Hans Tromp

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My interest is in applying fundamental colloid chemistry to food systems. Food systems are complex and usually not in thermodynamic equilibrium. Their broad ranges of characteristic time and distance scales pose an interesting challenge to systematic science. However, it is often surprising how well the physical behaviour of food systems can be described by relatively simple models.

In recent years, phase separation in aqueous biopolymer systems has been the focus of my research. In particular the interface between two aqueous incompatible liquids offers fascinating science. The central question is: how do we describe the stable interface between two liquids, which both contain more than 90% water?
Water-water interfaces are typically formed between solutions of polysaccharides and proteins. Stable interfaces suggest the possibility of stable emulsions. Such water-in-water emulsions would be ideal to replace oil-in-water emulsions. Below you find a movie (Movie 1) of a water-in-water emulsion under shear and restoring its quiescent structure after stopping the shear. The 'emulsion droplets' are aqueous droplets of pullulan solution (dark). They are immersed in an aqeuous solution of fish gelatine (light).
My other research interests include structure formation under shear (see Movies 2 and 3 below), the interaction between sound and liquid surfaces, and image analysis.


In the projects which I do for the customers of NIZO, I try to combine a practical approach with scientific progress. Both the customer and NIZO should profit from such an approach.

A confocal microscope image of peanut butter. Water-rich pockets are in green, and the continuous fat phase is dark. Fibers are red. Field of view 150 micron. 

Gelatine spheres, spontaneously formed after cooling from 50C a solution of gelatine and dextran. The typical size of the spheres is between 0.5 and 1 mm.

Movie 1. A phase separated mixture of aqueous solutions of pullulan and fish gelatine under a shear rate of 4.5/s, and the restoration of the structure after stopping the shear. Field of view 450 micron. The gelatine-rich phase was stained with rhodamine.

Movie 2. Custard under a shear rate of 1.5/s. Field of view 450 micron. The swollen starch granules are green, and particles of aggregated protein are red. Fat globules are visible as small black spheres. Note the different velocities of starch granules and fat globules suggesting different responses to shear stress of the weak network of starch granules and the serum in between. From observations like this NIZO tries to understand the role of serum viscosity in the creaminess of soft food products. 

Movie 3. A suspension of native tapioca starch granules in a whey protein solution. Field of view 450 micron. Shear rate 1.5/s. Note the usual rotational movement of the granules in a velocity gradient (which is perpendicular to the screen). This movement is made irregular by hydrodynamic interactions among the granules, and by transient clustering.