Heat release at the wetting front during capillary filling of cellulosic micro-substrates

Spontaneous imbibition in cellulosic materials is an expanding field of research due to the direct applicability in paper-based microfluidics. Here, we show experimentally, using simultaneous thermal and optical imaging that the temperature at the wetting front during capillary filling of paper is temporarily increased, even if the imbibed fluid and the cellulosic substrate are initially at isothermal conditions. Several liquids and two types of filter paper, characterised by scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis, were investigated demonstrating a significant temperature rise at the wetting front that cannot be neglected form the process. The temperature rise is found to be related to the energetics of imbibition compounds, including acid-base contributions, that result in electrostatic attractions as the liquid molecules are adhered on the fiber surfaces upon capillary contact.