Evaluation of Capillary Flow in Gels - The Liquid Uptake of Capillaries and Gelation Mechanisms in Alginate Gels
Licentiate thesis, 2016
For centuries it has been known that wetting liquids penetrate porous materials if the pores are sufficiently small. In some cases the liquid penetration is desired, like in kitchen paper or diapers, in other systems the penetration should be minimised or avoided, like in water repellent textiles or paper printing. Either way capillary flow has been studied extensively starting from smooth to rough surfaces and from single capillaries to porous systems. However, one field, which is lacking attention is the behaviour of capillary flow in porous gels. How does a semi-solid material influence capillary flow? Possible applications could be to absorb solutes or cell solutions in porous gels with the aim to get an even distribution of those. In this thesis alginate and agarose gels are used to study capillary flow. A thorough study of the gel characteristics including rheology measurements and investigation of the microstructure using two different gelation mechanisms gave the basis to study capillary action in air filled capillaries in alginate gels. A fast method to create a single capillary of tailored diameter in an alginate gel as well as in agarose gel was developed. Pure water penetrated the 5–6 cm long horizontal channel in an alginate gel of 630 µm, containing predominantly water in just 0.8 seconds. With that I showed first (i), that it is possible to get spontaneous capillary flow in gels containing large amounts of water and second (ii), I showed that the penetration dynamics follow the expected behaviour in that the squared travelled distance is proportional to the time, x^2∝t. As anticipated smaller capillaries exhibit slower uptake and higher viscosities decrease the speed in addition, as tested with a sucrose and a hydroxyethyl cellulose solution. Yet, the predictions from Lucas and Washburn theory optimised for hard systems, like glass result in times of only 0.2 seconds for water in a diameter of 630 µm and 5 cm length compared to the experimentally determined value of 0.8 seconds. Also all other tested liquids and diameters result in slower speeds than predicted for tested alginate and agarose gels. The most common reasons to address discrepancies to the prediction like inertial forces and using the dynamic contact angle instead of static, fail to explain the observed discrepancy, as I will show in this thesis. Other reasons are given and analysed. Since the spreading speed of a liquid drop on a soft surface has shown to be decreased and a wetting ridge at the three-phase contact line has been observed, it is hypothesised that the capillary flow is slowed down by such a wetting ridge, occurring in the front of the moving meniscus in the capillary. Besides the known viscous energy dissipation in the liquid, a viscoelastic energy dissipation due to the wall has to be added, which is the object of further investigations.