Former PhD student of Prof Paunov Marius did his PhD in Prof Paunov’s research group funded by EPSRC industrial CASE award. He graduated with PhD in Chemistry in 2014 and after doing a PDRA at North Carolina State University, he is now working as a Materials scientist at ABB Inc. in NC. PhD supervisors: Prof Vesselin Paunov (PI) and Prof Georg Mehl (Co-PI)

Research project 1 (PhD thesis):

Salt-hydrogel marbles and hollow-shell microcapsules

We designed a new method for preparation of liquid marbles by using hydrophilic particles [1]. Salt-hydrogel marbles were prepared by atomising droplets of hydrogel solution in a cold air column followed by rolling of the collected hydrogel microbeads in a bed of micrometre size salt particles. Evaporation of the water from the resulting salt marbles with a hydrogel core yielded hollow-shell salt microcapsules. The method is not limited to hydrophilic particles and could potentially be also applied to other materials, such as graphite, carbon, silica and others. The structure and morphology of the salt-hydrogel marbles were analysed with SEM and their particle size distributions were measured. We also tested the dissolution times of the dried salt marbles compared them to these for table salt samples at the same conditions. The high accessible surface area of the shell of salt microcrystals allows a faster initial release of salt from the hollow-shell salt capsules upon their dissolution in water than from the same amount of table salt. The results suggest that such hollow-shell particles could find applications as a table salt substitute in dry food products and salt seasoning formulations with reduced salt content without the loss of saltiness.

Fig. 1. Possible outcomes in the fabrication of salt marbles: Top mechanism shows production of the salt granules, where the liquid drop wets the hydrophilic salt particles to form salt granules. The bottom mechanism shows the preparation of the salt-hydrogel marbles, where a gel bead is formed before rolling onto the bed of salt microcrystals. Here, the salt crystals cannot penetrate into the hydrogel core and form a shell around it. Hollow-shell salt microcapsules are produced upon drying of the salt-hydrogel marbles.

References

1. Rutkevičius, M., Mehl, G.H. Petkov, J.T. Stoyanov, S.D. Paunov, V.N., Fabrication of salt–hydrogel marbles and hollow-shell microcapsules by an aerosol gelation technique, J. Mater. Chem. B, 3 (2015) 82-89.

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Research project 2:

Porous composites fabricated by a hydrogel templating technique

A non-conventional way of preparation of lightweight porous materials by templating hydrogels with a range of hydrophilic and hydrophobic scaffolding materials was explored [1]. Slurries of blended sub-millimetre hydrogel slurries of polyacrylamide and gellan gum were templated with aqueous slurries of cement, gypsum and clay-cement mixtures or alternatively, dispersed in curable polydimethylsiloxane (PDMS). After the solidification of the scaffolding material, the evaporation of structured hydrogel produced porous composite material whose pores mimic the hydrogel meso-structure. We studied the density, volume contraction and the compression strength of the formed porous materials as function of the hydrogel initial volume fraction. This versatile hydrogel templating method can be applied very inexpensively to a range of scaffolding materials to yield lightweight porous materials with a great potential for use in the building industry in heat and sound insulation panels, an alternative to aerated concretes, lightweight building blocks, porous rubber substitutes and foam shock absorbers.

Fig. 1. The process of templating of hydrogel slurries with aqueous slurries of (cement, gypsum or clays) of with a non-aqueous continuous phase (PDMS). The solidification of the continuous phase is followed by the evaporation of the water content of the hydrogel which leads to formation of porous composite materials.

Fig. 2.  Scanning Electron Microscopy (SEM) images of cement templated with 3% CaCl₂ containing polyachrylamide hydrogel: (a) control sample of solidified cement slurry (0% hydrogel), (b)-(c) 66% hydrogel, (d) 75% hydrogel, (e)-(f) 80% hydrogel.

References

1. Rutkevicius, M., Munusami, S.K., Watson, Z., Field, A.D., Salt, M., Stoyanov, S.D., Petkov, J., Mehl, G.H., Paunov, V.N., “Fabrication of novel lightweight composites by a hydrogel templating technique”, Mater. Res. Bull., 47 (2012) 980–986.

2. Rutkevicius, M., Mehl, G.H., Qin, Q., Rubini, P.A.,  Stoyanov, S.D., Paunov, V.N., Sound absorption properties of porous composites fabricated by a hydrogel templating technique, J. Mater. Research, 28 (2013) 2409-2414.

3. Rutkevicius, M., Austin, Z., Chalk, B., Mehl, G.H., Qin, Q., Rubini, P.A., Stoyanov, S.D., Paunov, V.N., Sound absorption of porous cement composites: effects of the porosity and the pore size, J. Materials Science, 50 (2015) 3495-3503.