Drying of colloidal dispersion and their consolidation into a particulate deposit is a common phenomenon. This process involves various physical processes such as diffusion of liquid molecules into the ambient atmosphere and advection of dispersed particles via evaporation driven flow. The colloidal particles forming a dried deposit exhibits distinct patterns and frequently possess structural defects such as desiccation cracks. This chapter gives an introductory review of the drying of colloidal dispersion and various associated phenomena. In principle, the drying of colloid dispersion, the process of their consolidation, and fluid-flow dynamics are all studied in numerous drying configurations. Here we explain drying induced phenomena concerning sessile drop drying. We begin with an introduction to colloids, provide background on the physics of drying, and then explain the formation of pattern and the desiccation cracks. The role of evaporation driven flows and their influence on particle accumulation, the impact of various physical parameters on pattern formation and cracks are all briefly illustrated.
A study of crack patterns in laponite films of different thickness is presented. Two cracking regimes are observed. The earlier with a higher rate of desiccation and shrinking has predominantly four-fold vertices in the crack network, while the later regime has less volume shrinking and forms mainly three-fold vertices. The pattern shows a self-similarity under coarse-graining. The graph of area covered by the cracks versus minimum crack-width resolved, scales with the film thickness. Curves representing crack area for different thickness can thus be made to collapse onto a single curve.
Polygonal desiccation crack patterns are commonly observed in natural systems. Despite their quotidian nature, it is unclear whether similar crack patterns which span orders of magnitude in length scales share the same underlying physics. In thin films, the characteristic length of polygonal cracks is known to monotonically increase with the film thickness, however, existing theories that consider the mechanical, thermodynamic, hydrodynamic, and statistical properties of cracking often lead to contradictory predictions. Here we experimentally investigate polygonal cracks in drying suspensions …
We study the crack patterns developed on desiccating films of an aqueous colloidal suspension of bentonite on a glass substrate. Varying the thickness of the layer $h$ gives the following new and interesting results: (i)We identify a critical thickness $h_{c}$, above which isolated cracks join each other to form a fully connected network. A topological analysis of the crack network shows that the Euler number falls to a minimum at $h_{c}$. (ii) We find further, that the total vertical surface area of the clay $A_v$, which has opened up due to cracking, is a constant independent of the layer th…
Consolidation of colloidal gels results in enormous stresses that are usually released in the formation of undesirable cracks. The capacity of a gel network to crack during drying depends on the existence and significance of a pressure gradient in the pore liquid; in addition it depends on the way the gel relaxes the resulting drying stresses. In this paper the effect of a binary mixture of solvents saturating the gel network on the crack patterns formation is investigated. Indeed, incorporation of a small quantity of non-volatile cosolvent, i.e. glycerol, inhibits drying-induced cracks; moreo…
In this work, we continue to study the formation of particle chains (clusters) inside the annular sediment during the drying of a colloidal droplet on a substrate. The average value of the cluster size was determined after processing experimental data from other authors. We performed a series of calculations and found the value of the model parameter allowed to get numerical results agreed with the experiment. Also, a modification of the previously proposed algorithm is analyzed here.
Cracking and peeling of a layer of clay on desiccation has been simulated using a spring model. A vertical section through the layer with finite thickness is represented by a rectangular array of nodes connected by linear springs on a square lattice. The effect of reduction of the natural length of the springs, which mimics the drying is studied. Varying the strength of adhesion between sample and substrate and the rate of penetration of the drying front produces an interesting phase diagram, showing cross-over from peeling to cracking behavior. Changes in the number and width of cracks on var…
Drying of bacterial suspensions is frequently encountered in a plethora of natural and engineering processes. However, the evaporation-driven mechanical instabilities of dense consolidating bacterial suspensions have not been explored heretofore. Here, we report the formation of two different crack patterns of drying suspensions of \textit{Escherichia coli} (\textit{E. coli}) with distinct motile behaviors. Circular cracks are observed for wild-type \textit{E. coli} with active swimming, whereas spiral-like cracks form for immotile bacteria. Using the elastic fracture mechanics and the poroela…
The drying of a colloidal dispersion can result in a gel phase defined as a porous matrix saturated in solvent. During the drying process, high mechanical stresses are generated. When these stresses exceed the strength of the material, they can be released in the formation of cracks. This process strongly depends on both the mechanical properties of the material and the way the gel consolidates. In this report, we give experimental evidences that the number of cracks formed in the consolidating film depend on the drying rate, the nature of the solvent and the mechanical properties of the collo…
A possible clarification of memory effect observed in crack patterns of drying pastes [A. Nakahara and Y. Matsuo, J. Phys. Soc. Japan 74, 1362 (2005)] is presented in terms of a macroscopic elastoplastic model of isotropic pastes. We study flows driven by steady gravitational force instead of external oscillation. The model predicts creation of residual tension in favor of cracks perpendicular to the flow direction, thus causing the same type of memory effect as that reported by Nakahara and Matsuo for oscillated CaCO_3 pastes.
In the drying process of paste, we can imprint into the paste the order how it should be broken in the future. That is, if we vibrate the paste before it is dried, it remembers the direction of the initial external vibration, and the morphology of resultant crack patterns is determined solely by the memory of the direction. The morphological phase diagram of crack patterns and the rheological measurement of the paste show that this memory effect is induced by the plasticity of paste.
When colloidal suspensions dry, stresses build up and cracks often occur - a phenomenon undesirable for important industries such as paint and ceramics. We demonstrate an effective method which can completely eliminate cracking during drying: by adding emulsion droplets into colloidal suspensions, we can systematically decrease the amount of cracking, and eliminate it completely above a critical droplet concentration. Since the emulsion droplets eventually also evaporate, our technique achieves an effective function while making little changes to the component of final product, and may therefo…
-- A scientific hurdle in manufacturing solid films by drying colloidal layers is preventing them from fracturing. This paper examines how the drying rate of colloidal liquids influences the particle packing at the nanoscale in correlation with the crack patterns observed at the macroscale. Increasing the drying rate results in more ordered, denser solid structures, and the dried samples have more cracks.Yet, introducing a holding period (at a prescribed point) during the drying protocol results in a more disordered solid structure with significantly less cracks. To interpret these observation…
A densely packed colloidal suspension with plasticity, called paste, is known to remember directions of vibration and flow. These memories in paste can be visualized by the morphology of desiccation crack patterns. Here, we find that paste made of charged colloidal particles cannot remember flow direction. If we add sodium chloride into such paste to screen the Coulombic repulsive interaction between particles, the paste comes to remember flow direction. That is, one drop of salt water changes memory effect in the paste and thereby we can tune the morphology of desiccation crack patterns more …
The formation of three-dimensional prismatic cracks in the drying process of starch-water mixtures is investigated numerically. We assume that the mixture is an elastic porous medium which possesses a stress field and a water content field. The evolution of both fields are represented by a spring network and a phenomenological model with the water potential, respectively. We find that the water content distribution has a propagating front which is not explained by a simple diffusion process. The prismatic structure of cracks driven by the water content field is observed. The depth dependence a…
A deposited drop of bovine serum albumin salt solution experiences both gelation and fracturation during evaporation. The cracks appearing at the edge of the gelling drop are regularly spaced, due to the competition between the evaporation-induced and relaxation-induced stress evolution. Subsequently, the mean crack spacing evolves in an unexpected way, being inversely proportional instead of proportional to the deposit thickness. This evolution has been ascribed to the change with time of the average shrinkage stress, the crack patterning being purely elastic instead of evaporation-controlled…
We report an experimental study of the crack pattern formed during the drying of a colloidal suspension. A horizontal fiber, which provides a one dimensional, boundary-free substrate, is coated by a film of micronic thickness. The geometry imposes a remarkable annular crack pattern and allowing precise measurements of the crack spacing over a short range of film thickness (between 2 and 10 $μ$m) which varies linearly with the film height. We compare our experimental data with a model proposed by Kitsunezaki which suggests that the variation of the crack spacing with the film thickness depends …
In this study, we experimentally investigated the time dependence of the statistical properties of two-dimensional drying crack patterns to determine the functional form of fragment size distribution. Experiments using a thin layer of a magnesium carbonate hydroxide paste revealed a "dynamical scaling" property in the time series of the fragment size distribution, which has been predicted by theoretical and numerical studies. Further analysis results based on Bayesian inference show the transition of the functional form of the fragment size distribution from a log-normal distribution to a gene…
Experiments involving vibrating pastes before drying were performed for the purposes of controlling the crack patterns that appear in the drying process. These experiments revealed a transition in the direction of lamellar cracks from perpendicular to parallel when compared with the direction of the initial external vibration as the solid volume fraction of the paste is decreased. This result suggests a transition in the "memory" in paste, which is visually represented as morphological changes in the crack pattern. Given that the memory in paste represents the flow pattern induced by the initi…
We investigate the formation of spiral crack patterns during the desiccation of thin layers of precipitates in contact with a substrate. This symmetry-breaking fracturing mode is found to arise naturally not from torsion forces, but from a propagating stress front induced by the fold-up of the fragments. We model their formation mechanism using a coarse-grain model for fragmentation and successfully reproduce the spiral cracks. Fittings of experimental and simulation data show that the spirals are logarithmic, corresponding to constant deviation from a circular crack path. Theoretical aspects …
The fracture mechanics was widely employed to explain the crack propagation in the deposition produced by drying colloidal suspension. However, more complex than conventional fracture, those cracks periodically distribute and make up a unique pattern. This still remains mysterious so far. Inspired by the concept of spinodal decomposition, here, we develop the theory to elucidate the spatial arrangement of the cracks, which indicates that the crack pattern is generated by the phase separation of colloidal clusters and water. It concludes that the crack spacing results from the wavelength of the…
Many clays, soils, biological tissues, foods, and coatings are shrinkable, granular materials: they are composed of packed, hydrated grains that shrink when dried. In many cases, these packings crack during drying, critically hindering applications. However, while cracking has been widely studied for bulk gels and packings of non-shrinkable grains, little is known about how packings of shrinkable grains crack. Here, we elucidate how grain shrinkage alters cracking during drying. Using experiments with model shrinkable hydrogel beads, we show that differential shrinkage can dramatically alter c…
Pattern formation in drying protein droplets continues to attract considerable research attention because it can be linked to specific protein-protein interactions. An extensive study of the drying evolution and the final crack patterns are presented, highlighting the concentration dependence (from 1 to 13 wt%) on two globular proteins, lysozyme (Lys) and bovine serum albumin (BSA), in de-ionized water. The drying evolution starts with a constant contact radius mode and shifts to a mixed mode where both fluid front and contact angle changes. The contact angle monotonically decreases, whereas, …
Image processing and pattern recognition offer a useful and versatile method for optically characterizing drops of a colloidal solution during the drying process and in its final state. This paper exploits image processing techniques applied to cross-polarizing microscopy to probe birefringence and the bright-field microscopy to examine the morphological patterns. The bio-colloidal solution of interest is a mixture of water, liquid crystal (LC) and three different proteins [lysozyme (Lys), myoglobin (Myo), and bovine serum albumin (BSA)], all at a fixed relative concentration. During the dryin…
We investigate the initiation of cracks in vertically freestanding water-saturated colloidal pillars constructed using a direct-write technique. Paradoxically, the cracks form during drying at the free end, far from the substrate, where the particle network is unconstrained in contracting its volume as it bears compression by a uniform capillary pressure acting at its outer surface. This is explained by a dominant balance of wetting energy terms, from which follows a simple relationship between the particle size and pillar dimensions that captures the presence or absence of cracks. This relati…
We investigate the desiccation crack patterns on the surface of a drying paste made of calcium carbonate (CaCO$_3$) powder and distilled water. Forced vibration of the CaCO$_3$ paste prior to drying results in an anisotropic crack pattern, in which many long cracks develop along a specific preferred direction. We reveal that the preferred direction changes from perpendicular to parallel to the vibration direction at the threshold velocity of vibration. The transition is attributed to the reorientation of constituent particles subjected to the forced oscillatory flow of fluid in the paste.
We present an experimental investigation of drying suspensions of both hard and soft nanolatex spheres. The crack formation is examined as a function of the proportion of hard and soft deformable particles, leading to tunable elastic properties of the drying film. In our experiments corresponding to a given layer thickness, no crack formation could be observed below an onset value of the proportion in hard spheres around 0.45. During the drying process, the mass of films with various compositions in hard and soft spheres is measured as a function of time. The results suggest that the soft part…
The work is devoted to particles dynamics simulation in a colloidal drop, when it dries on a substrate and the triple-phase boundary is fixed. Experimental observations [Deegan R. D. et. al., 2000] show a ring deposition on a solid substrate after full droplet desiccation. This phenomenon of macrolevel is known as coffee ring effect. There are other experiments that show a co-effect at the microlevel. We are talking about the formation of a quasicrystalline structure on the outer part of the ring and occurrence amorphous inner zone of ring [Mar\'ın Á. G. et. al., 2011]. The goal of this work i…
We report an experimental study on the drying of silica colloidal dispersions. Here we focus on a surface instability occurring in a drying paste phase before crack formation which affects the final film quality. Observations at macroscopic and microscopic scales reveal the occurrence of the instability, and the morphology of the film surface. Furthermore, we show that the addition of adsorbing polymers on silica particles can be used to suppress the instability under particular conditions of molecular weight and concentration. We relate this suppression to the increase of the paste elastic mo…
When a colloidal gel dries through evaporation, cracks are usually formed, which often reveal underlying processes at work during desiccation. Desiccating colloid droplets of few hundred $μl$ size show interesting effects of pattern formation and cracking which makes this an active subject of current research. Since aqueous gels of clay are known to be strongly affected by an electric field, one may expect crack patterns to exhibit a field effect. In the present study we allow droplets of laponite gel to dry under a radial electric field. This gives rise to highly reproducible patterns of crac…
A fascinating class of patterns, often encountered in nature as meandering cracks on rocks, dried-out fields and tectonic plates is produced by the fracture of solids. Here we report the observation and modeling of an unusual type of patterns consisting of spiral cracks within fragments of a thin layer of drying precipitate. We find that this symmetry-breaking cracking mode arises naturally not from twisting forces, but from a propagating stress front induced by the fold-up of the fragments.
Direct observations of the surface and shape of model nano-colloidal gels associated with measurements of the spatial distribution of water content during drying show that air starts to significantly penetrate the sample when the material stops shrinking. We show that whether the material fractures or not during desiccation, as air penetrates the porous body, the water saturation decreases but remains almost homogeneous throughout the sample. This air-invasion is at the origin of another type of fracture due to capillary effects, these results provide a new insight in the liquid dynamics at th…
In a uniform layer consisting of a mixture of granular material and liquid, it is known that desiccation cracks exhibit various anisotropic patterns that depend on the nature of the shaking that the layer experienced before drying. The existence of this effect implies that information regarding the direction of shaking is retained as a kind of memory in the arrangements of granular particles. In this work we make measurements in paste composed of Lycopodium powder using microfocus x-ray computerized tomography ($μ$CT) in order to investigate the three-dimensional arrangements of particles. We …
The process of drying is a simple physical mechanism that drives a system to relax from one equilibrium point to another. The native states of the constituent particles in the droplets can be linked to the emergent morphological patterns via this drying process. This paper explores the interplay between different initial concentrations of a globular protein lysozyme ($ϕ_p$) and the salts ($ϕ_s$) in the phosphate buffer saline (PBS). The $ϕ_s$ = 0 wt% embodies the lysozyme solution prepared in de-ionized water. The samples at $ϕ_s$ = 0.9 wt% display a dark texture in the central region. We exam…
Substrate wettability alteration induced control over crack formation process in thin colloidal films has been addressed in the present study. Colloidal nanosuspension (53nm, mean particle diameter) droplets have been subjected to natural drying to outline the effects of substrate surface energies over the dry-out characteristics with emphasis on crack dynamics, crack morphology and underlying particle arrangements. Experimental findings indicate that number of cracks formed decreases with increase in substrate hydrophobicity. These physical phenomena have been explained based on the magnitude…
Patterns are quotidian in nature. Distinct multiscale patterns are generally a consequence of nonequilibrium dynamical processes associated with mechanical or hydrodynamic instabilities. In this thesis, I report experimental investigations on pattern formation in a few examples of fluid and granular systems, and uncover the underlying mechanisms that give rise to those patterns. Leidenfrost drops are known to experience star-shaped oscillations with little damping. However, the underlying mechanism remains unclear. Here I report that the hydrodynamic coupling between the rapid evaporated vapor…
The ordering of particles in the drying process of a colloidal suspension is crucial in determining the properties of the resulting film. For example, microscopic inhomogeneities can lead to the formation of cracks and defects that can deteriorate the quality of the film considerably. This type of problem is inherently multiscale and here we study it numerically, using our recently developed method for the simulation of soft polymeric capsules in multicomponent fluids. We focus on the effect of the particle softness on the film microstructure during the drying phase and how it relates to the f…
Directional drying of a colloidal dispersion: quantitative description with water potential measurements using water clusters in a poly(dimethylsiloxane) microfluidic chip
Drying of colloidal droplets on solid, rigid substrates is associated with a capillary pressure developing within the droplet. In due course of time, the capillary pressure builds up due to droplet evaporation resulting in the formation of a colloidal thin film that is prone to crack formation. In this study, we show that introducing a minimal amount of nematic liquid crystal (NLC) can completely suppress the crack formation. The mechanism behind the curbing of the crack formation may be attributed to the capillary stress-absorbing cushion provided by the elastic arrangements of the liquid cry…
The consolidation of colloidal particles in drying colloidal dispersions is influenced by various factors such as particle size and shape, and inter-particle potential. The capillary pressure induced by the menisci, formed between the top layer of particles in the packed bed, compresses the bed of particles while the constraints enforced by the boundaries result in tensile stresses in the packing. Presence of flaws or defects in the bed determines its ultimate strength under such circumstances. In this study, we determine the asymptotic stress distribution around a flaw in a two dimensional co…
When a flat sample of medium density fibreboard (MDF) is exposed to radiant heat in an inert atmosphere, primary crack patterns suddenly start to appear over the entire surface before pyrolysis and any charring occurs. Contrary to common belief that crack formation is due to drying and shrinkage, it was demonstrated for square samples that this results from thermomechanical instability. In the present paper, new experimental data are presented for circular samples of the same MDF material. The sample was exposed to radiant heating at 20 or 50 kW/m2, and completely different crack patterns with…
