Tailoring crack morphology in coffee-ring deposits via substrate heating

Abstract

The drying of a sessile drop consisting of colloidal particles and the formation of particulate deposits with spatially periodic cracks were ubiquitous. The drying induced stress, which is generated during the evaporation of a colloidal drop, is released by the formation of cracks. We find that the morphology of cracks formed in particulate films dried at substrate temperature, T = 25 °C is markedly different from that of cracks formed at T > 45 °C. The cracks are disordered in the former case, but ordered and periodic in the latter. The disorderedness of cracks observed at T = 25 °C is mainly due to the formation of a coffee-ring like particle deposit that exhibits a larger height gradient. The ultimate deposit pattern after complete drying is observed to be different for colloidal dispersion drops evaporated at different substrate temperatures. This is attributed to temperature-dependent solvent flow mechanisms and capillary-driven flow, which occur inside the colloidal drop during the course of drying. In addition, for the coffee-ring-like particulate deposit obtained at T ≤ 45 °C, the ratio between the width of the deposit w and the radius of the ring R scales with the volume fraction of the colloids φ, w/R ∼ φ, in the range of volume fractions studied in this work. The deposited patterns obtained at temperature T > 45 °C are largely dominated by the capture of particles by the receding liquid-vapor interface. This is due to the faster rate of decrease of the liquid-vapor interface position with an increase in substrate temperature.

Crack morphologies in drying suspension drops

“…The central part of the drop contains almost no particles and no clear pattern of cracks is visible in the deposit. As the volume fraction is increased to = 0.75%, a ring pattern remains 28,29 , as seen in Fig. 2b.…”

Analysis of profile and morphology of colloidal deposits obtained from evaporating sessile droplets

“…A SEM image of the cracks at d = 0.1 μm, c = 0.1 %(Figure 4(a)) shows a dense packing of the particles at the two interfaces of the crack. As pointed to in Refs [18,35,44,45],. a growing ring exhibits a gel-like behavior with a dense packing of the particles.…”

“…By lowering the pH [11] or by using ellipsoidal particles [12], or by applying electrowetting [13,14] the particles deposit uniformly across the wetted area. Indeed, the deposit pattern could alter substantially depending upon the system used and some examples of the patterns include, a uniform deposition [15], an inner deposit [16], a stick-slip pattern resulting in multiple rings [17], fingering-like pattern [3], patterns displaying cracks [18], a crystalline pattern [19], a ring with an inner deposit [10] and patterns combining more than one of these features.…”

Droplet Drying Patterns on Solid Substrates: From Hydrophilic to Superhydrophobic Contact to Levitating Drops

“…Patterns formed can range from a simple ring at the periphery of the droplet, the so-called coffee ring [22,23], to multiple rings forming bands, fractal and multifractal aggregates of salt crystals, or nanoparticles [24][25][26][27]. In addition, the dried drop may develop crack patterns [8,[28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43], which can also be induced by external fields [35,44]. It is important to realize that the shape of the droplet plays a crucial role in generating these patterns.…”