Effect of Shear on Pumped Capillary Foams

Foam flow in many applications, like firefighting and oil recovery, requires stable foams that can withstand the stress and aging that result from both shear and thermodynamic instability. Events of drainage and coarsening drive the collapse of foams and greatly affect foam efficacy in processes relying on foam transport. Recently, it was discovered that foams can be stabilized by the synergistic action of colloidal particles and a small amount of a water-immiscible liquid that mediates capillary forces. The so-called capillary foams contain gas bubbles that are coated by a thin oil-particle film and integrated in a network of oil-bridged particles; the present study explores how this unique architecture impacts the foams’ flow dynamics. We pumped capillary foams through millimeter-sized tubing (ID: 790 μm) at different flow rates and analyzed the influence of stress and aging on capillary foam stability. We find that the foams remain stable when pumped at higher flow rates but undergo phase separation when pumped at low flow rates. Our observations further show that the particle network is responsible for the observed stability in capillary foams and that network strength and stability of an existing foam can be increased by shearing.


Capillary Foam Bubbles Under UV Light Excitation
Capillary foams (CFs) were prepared from a suspension of silica particles (0.68 vol. %) containing Rhodamine 6G dye and Trimethylolpropane trimethacrylate (TMPTMA oil). Foam flow through the tubing was imaged using fluorescence excitation on an upright microscope. The particle network under excitation in Fig. S1 appears grey because of the dye and the bubbles are dark. Figure S1: Image of capillary foam showing the bubbles enclosed within the particle network.

Capillary Foam Stability
CFs were observed to coarsening over time and bubbles can be seen in a 20 ml vial after 6 h.
3 Figure S2: Images of bulk capillary foams ( = 0.68 vol. %; = 1.4) showing foam evolution from a) when t = 0 h to b) when t = 6 h after preparation of the foam.
The bubble distribution of CFs was observed to increase with time as large bubbles grow and small bubbles shrink due to diffusive coarsening.  Amplitude sweep experiments under oscillatory tests were conducted on capillary foams prepared with silica particles at a volume fraction of 0.92 % and an oil-particle ratio of 1. Fig. S4b shows the raw storage and loss modulus plots obtained from the amplitude sweep experiments. The plot shows that G' is an order of magnitude higher than G" at low strains and indicates solid like behavior in CFs at low strains. At higher stains, when ≥ 0.1, G' decreases and eventually falls below G" because the CF flows at higher strains.

Capillary Foam Flow at Low Shear Rate
Capillary foams pumped at a low shear rate were observed to undergo phase separation in the tube.
The foam separated into plugs of water, gas and foam during pumping and the recovered foam was lower in volume than the foam volumes recovered at higher shear rates. Furthermore, we observed that when the capillary foam is pumped in an upward flow protocol as shown in Fig. S6 below, water can flow through the foam, breakup the particle network and thus reduce the foam volume recovered downstream. The image in Fig. S6 shows that the particle network dislodged from the foam head, as water flow through the foam, settles on the piston in the syringe.
6 Figure S6: Image showing particles dislodged from capillary foam network settling at the top of the syringe piston.

Foam Stability in Presence of Crude Oil
Bulk foam stability in the presence of crude oil was evaluated by the addition of 0.8 ml of crude oil to surfactant foam (SF) and capillary foam. The surfactant foam was prepared by frothing a 2 vol. % suspension of sodium dodecyl sulfate (SDS) in a vortex mixer. The capillary foam was prepared by frothing a silica suspension (0.92 vol. %) containing 5 mM sodium chloride with TMPTMA at an oil-particle ratio of 0.5. Fig. S7 shows a time series of images of both foams after the addition of crude oil. In surfactant foams, the initial volume of the foam head is halfed 10 mins after crude oil is added and continues to degrade with time. We observe that most of the foam head is gone about 2 h following the addition of crude oil. In CFs on the other hand, Fig. S7 shows that the CF head remains stable in the presence of crude oil for at least 48 h after the addition of crude 7 oil. We suspect that the crude oil does not affect the CF because the CF is stabilized oil that bridges the particles in a strong network and coats the bubbles.