Detaching Microparticles from a Liquid Surface

The work required to detach microparticles from fluid interfaces depends on the shape of the liquid meniscus. However, measuring the capillary force on a single microparticle and simultaneously imaging the shape of the liquid meniscus has not yet been accomplished. To correlate force and shape, we combined a laser scanning confocal microscope with a colloidal probe setup. While moving a hydrophobic microsphere (radius $5–10\mu \mathrm{m}$) in and out of a $2–5\mu \mathrm{m}$ thick glycerol film, we simultaneously measured the force and imaged the shape of the liquid meniscus. In this way we verified the fundamental equations [D. F. James, J. Fluid Mech. 63, 657 (1974); A. D. Scheludko, A. D. Nikolov, Colloid Polymer Sci. 253, 396 (1975)] that describe the adhesion of particles in flotation, deinking of paper, the stability of Pickering emulsions and particle-stabilized foams. Comparing experimental results with theory showed, however, that the receding contact angle has to be applied, which can be much lower than the static contact angle obtained right after jump in of the particle.

Figure 1

(a) Schematic setup combining a colloidal probe setup with a confocal microscope. Position sensitive device (PSD). (b) Schematic of a particle in contact with a liquid film on a solid substrate.

Figure 2

(a) Representative force curve. A silica microsphere ($R=6.9\mu \mathrm{m}$, $k_c=80.5\mathrm{N}/\mathrm{m}$, approaching speed $0.5\mu \mathrm{m}/\mathrm{s}$) approached a film of glycerol of $h_0=2.5\mu \mathrm{m}$ thickness. Approaching (black) and retracting (red) parts are indicated by arrows. The open circles are forces calculated from the contact angle and contact line radius with Eq. (1). Positive forces are repulsive, negative forces attractive. (b) Shape of the liquid surface and the position of the particle from confocal microscopy images. The red dashed lines indicate the film thickness at large distance. The numbers indicate the time at which a specific force or profile was recorded.

Figure 3

(a) Position of the contact line ($\beta$, red), change of the contact angle ($\mathrm{\Theta }$, black), and angle of the liquid surface with the horizontal ($\alpha$, green) during one representative force experiment. The three regimes of one force cycle are the free approach of the particle until 18 s, the contact of the particle with the glass substrate until 28 s and afterwards the retraction of the particle. For better comparison the same data as in Fig. 2 were used. (b) Measured force versus time with the approach (black) and retraction (red).