Emulsion stability

Maxim Khazimullin, Dinis Samigullin

Objective:

Investigation of the emulsion stability in microchannel flows

Emulsions are mixtures of two immiscible liquids which form a suspension of droplets of one liquid in the other. Emulsions are widely encountered in nature and are used in medicine, cosmetics, oil industry, and in polymer production. The emulsion stability is the main physical problem, which boils down to the question under what conditions a mixture of two liquids will form a stable suspension of droplets?

Fig. 1. Droplets in a microchannel

The physical properties can be measured under external action on an object. Generation of an emulsion flow results in hydrodynamic forces acting on the droplets -- while small forces deform the droplets; larger forces may destroy them. Therefore, investigation of the behavior of droplets in hydrodynamic flows is an important tool to study the emulsion stability. Individual emulsion droplets can be observed quite easily by an optical microscope due to the difference in the optical properties of the two liquids and the droplet shape, which deflect rays of transmitted or reflected light. For studying the emulsion in microchannels at high flow rates it is essential to use high-speed digital cameras (up to 700,000 frames per second), which enables observations of the processes of droplet deformation at flow rates of about 1-10 m/sec.

Fig. 2. The dependence of the deformation on the droplet position in the microchannel

We created an experimental set-up enabling studies of the droplet shape in microchannels at different flow rates. As a model emulsion we used a mixture of the hydrocarbon – tetradecane (ρ = 0.762 g/cm3) and water, stabilized by surfactant Span 80. The microchannel was fabricated using photolithography from polydimethylsiloxane (PDMS). The thickness of the microchannel is 40 microns and the width is 50 microns. An emulsion prepared by mechanical stirring can be fed into the channel under the pressure drop adjustable by pressure controller that provides a constant flow rate of the emulsion. Figure 1 shows a typical microscopic image of the flow of the emulsion droplets in a microchannel with an external pressure of 45 kPa. As it can be seen, a large droplet (15 microns) has an ellipsoidal shape unlike small droplets (<10 microns), which are spherical. We have developed an algorithm of digital image processing to determine the contours of the droplets (red lines in Fig. 1), which allowed us to determine the degree of deformation depending on the position of the droplet in the microchannel (Fig. 2).