Length of the deformed droplet (mm)

N :

Number of images (–)

Δp :

Homogenization pressure (bar)

Δp max :

Maximum pressure loss (bar)

r :

Radius of the droplet (m)

Re :

Reynolds number (–)

t def :

Critical deformation time (s)

Δt :

Interframing time between two images (s)

u :

Mean axial velocity (m/s)

u a :

Average velocity (m/s)

u m,c :

Mean centerline velocity (m/s)

\(\overline{u^{{\prime }}}\) :

Velocity fluctuations axial direction (m/s)

\(\overline{u^{{{\prime }2}}} /\overline{u_{m,c}^{2}}\) :

Normalized velocity fluctuations axial direction (–)

x :

Streamwise or axial coordinate (mm)

x 50.3 :

Median droplet size (m)

x in/d :

Normalized distance to the orifice inlet (–)

x out/d :

Normalized distance to the orifice outlet (–)

y :

Lateral or radial coordinate (mm)

W :

Width of the deformed droplet (mm)

y/d :

Normalized diameter of the orifice (–)

z :

Height coordinate (mm)

α :

Flow parameter (–)

γ :

Surface tension (N/m)

\(\dot{\varvec{\gamma }}\) :

Shear rate (1/s)

\(\dot{\varvec{\varepsilon }}\) :

Elongational rate (1/s)

λ :

Viscosity ratio (–)

η c :

Dynamic viscosity of the continuous phase (mPa s)

η d :

Dynamic viscosity of the dispersed phase (mPa s)

ρ c :

Density of the fluid (kg/m3)

Φ :

Dispersed phase fraction (%)

τ :

Stress (kg/m s2)

DOC:

Depth of correlation

µPIV:

Micro particle image velocimetry

LV:

Laminar viscous regime

PEG:

Polyethylene glycol

TI:

Turbulent inertia regime

TV:

Turbulent viscous regime

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Acknowledgments

The authors are grateful to Prof. C. Kähler and his group (Universität der Bundeswehr München) for the useful discussions. The support in manufacturing the optical accessible orifices of P. Hoppen and the team in the workshops of the wbk Institute of Production Science (Karlsruher Institute of Technology) is gratefully acknowledged. The valuable experimental help by K. Melter is also deeply appreciated. This project is part of the JointLab IP3, a joint initiative of KIT and BASF. Financial support by the ministry of science, research and the arts of Baden-Württemberg (Az. 33-729.61-3) is gratefully acknowledged.

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  1. Institut für Bio- und Lebensmitteltechnik Bereich I: Lebensmittelverfahrenstechnik (LVT), Karlsruhe Institute of Technology, Kaiserstr. 12, 76131, Karlsruhe, Germany

    K. Kelemen & Heike P. Schuchmann

  2. Institut für Thermische Strömungsmaschinen (ITS), Karlsruhe Institute of Technology, Kaiserstr. 12, 76131, Karlsruhe, Germany

    S. Gepperth, R. Koch & H.-J. Bauer

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  1. K. Kelemen
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Correspondence to Heike P. Schuchmann.

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Kelemen, K., Gepperth, S., Koch, R. et al. On the visualization of droplet deformation and breakup during high-pressure homogenization. Microfluid Nanofluid 19, 1139–1158 (2015). https://doi.org/10.1007/s10404-015-1631-z

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