An experimental investigation of roll coating phenomena

This thesis gives an account of an extensive experimental investigation of the operation of a twin roll coater. Two distinct modes of operation are identified: Classical (fullyflooded inlet) and Meniscus (ultra-starved inlet). The former has been the subject of investigation for a number of years; the latter is less well known and would appear to have escaped the attention of the Coating Community at large prior to the research work reported here being carried out. A detailed description of how the key features of an industrial roll coater can be reproduced using a piece of simple but well designed experimental apparatus, which encapsulates all the necessary elements for an in-depth study of the flow, is presented. Various methods are used to visualise the flow. These include dye injection and novel computerised particle tracking techniques, coupled with state-of-the-art image processing and High Speed Video photography. Experiments reveal that the flow associated with the Classical mode of operation is essentially one-dimensional throughout the nip; Meniscus coating flow, on the other hand, is uniquely two-dimensional, containing large vortical structures. Also the pressure distributions are found to be quite distinct. A fully-flooded nip results in a pressure profile which exhibits a characteristic maximum and minimum, while an ultra-starved nip produces one which is linear and entirely sub-ambient. The transformation of the flow from one mode of operation to the other is then considered, a key feature of which is the behaviour of the upstream free surface which movesin to a minimum point and then out again as the flux is reduced, giving a non-singular result, that is, there are two non-dimensional values of the flux for each free surface position. Finally, the subject of instability in roll coating is addressed, for both the Classical and Meniscus regimes. A number of new instabilities were observed using High Speed Video photography and tentative explanations for their occurrence are given