Processable high internal phase Pickering emulsions using depletion attraction

High internal phase emulsions have been widely used as templates for various porous materials, but special strategies are required to form, in particular, particle-covered ones that have been more difficult to obtain. Here, we report a versatile strategy to produce a stable high internal phase Pickering emulsion by exploiting a depletion interaction between an emulsion droplet and a particle using water-soluble polymers as a depletant. This attractive interaction facilitating the adsorption of particles onto the droplet interface and simultaneously suppressing desorption once adsorbed. This technique can be universally applied to nearly any kind of particle to stabilize an interface with the help of various non- or weakly adsorbing polymers as a depletant, which can be solidified to provide porous materials for many applications.


Supplementary
. Emulsion destabilization by reducing the PEG concentration from 3.3 to 0.00033 wt%. With the presence/absence of 0.4 v% of 1 μm-silica particles, emulsions (70% oil fraction) are produced with 3.3 wt% of PEG. The concentration of 3.3 wt% PEG in a water phase is then reduced to 0.33 wt% by the addition of fresh DI water. By removal of the PEG solution (0.33 wt%) and the addition of fresh water again, the concentration of PEG reaches 0.033 wt%, thereby enabling the emulsion with 0.4 v% of particles to remain stable, while the emulsions without particles destabilize completely. These processes are repeated twice more to obtain a 0.00033 wt% concentration of PEG, and this emulsion still survives even after 24 hours. This delayed destabilization by irreversible adsorption is likely due to contact line pinning at the rough silica surface 2 . During the force calculation for the particle detachment, it is impossible to determine the contact angle, θ that determines the magnitude of the detachment force, but a maximum magnitude of the force could be simply estimated. Here, depletion pressure, P in 3.3 wt % is ~ 100 and interfacial tension between oil-water with 3.3 wt % PEG, γ, is ~ 30 mN/m, as indicated in Fig. 2c. Thus, the magnitude of the detachment force that is balanced with the capillary force is ~ πD * cosα * γ * cos π α θ 3,4 . Therefore, the maximum magnitude of the detachment force is ~ πD * γ ~ 100 nN, whereas the magnitude of depletion force is ~ πD P ~ 3.14 * 1 μm * 100 ~ 300 nN. 85 % of oil contents). In this rheological experiment, ubiquitous similarity is also observed. G' shows weak dependence on frequency (ω), and G'' increases with ~ ω . dependence, especially for a high-frequency regime. This universality also indicates that HIPPEs and emulsions are gel-like soft materials 6 .

Confocal imaging for visualization of internal structure
Nile red molecules (≥ 98.0%, Sigma Aldrich) dissolved in hexadecane (5 μg/mL) are used for visualization of the oil droplets (Fig 1, c and d). To excite the Nile red molecules, a 488-nm laser (Coherent) is applied, and the emitted light is captured to visualize the microscopic structures using a customized confocal microscopy system 1 . It has a 75 μmdiameter pinhole and an extra-long working distance with a 50× objective lens with 0.5 N.A. (Olympus). Micrographs of 3 μm fluorescence silica particles (Biotech GmbH & Co. KG, excitation: 569 nm, emission: 585 nm), shown in Fig. 1e, are also visualized by the same confocal microscopy system, but with a different excitation laser (594 nm, Coherent).

Droplet size in emulsions
To measure the average droplet size, two HIPPEs with 0.18 v% of 1-μm diameter silica microspheres and 1 wt% PEG are prepared by low-and high-energy emulsification, respectively. Confocal micrographs of the HIPPEs are then taken and converted to binary images in only black (oil droplets) and white (continuous phase). A size-range of 4 µm for histograms and a drop population of ~200 are used to establish the size-distribution curve of both emulsions produced by high-and low-energy emulsification. All image processing and analysis are conducted using Image J software (NIH).

Scanning electron microscopy (SEM)
Internal structures of HIPPEs are investigated by scanning electron microscopy (SEM, S-4800, Hitachi). In order to obtain SEM images, HIPPEs samples are first prepared using 0.06-0.54 v% of silica (or TiO2) particles (in total volume of emulsion), 0-10 wt% of PEG (in a water phase), 3-10 wt% of PEGDA (in a water phase) and 0-26.5 v% of acrylic acid (in a water phase, Sigma-Aldrich) with photo-initiator (15 wt% to PEGDA, 2-Hydroxy-2-methylpropiophenone, Sigma-Aldrich). Here, the addition of PEG is to keep the depletion pressure because PEGDA are consumed by the crosslinking. Then a UV light (Xcite-120Q, Lumen dynamics) is shined to the emulsion for 1 h to crosslink the continuous phase. Next, the prepared HIPPEs are freeze-dried (FDU-1200, EYELA) for 14 h, and these well dried samples are then loaded onto a carbon tape. Finally, the microstructures of the HIPPEs are visualized using SEM at a relatively low voltage (2-5 kV). To visualize the material surface in which TiO2 particles are adsorbed, 8 nm thick platinum thin film is deposited on the material surface.

Phase diagram
Our emulsions are classified into various kinds depending on their stability. First, 'emulsion only' indicates that a stable emulsion could be formed only with oil fractions up to 70%, but the addition of more oil (emulsification of 80% oil fraction) immediately destabilizes the emulsion. 'Unstable emulsion' denotes when even emulsions with 70% oil fraction are not stable. 'Instant HIPE' and 'HIPE' refers to HIPPEs that could only survive for a few hours with a gradual destabilization and are stable for at least several days, respectively. For the phase diagram of stable HIPPEs, 1-μm silica microspheres and 10k Da PEG are used.