Using Contactless Interfacial Rheology to Probe Interfacial Mechanics for Compositional Ripening

Abstract

In this study, we investigate the impact of modifying colloid-colloid interactions on the rheological properties of a layer of poly(methyl methacrylate) PMMA colloids at a dodecane-water interface. Toluene is introduced into the oil phase in order to modify attractive interactions between colloids. We first make qualitative observations of water-in-oil emulsions undergoing compositional ripening, demonstrating how the addition of toluene modifies the evolution. Without toluene, water droplets finally "explode"; with the addition of toluene, they instead form connected colloidal structures. We secondly employ a novel contactless interfacial setup to probe the rheological properties of a PMMA colloid-laden water-dodecane interface, examining the effects of toluene addition. We find that the interface becomes significantly weaker and more flexible following addition of toluene, contrary to what one might expect for increasing interparticle attractions for high surface coverage interfaces.

Figure 1

Confocal 3-D projection of a water-in-dodecane droplet stabilized by 360 nm radii PMMA before (left) and after (right) the addition of toluene. Scale bar 20 μm.

Figure 2

Compositional ripening over 4 h observed at (a) 0 min, (b) 48 min, (c) 96 min, (d) 144 min, (e) 192 min, and (f) 240 min. Toluene-treated water droplets (yellow) stabilized by PMMA (yellow) undergo ripening losing mass to the sugar droplets (dark purple) (a–d). After sufficient mass is lost, the final structure remains the same (e,f). Scale bar 100 μm.

Figure 3

Confocal slices through a toluene-treated water droplet remnant, acquired at 4.5 μm intervals, after 15 h of compositional ripening, showing connected PMMA structures (yellow). Scale bar 25 μm.

Figure 4

Confocal micrographs of a PMMA dispersion in pure dodecane (left) and after rollerbank overnight in 50:50 dodecane/toluene mixture (right). In the bulk oil phase the behavior of these colloids do not appear to be significantly altered via the addition of toluene. Scale bar 10 μm.

Figure 5

A Creep-recovery profile for an interface showing flowing behavior (blue) reaching a steady strain rate, γ̇. Illustrated are the maximum, γ_max, recoverable, γ_rec, and irrrecoverable, γ_irr strains. The strain rate, γ̇, is determined by taking the gradient of the steady strain toward the end of the applied stress, over a 20s window (red dotted lines). Also shown is the creep-recovery curve for an interface showing very elastic behavior (orange), illustrating considerably lower values.

Figure 6

Effect of increasing the stress, via the rotational speed, ω, on the interfacial creep recovery profile. Left: the interface shows a fairly elastic response with a high recoverable strain (ω = 3 rpm; 1.1 × 10^–6 Pa m) and begins to show plastic behavior as the stress is increased . Right: the interfaces become dominated by plastic strain and flow with a clear constant strain rate for ω higher than 10 rpm; 3.5 × 10^–6 Pa m. See main text for details.

Figure 7

Effects of shear history on the interfacial shear response on a PMMA (362 nm radius) water-dodecane interface without toluene. The crosses indicate the initial response when the interfacial stress, σ^s, is increased. The orange squares are measurements made after σ^s = 5.3 × 10^–6 Pa m. The triangles are measurements made after increasing to σ^s = 6.5 × 10^–6 Pa m.

Figure 8

Left: the maximum, (closed), and recoverable, (open), strains as a function of interfacial stress. Right: The ratio of / before (blue circles) and after (red square) the addition of toluene to the interface.

Figure 9

Left: The effects of increasing stress on the interfacial viscosity before (blue circles) and after (red squares) the addition of toluene. Right: A closer look at the viscosity-stress curve, where the variation in the toluene data is more evident.

Figure 10

Maximum (closed) and recoverable (open) strains as a function of interfacial stress before (blue circles) and after (red squares) the addition of toluene to the interface for 3 different runs (top to bottom). Straight lines are also fitted to the recoverable strain to derive an “effective” shear modulus.