https://doi.org/10.1140/epje/s10189-021-00143-5
Regular Article - Flowing Matter
Aggregation patterns of copper sulphate salt via droplet drying: mediation by surface hydrophobicity and salt concentration
1
Physics Department, St. Xavier’s College, 700016, Kolkata, India
2
Condensed Matter Physics Research Centre, Physics Department, Jadavpur University, 700032, Kolkata, India
3
Physics Department, Bhawanipur Education Society College, 700020, Kolkata, India
c tapati_mithu@yahoo.com, tapati_dutta@sxccal.edu
Received:
1
July
2021
Accepted:
27
October
2021
Published online:
15
November
2021
Patterns in drying droplets formed from colloidal solution of copper sulphate and gelatin are investigated with respect to variation of substrate hydrophobicity and salt concentration. Hydrophilic substrates as (i) glass, (ii) quartz and hydrophobic substrate as (iii) polypropylene (PP) have been used. It is observed that the dry residue pattern of salt crystals shows curved branches of crystalline aggregate growth about droplet centre for hydrophilic substrates, while thick, light and dark concentric bands of aggregates are observed for hydrophobic substrates. The geometry and topology of the patterns have been characterized through an analysis of fractal dimension and the topological measure, Euler characteristic. The fractal dimension of the deposit increases substantially with salt concentration for hydrophilic substrates, but decreases with concentration for hydrophobic substrate. Our analysis leads us to propose that an optimal viscosity contrast that facilitates prominent viscous fingers is a function of contact angle and salt concentration. We propose that substrate hydrophobicity and salt concentration together are responsible for DLA-like aggregation in evaporating droplets.
© The Author(s), under exclusive licence to EDP Sciences, SIF and Springer-Verlag GmbH Germany, part of Springer Nature 2021
