Colloidal interfaces in microfluidics

The rapidly evolving field of microfluidics, in which fluids are manipulated at a micro- or even nanoscopic scale, finds many applications in industries, ranging from the pharmaceutical to the oil-industry. It also raises new questions about the behaviour of liquids at increasingly small scales. One particular topic of interest is the behaviour of interfaces in confinement and under flow, which is for example essential when carrying out chemical reactions in liquid mixtures. An exciting way to address these questions is by making use of colloidal suspensions. Here, colloidal particles of size roughly between 1 nm and 1 micron are dispersed in a molecular solvent and the suspensions display rich phase and interface behaviour in many ways analogously to molecular or atomic systems. However, the length- and timescales in colloidal suspensions are such that many interesting phenomena become much more accessible, for example because they happen relatively slowly or at more convenient lengths. Furthermore, modern colloid science offers tailoring the interactions in colloidal suspensions to a level which is simply impossible in molecular systems. We propose to combine colloid science with microfluidics to study flow instabilities of fluid interfaces in microfluidic devices. Such hydrodynamic instabilities are frequently observed in every day life, for example when turning on a tap and watching drops form. We have developed a colloidal system, which shows similar behaviour as molecular fluids, such as oil-water mixtures, but with the large advantage that the interfacial tension, which characterizes the cost to create extra interface, is about a million times smaller than in ordinary molecular systems. As as consequence, certain features of interface behaviour become very pronounced and can be easily studied. We will focus on two types of instabilities; the first one occurs whenever a fluid with a low viscosity displaces a fluid with a high viscosity in a flat, narrow channel. The second one always takes place when a heavy fluid lies on top of a light one. Without possibly realizing it, such instabilities can be observed almost on a daily basis, and they are hugely important in many technological and industrial processes. Our proposed work will lead to a better understanding of the instabilities and it may shed light on new potential applications of microfluidics in the chemical industry.

The proposed research will lead to the development of new methodologies and techniques to explore and exploit interfacial instabilities of complex fluids in microfluidic channels. We expect the outcome of the research to be used in both science and technology. The academic side has largely been addressed in the 'Academic Beneficiaries' section and here we focus on the technological/industrial side. Most fluids used in technology and industry can be classified as complex fluids consisting of nano- or micron-sized objects suspended in a molecular solvent. Moreover, microfluidic devices are now playing a more and more dominant role in companies dealing with complex fluids. Examples easily range from foods (Unilever), to medicines (DSM), to oils (Schlumberger). The proposed research can be exploited to manipulate complex fluids, for example to mix or structure products at the micron scale in microfluidic devices. This is a notorious problem in microfluidics and in meetings with colleagues from industry we have learnt that the technique we use and develop is of direct interest to them; we have discussed this with Unilever, DSM, Schlumberger, Beiersdorf, and P&G. The companies will benefit both from the technological knowledge we built up and from the people we train in this field. Unilever, for example, recognizes that it is 'powered by people' and that it are their people who give the company its 'energy, culture and ideas' (quotes from Unilever website). One of the outcomes of our work will thus be the delivery of highly trained people. This will happen on a timescale of a few years. All of the above will strengthen the companies' strategic position with immediate financial consequences for them and the UK as a whole, and it may improve their products in making them cheaper and/or better, both to the benefit of the customer. Next, we give two more detailed examples of possible impact of the proposed work: We have a project with Unilever entitled 'Utilizing polyphenol-biopolymer interactions for structural design of food for diet and health'. This project will benefit from the proposed research: the instabilities we propose to study can be optimized to structure complex fluids on the micron scale. Optimizing or tailoring people's diets will for example help fighting obesity, which 'has now become one of the most serious medical problems of the Western world' (quote from the NHS website). This will clearly enhance the quality of life/health and may bring a strategic, economic advantage to UK based Unilever. A PhD student is involved, which ensures the transfer of knowledge between the university and the company. Another example worth highlighting lies in the study of the Saffman-Taylor instability, which is a classic instability encountered in the oil industry. A better understanding of this instability, of the three dimensional aspects, and the role of wetting and obstacles, is most likely to occur in the next year, especially since we also attack the problem by means of computer simulations. This may ultimately lead to a better control of the instability, which in its turn may improve oil recovery from oil wells. A small improvement will have an enormous impact both on economic competitiveness and on energy crisis management of the UK. The applicant has visited Schlumberger Cambridge Research a couple of times and has established contacts with a number of their researchers. It should be clear that the applicant has an extensive range of contacts with industry. This First Grant and the work it makes possible will help to establish and deepen collaborations with companies. The Impact Plan will address how this will be done such that all beneficiaries have the opportunity to benefit from the research.