Spinning Drop Tensiometry

Spinning drop tensiometry is the technology of choice for measuring extremely small interfacial tensions. The method is based on the optical contour analysis of a drop. This drop, instead of hanging from a dosing needle and being exposed to gravitation, is located inside of a rotating capillary.

Inside of the capillary the centrifugal force pushes the denser liquid surrounding the drop outwards while the less dense drop gets pushed towards the rotational axis. Hence the drop is deformed cylindrically and its interfacial area increases. The interfacial tension counteracts this area increase and thus can be determined by analysing the equilibrium drop shape.

Vonnegut evaluation

An easy and established evaluation method for strongly elongated drops is the interfacial tension calculation by Vonnegut. Here the shape that the drop adopts at rotation with the angular velocity ω is approximated by a cylinder. The cylinder has a rotational energy that decreases with decreasing radius, which compensates the increase in interfacial energy due to increasing elongation and interfacial area. In the equilibrium state an energy minimum is reached. Vonnegut equation:

Enhanced oil recovery (EOR)

Ultra-low interfacial tensions occur particularly in microemulsions which are used, for example, in enhanced oil recovery. At suitable conditions microemulsions form spontaneously from water, oil and surfactant. The thermal stability of the microemulsion plays a crucial role as temperatures in the oil reservoir can reach well beyond the boiling point of water. Thus, measurements with superheated aqueous solutions of up to 130 °C are an important step in the research of appropriate surfactants and mixture compositions.

Interfacial rheology

With spinning drop tensiometry it is also possible to analyse how an interface reacts when its area is enlarged or reduced (interfacial rheology). For example, how fast can surfactants from the bulk adsorb at the interface in order to stabilise a newly formed emulsion? To investigate such questions the rotational velocity is varied in a spinning drop experiment, typically in an oscillating manner, which directly results in a change of size of the interfacial area.

The SVT 25

The Spinning drop video tensiometer SVT 25 is a compact optical measuring instrument for the determination of low to ultra-low interfacial tensions and rheological interface properties. Highest precision and reproducibility of the measuring results is guaranteed by a well-established measuring technique and first grade mechanical and optical components. Due to state of the art electronic components the SVT 25 can be operated intuitively via the TP 50 control panel.

For an efficient measuring procedure the measuring cell, with the rotating capillary, is mounted on a tilting table, which allows the operator to easily move the drop of interest into the camera view. The fast 6.5-fold zoom lens with fine focus enables an analysis of small and large drops alike.

Due to the high-performance camera with USB 3 interface the spinning drop can be analysed with exceptional resolution. Fast processes e.g. during oscillation experiments can be captured with up to 3250 frames/s. Once detected the drop is automatically tracked by the movable camera.

A highly dynamic measurement drive creates a uniform rotation even at highest rotational velocities of up to 20 000 rev/min. In addition it allows for sinusoidal changes in revolution speed for oscillation measurements and for rapid rotational velocity steps with a max. acceleration of up to 500 rev/s².

Samples can be easily placed inside the SVT 25, within seconds, due to fast-exchange capillaries with perfect fitting accuracy. Hence the highest throughput rates can be assured. In order to avoid extensive cleaning procedures disposable glass tubes with corresponding holders can also be used for the measurement.

The observation windows of the measuring cells for the SVT 25 can be easily removed by opening just two knurled screws and are easy to clean in case they came into contact with sample material.

Temperatures within the range of -30 °C to 180 °C can be reached inside the temperature controlled measuring cells. This facilitates the recreation of many different environmental conditions. Due to a specially designed capillary and a resulting higher pressure tolerance the SVT 25 can even be used with water-based solutions at up to 130 °C.

Main features

• Measurement of low to ultra-low interfacial tensions based on the contour of spinning drops according to models by Vonnegut, Cayias-Schechter-Wade or Young-Laplace
• highly dynamic measurement drive with 0 … 20000 rev/min
• oscillation measurements with oscillation periods of 0.5 s …∞ and max. acceleration of 500 rev/s²
• motorised tilting table
• fast 6.5-fold zoom lens with integrated continuous fine focus
• motorised movable optics for automatically following moving drops
• high-performance camera with USB 3 interface, 2/3″ sensor, max. 2048 x 1088 pixel, max. 3250 frames/s
• precise, fast and easy to use capillary exchange system
• LED-lighting with strobe function and adjustable slit diaphragm
• temperature controlled measuring cells for temperatures ranging from -30 °C to 180 °C
• special capillary for measuring water-based solutions at up to 130 °C
• automatic calibration function for correcting optical distortions caused by the capillaries (cylindrical lens effects)
• TP 50 control panel with touch screen and precision control wheel

Software

The SVTSoftware is available in various independently usable modules. The available software modules for the SVT 25 are:

• SVTS 20 — interfacial tension
• SVTS 21 — oscillation
• SVTS 22 — membrane covered drops

Learn more about the SVTS modules.

Accessories

Due to the modular product design the SVT 25 can be combined with a multitude of options of the versatile SVT accessories range. Learn more about the available accessories.

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