Institute of Mechanical Process Engineering and Mechanics

Drop shape analysis

Links: Aufbau des Messgeräts; mitte: Beispiel eines sitzenden Tropfens; rechts: Beispiel eines hängenden Tropfens.

Fig. 1: Setup of device (left); example of a sessile drop image (middle); example of a pendant drop image (right).

Objective

Interfacial tensions between two immiscible fluids and surface tensions between a fluid and air can be measured using the pendant drop method. For detecting the contact angle a fluid forms with a solid surface in air or in an immiscible fluid is measured by the sessile drop method. Both methods are based on image analysis. Interfacial tensions and contact angles are important parameters for many product formulations like emulsions, dispersions, foams and coating applications.

Operating principles

Interfacial or surface tensions are detected by evaluating the outline of a pendant drop via image analysis. The drop is created manually with a syringe and a picture is taken with a CCD camera. The camera allows the evaluation of singe images or videos sequences with 84 fps. Video analysis makes it possible to detect changes in surface tension over time, which can be used e.g. for protein or surfactant adsorption. Temperature can be adjusted in a wide range by using a tempered glass box. The contact angel is also detected using image analysis. The setup is changed depending on substrate (powder or solid) and fluid or air environment. For measuring a contact angle on a powder substrate, the powder is pressed to a tablet using a hand press machine. Direct measurement of a three-phase contact angle (solid/liquid/liquid) can be achieved as well as temperatures up to 70°C.

Specifications

Required sample quantity: 10 mL
Temperature range: 20- 80°C
Camera:
Stingray F-033
CCD Progressive
84 fps, 656 x 492 px
Magnification: 1 to 20

Applications

Measurement of surface properties for various applications like:
  • Emulsions
  • Foams
  • Adhesives
  • Wetting and rewetting properties
  • Three-phase contact angle for Pickering emulsions or capillary suspensions

Literatur

  1. Hansen F.K. Rødsrud G. (1991). Surface tension by pendant drop: I. A fast standard instrument using computer image analysis. Journal of Colloid and Interface Science, 141, 1–9.
  2. Huh C. Reed R.L. (1983). A method for estimating interfacial tensions and contact angles from sessile and pendant drop shapes. Journal of Colloid and Interface Science, 91, 472–484.