Hamza did his PhD in Prof Paunov research group 2011-2015. He was granted his PhD in December 2015. Currently, he is looking for a post-doc position in Saudi Arabia. PhD supervisors:  Prof. Vesselin N. Paunov (PI) and Dr Tommy S. Horozov (Co-PI)

Research project 1:

Wettability of Anisotropic and Porous Particles Adsorbed to Fluid Interfaces

 We have studied the attachment and orientation of anisotropic and porous microparticles at liquid surfaces by using the Gel Trapping Technique (GTT). This technique involves spreading of the microparticles of interest at the liquid interface, subsequent setting of the aqueous phase to a hydrogel thus “arresting” the particle positions at the liquid surface, and further replication of the hydrogel surface with curable polydymethilsiloxane (PDMS). The advantage of the GTT comes from the possibility to look at the PDMS replica with scanning electron microscopy (SEM) or atomic force microscopy (AFM), which allows even sub-micrometer particles to be studied at the air-water and the oil-water interface. Here we report our results on the adsorption of non-spherical anisotropic particles at liquid surfaces using the GTT. Although the GTT has been originally designed to measure three-phase contact angles of spherical colloid particles, here we used this technique to reveal the orientation of a variety of shape-anisotropic and porous microparticles of practical interest at both the air-water and decane-water interfaces. We show results on typical attachment and orientation of needle-like (aragonite), rhombohedra-like (calcite) microcrystals, ethyl cellulose micro-rods, as well as highly porous hydrophilic and hydrophobic silica microparticles at these liquid interfaces. The results are important for understanding the adsorption behaviour of shape-anisotropic particles as well as porous microparticles which are used in industrial formulations as fillers, foam stabilisers and emulsifiers.

Fig. 1 (A) A solid porous microparticle adsorbed at the air-water (A) and (B) the oil-water interface. The position of the porous particle adsorbed at the liquid interface depends on the effective contact angle, θ_eff, which is different from the contact angle of the individual particles in the aggregate. Needle-like (C) and rhombohedra-like microcrystals (D) and microfibrers (E-F) may have several possible positions or orientations at the liquid interface.

Fig. 2. SEM images of needle-like aragonite and rhombohedra-like calcite microcrystals as well as ethyl cellulose fibres, adsorbed at the air-water interface (top-row of images, a-c) followed by its replication with PDMS by using the GTT. The parts of the particles immersed in the PDMS have been exposed to the air phase when adsorbed at the air-water interface. The bottom row of SEM images (d-f) SEM images show the same particles adsorbed at the decane-water interface followed by its replication with PDMS by using the GTT.

References

1) Sharp, E.L., Al-Shehri, H., Horozov, T.S., Stoyanov, S.D., Paunov, V.N., Adsorption of shape-anisotropic and porous particles at the air–water and the decane–water interface studied by the gel trapping technique, RSC Advances, 4 (2014) 2205-2213.

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Research project 2:

Adsorption of carboxylic modified latex particles at liquid interfaces studied by the gel trapping technique

We have studied how carboxylic modified latex (CML) microparticles adsorb at liquid surfaces and the preferred type of emulsion they can stabilise depending on the particle size and the surface density of carboxylic groups. We measured the particle contact angle by using the gel trapping technique (GTT) for CML particles adsorbed at air-water and oil–water interfaces. Using this method we obtained scanning electron microscopy (SEM) micrographs of polydimethylsiloxane (PDMS) replicas of the liquid interface with the particles, where the PDMS replicates the non-polar phase and measured the particle contact angle. We discovered that the particle wettability correlates well with their surface density of the carboxylic groups but is not very sensitive on the presence of electrolyte in the aqueous phase and the value of the particle zeta potential. We demonstrated that CML microparticles of high surface density of COOH groups stabilise oil-in-water (O/W) emulsions while these with the lowest coverage with COOH groups favour the formation of water-in-oil (W/O) emulsions. We found that this corresponds to a change of the CML particle contact angle from lower than 90^o to higher than 90^o upon decrease of the surface density of COOH groups. The findings confirm that the surface density of polar groups has much bigger effect on the particle wettability and the preferred emulsion than the particle surface charge and zeta potential.  Our results on the type of stabilised Pickering emulsion agree with other experimental studies with different particle materials. We propose an alternative explanation of the link between the particle contact angle and the type of stabilised Pickering emulsion.

Fig. 1. Optical photographs of emulsion made from 50:50 dodecane:water stabilised by 5 wt% CML particles in 1mM NaCl immediately after emulsification. The emulsification was done by using the hand-shaking method. The type of the emulsion changed from (W/O) water-in-oil emulsion to oil-in-water (O/W) emulsions is not due to the different size of the particles but due to the decreasing area per COOH group on the particle surface which switches the particles from hydrophobic to hydrophilic. The scale bar 200 µm.

References

Al-Shehri, H., Horozov, T.S., Paunov, V.N., Adsorption of carboxylic modified latex particles at liquid interfaces studied by the gel trapping technique, Soft Matter, 10 (2014) 6433-6441.