Spiral cracks in drying precipitates

Effect of Colloidal Surface Charge on Desiccation Cracks

We report the effect of polarity and surface charge density on the nucleation and growth kinetics of desiccation cracks in deposits of colloids formed by drying. We show that the average spacing between desiccation cracks and crack opening are higher for the deposit of positively charged colloids than that of negatively charged colloids. The temporal evolution of crack growth is found to be faster for positively charged particle deposits. The distinct crack patterns and their kinetics are understood by considering the spatial arrangement of particles in the deposit, which is strongly influenced by the substrate-particle and particle-particle interactions. Interestingly, the crack spacing, the crack opening, and the rate at which the crack widens are found to increase upon decreasing the surface charge of the colloids.

Recent progress on crack pattern formation in thin films

Fascinating pattern formation by quasi-static crack growth in thin films has received increasing interest in both interdisciplinary science and engineering applications. The paper mainly reviews recent experimental and theoretical progress on the morphogenesis and propagation of various quasi-static crack patterns in thin films. Several key factors due to changes in loading types and substrate confinement for choosing crack paths toward different patterns are summarized. Moreover, the effect of crack propagation coupled to other competing or coexisting stress-relaxation processes in thin films, such as interface debonding/delamination and buckling instability, on the formation and transition of crack patterns is discussed. Discussions on the sources and changes in the driving force that determine crack pattern evolution may provide guidelines for the reliability and failure mechanism of thin film structures by cracking and for controllable fabrication of various crack patterns in thin films.

Crack patterns of drying dense bacterial suspensions

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 Escherichia coli (E. coli) with distinct motile behaviors. Circular cracks are observed for wild-type E. coli with active swimming, whereas spiral-like cracks form for immotile bacteria. Using the elastic fracture mechanics and the poroelastic theory, we show that the formation of the circular cracks is determined by the tensile nature of the radial drying stress once the cracks are initiated by the local order structure of bacteria due to their collective swimming. Our study demonstrates the link between the microscopic swimming behaviors of individual bacteria and the mechanical instabilities and macroscopic pattern formation of drying bacterial films. The results shed light on the dynamics of active matter in a drying process and provide useful information for understanding various biological processes associated with drying bacterial suspensions.

Effect of the Shape of the Confining Boundary and Particle Shape Anisotropy on the Morphology of Desiccation Cracks

The control of the morphology of desiccation cracks is fascinating not only from the application point of view but also from the rich physics behind it. Here, we present a seemingly simple method to tailor the morphology of desiccation cracks by exploitation of the combined effect of particle shape anisotropy and the shape of the confining boundary. This allows us to make circular, square, and triangular-shaped desiccation cracks in the vicinity of the confining boundaries. As the colloidal dispersion dries in confined wells, a drying front appears at the center of the well. With further evaporation, the drying front recedes toward the boundary from the center of the well. We show that the temporal evolution of the drying front is strongly influenced by the shape of the well. Subsequently, desiccation cracks appear in the penultimate stage of drying, and the morphology of the cracks is governed by the shape of the drying front and hence by the shape of the wells. The spatial evolution of the crack pattern is quantified by estimation of the curvature of the cracks, which suggests that the influence of the confining boundary on crack formation is long-ranged. However, the cracks in the dried deposit consisting of spherical particles remain unaffected by the shape of the well, and the cracks are always radial. We establish a one-to-one correspondence between the shape of the drying front and the morphology of the crack pattern in the final dried deposit of ellipsoids.

Aggregation Properties of Albumin in Interacting with Magnetic Fluids

In this study, interactions of Fe3O4 magnetic nanoparticles with serum albumin biomolecules in aqueous solutions were considered. The studies were conducted with the laser correlation spectroscopy and optical analysis of dehydrated films. It was shown that the addition of magnetite to an albumin solution at low concentrations of up to 10-6 g/L led to the formation of aggregates with sizes of up to 300 nm in the liquid phase and an increase in the number of spiral structures in the dehydrated films, which indicated an increase in their stability. With a further increase in the magnetite concentration in the solution (from 10-4 g/L), the magnetic particles stuck together and to albumin, thus forming aggregates with sizes larger than 1000 nm. At the same time, the formation of morphological structures in molecular films was disturbed, and a characteristic decrease in their stability occurred. Most stable films were formed at low concentrations of magnetic nanoparticles (less than 10-4 g/L) when small albumin-magnetic nanoparticle aggregates were formed. These results are important for characterizing the interaction processes of biomolecules with magnetic nanoparticles and can be useful for predicting the stability of biomolecular films with the inclusion of magnetite particles.

Ordered ring-shaped cracks induced by indentation in metal films on soft elastic substrates

Ordered crack patterns contain plentiful physical mechanisms and are useful for technological applications such as lithography, template, and biomimicry. Here we report on ordered multiple ring-shaped cracks induced by indentation in metal films on soft elastic polydimethylsiloxane (PDMS) substrates. It is shown that the indentation triggers the deformation of PDMS substrate and generates a radial tensile stress in the film, leading to the formation of ring-shaped cracks with a nearly uniform spacing. The morphological characteristics and evolution behaviors of the multiple ring-shaped cracks are revealed by optical microscopy, atomic force microscopy, and scanning electron microscopy. Their formation mechanisms are discussed by theoretical analysis based on the fracture mechanics. The report in this work can promote better understanding of the indentation-induced stress anisotropy and mode competition in rigid-film-soft-substrate systems and provide a facile strategy to control the crack patterns by simple mechanical loading.

Machine Learning Analysis for Quantitative Discrimination of Dried Blood Droplets

One of the most interesting and everyday natural phenomenon is the formation of different patterns after the evaporation of liquid droplets on a solid surface. The analysis of dried patterns from blood droplets has recently gained a lot of attention, experimentally and theoretically, due to its potential application in diagnostic medicine and forensic science. This paper presents evidence that images of dried blood droplets have a signature revealing the exhaustion level of the person, and discloses an entirely novel approach to studying human dried blood droplet patterns. We took blood samples from 30 healthy young male volunteers before and after exhaustive exercise, which is well known to cause large changes to blood chemistry. We objectively and quantitatively analysed 1800 images of dried blood droplets, developing sophisticated image processing analysis routines and optimising a multivariate statistical machine learning algorithm. We looked for statistically relevant correlations between the patterns in the dried blood droplets and exercise-induced changes in blood chemistry. An analysis of the various measured physiological parameters was also investigated. We found that when our machine learning algorithm, which optimises a statistical model combining Principal Component Analysis (PCA) as an unsupervised learning method and Linear Discriminant Analysis (LDA) as a supervised learning method, is applied on the logarithmic power spectrum of the images, it can provide up to 95% prediction accuracy, in discriminating the physiological conditions, i.e., before or after physical exercise. This correlation is strongest when all ten images taken per volunteer per condition are averaged, rather than treated individually. Having demonstrated proof-of-principle, this method can be applied to identify diseases.

A comparative study of the drying evolution and dried morphology of two globular proteins in de-ionized water solutions

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 is presented, highlighting the concentration dependence (from 1 to 13 wt%) of 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, the fluid front exhibits two regimes: an initial linear regime and a later non-linear regime. Unlike the linear regime, the non-linear regime is faster for Lys droplets. This results in the formation of a “mound”-like structure in the central region. A new feature, a “dimple” is observed in this mound which is found to be dependent on the initial concentration. The different crack morphology of BSA and Lys depends strongly on the initial state of the solution and can be interpreted using a simple mechanical model. In fact, when dried under uniform conditions (surface, humidity, temperature, droplet diameter, etc.), the evolution and the final pattern displays as a fingerprint of the initial state.

Signature pattern formation in drying globular protein solution droplets: understanding self-assembled macroscopic states as indicators of the initial microscopic states.

Hierarchical crack patterns of metal films sputter deposited on soft elastic substrates

Controlled cracks are useful in a wide range of applications, including stretchable electronics, microfluidics, sensors, templates, biomimics, and surface engineering. Here we report on the spontaneous formation of hierarchical crack patterns in metal (nickel) films sputter deposited on soft elastic polydimethylsiloxane (PDMS) substrates. The experiment shows that the nickel film generates a high tensile stress during deposition, which is relieved by the formation of disordered crack networks (called primary cracks). Due to the strong interfacial adhesion and soft substrate, the cracks can penetrate into the PDMS substrate deeply. The width and depth of the primary cracks both increase with increasing film thickness, whereas the crack spacing is insensitive to the film thickness. The film pieces dividing by the primary cracks can fracture further when they are triggered by an external disturbance due to the residual tensile stress, resulting in the formation of fine crack networks (called secondary cracks). The width and spacing of the secondary cracks show different behaviors in comparison to the primary cracks. The morphological characteristics, growth behaviors, and formation mechanisms of the primary and secondary cracking modes have been discussed in detail. The report in this work could provide better understanding of two distinct cracking modes with different sizes and morphologies.

Universal scaling of polygonal desiccation crack patterns

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 of micron-sized particles by varying film thickness, boundary adhesion, packing fraction, and solvent. Although polygonal cracks were observed in most systems above a critical film thickness, in cornstarch-water mixtures, multiscale crack patterns were observed due to two distinct desiccation mechanisms. Large-scale, primary polygons initially form due to capillary-induced film shrinkage, whereas small-scale, secondary polygons appear later due to the deswelling of the hygroscopic particles. In addition, we find that the characteristic area of the polygonal cracks, A_{p}, obeys a universal power law, A_{p}=αh^{4/3}, where h is the film thickness. By quantitatively linking α with the material properties during crack formation, we provide a robust framework for understanding multiscale polygonal crack patterns from microscopic to geologic scales.

Harnessing Colloidal Crack Formation by Flow-Enabled Self-Assembly

Self-assembly of nanomaterials to yield a wide diversity of high-order structures, materials, and devices promises new opportunities for various technological applications. Herein, we report that crack formation can be effectively harnessed by elaborately restricting the drying of colloidal suspension using a flow-enabled self-assembly (FESA) strategy to yield large-area periodic cracks (i.e., microchannels) with tunable spacing. These uniform microchannels can be utilized as a template to guide the assembly of Au nanoparticles, forming intriguing nanoparticle threads. This strategy is simple and convenient. As such, it opens the possibility for large-scale manufacturing of crack-based or crack-derived assemblies and materials for use in optics, electronics, optoelectronics, photonics, magnetic device, nanotechnology, and biotechnology.

Blood drop patterns: Formation and applications

The drying of a drop of blood or plasma on a solid substrate leads to the formation of interesting and complex patterns. Inter- and intra-cellular and macromolecular interactions in the drying plasma or blood drop are responsible for the final morphologies of the dried patterns. Changes in these cellular and macromolecular components in blood caused by diseases have been suspected to cause changes in the dried drop patterns of plasma and whole blood, which could be used as simple diagnostic tools to identify the health of humans and livestock. However, complex physicochemical driving forces involved in the pattern formation are not fully understood. This review focuses on the scientific development in microscopic observations and pattern interpretation of dried plasma and whole blood samples, as well as the diagnostic applications of pattern analysis. Dried drop patterns of plasma consist of intricate visible cracks in the outer region and fine structures in the central region, which are mainly influenced by the presence and concentration of inorganic salts and proteins during drying. The shrinkage of macromolecular gel and its adhesion to the substrate surface have been thought to be responsible for the formation of the cracks. Dried drop patterns of whole blood have three characteristic zones; their formation as functions of drying time has been reported in the literature. Some research works have applied engineering treatment to the evaporation process of whole blood samples. The sensitivities of the resultant patterns to the relative humidity of the environment, the wettability of the substrates, and the size of the drop have been reported. These research works shed light on the mechanisms of spreading, evaporation, gelation, and crack formation of the blood drops on solid substrates, as well as on the potential applications of dried drop patterns of plasma and whole blood in diagnosis.

Intertwined Multiple Spiral Fracture in Perforated Sheets

We study multiple tearing of a thin, elastic, brittle sheet indented with a rigid cone. The n cracks initially prepared symmetrically propagate radially for n≥4. However, if n<4 the radial symmetry is broken and fractures spontaneously intertwine along logarithmic spiral paths, respecting order n rotational symmetry. In the limit of very thin sheets, we find that fracture mechanics is reduced to a geometrical model that correctly predicts the maximum number of spirals to be strictly 4, together with their growth rate and the perforation force. Similar spirals are also observed in a different tearing experiment (this time up to n=4, in agreement with the model), in which bending energy of the sheet is dominant.

Drying-mediated assembly of colloidal nanoparticles into large-scale microchannels

Large-scale highly ordered microchannels were spontaneously and rapidly created by simply drying the colloidal nanoparticle suspension on a rigid substrate. Interestingly, free evaporation of colloidal suspension yielded radially aligned microchannels, while constrained evaporation that was rendered by the use of confined geometries composed of either two nearly parallel plates or a slide placed perpendicular to a rigid substrate imparted the formation of periodic arrays of parallel microchannels in a controllable manner. The microchannels were formed as a result of the competition between stress relaxation due to crack opening that ruptured the film and stress increase due to the loss of solvent. Quite intriguingly, these patterned microchannels can be exploited as templates to craft well-ordered metallic stripes. This facile and scalable approach may offer a new paradigm of producing microscopic patterns over large areas with unprecedented regularity at low cost that can serve as scaffolds for use in microelectronics and microfluidic-based biochips, among other areas.

Drying of thin colloidal films

When thin films of colloidal fluids are dried, a range of transitions are observed and the final film profile is found to depend on the processes that occur during the drying step. This article describes the drying process, initially concentrating on the various transitions. Particles are seen to initially consolidate at the edge of a drying droplet, the so-called coffee-ring effect. Flow is seen to be from the centre of the drop towards the edge and a front of close-packed particles passes horizontally across the film. Just behind the particle front the now solid film often displays cracks and finally the film is observed to de-wet. These various transitions are explained, with particular reference to the capillary pressure which forms in the solidified region of the film. The reasons for cracking in thin films is explored as well as various methods to minimize its effect. Methods to obtain stratified coatings through a single application are considered for a one-dimensional drying problem and this is then extended to two-dimensional films. Different evaporative models are described, including the physical reason for enhanced evaporation at the edge of droplets. The various scenarios when evaporation is found to be uniform across a drying film are then explained. Finally different experimental techniques for examining the drying step are mentioned and the article ends with suggested areas that warrant further study.

Plasticity and fracture in drying colloidal films

Cracks in drying colloidal dispersions are typically modeled by elastic fracture mechanics, which assumes that all strains are linear, elastic, and reversible. We tested this assumption in films of a hard latex, by intermittently blocking evaporation over a drying film, thereby relieving the film stress. Here we show that although the deformation around a crack tip has some features of brittle fracture, only 20%-30% of the crack opening is relieved when it is unloaded. Atomic force micrographs of crack tips also show evidence of plastic deformation, such as microcracks and particle rearrangement. Finally, we present a simple scaling argument showing that the yield stress of a drying colloidal film is generally comparable to its maximum capillary pressure, and thus that the plastic strain around a crack will normally be significant. This also suggests that a film's fracture toughness may be increased by decreasing the interparticle adhesion.

Fracture patterns in thin films and multilayers

While there are many stress relief mechanisms observed in thin films, excessive residual and externally applied stresses cause film fracture. In the case of tensile stress a network of through-thickness cracks forms in the film. In the case of compressive stress thin film buckling is observed in the form of blisters. Thin film delamination is an inseparable phenomenon of buckling. The buckling delamination blisters can be either circular, straight, or form periodic buckling patterns commonly known as telephone cord delamination morphology.

While excessive biaxial residual stress is the key for causing thin film fracture, either in tension, or compression, it is the influence of the external stress that can control the final fracture pattern. In this paper we consider phone cord buckling delamination observed in compressed W/Si and TiWN/GaAs thin film systems, as well as spiral and sinusoidal though-thickness cracks observed in Mo/Si multilayers under 3-point high-temperature bending in tension.

Desiccation of a clay film: cracking versus peeling

We report a simulation study on competition between cracking and peeling, in a layer of clay on desiccation and how this is affected by the rate of drying, as well as the roughness of the substrate. The system is based on a simple 2-dimensional 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 varying the layer thickness is observed to reproduce experimental reports.

Coupling between precipitation and contact-line dynamics: multiring stains and stick-slip motion

The contact line in an evaporating drop can stay pinned to form a single ring or can shrink in a discontinuous stepwise manner and generate multiple rings. We demonstrate the latter with DNA solutions and attribute it to a pinning-depinning cycle that generates new contact lines. The new contact line recedes after depinning and is repinned at an internal precipitate ring that determines the location of the next contact line. Each precursor ring is formed when DNAs are trapped by an internal microstagnation flow and precipitation dynamics hence control this unsteady drop motion.

Shake-induced order in nanosphere systems

Self-assembled patterns obtained from a drying nanosphere suspension are investigated by computer simulations and simple experiments. Motivated by the earlier experimental results of Sasaki and Hane and Schöpe, we confirm that more ordered triangular lattice structures can be obtained whenever a moderate intensity random shaking is applied on the drying system. Computer simulations are realized on an improved version of a recently elaborated Burridge-Knopoff-type model. Experiments are made following the setup of Sasaki and Hane, using ultrasonic radiation as source for controlled shaking.

Dynamics of fracture in drying suspensions

We investigate the dynamics of fracture in drying films of colloidal silica. Water loss quenches the nanoparticle dispersions to form a liquid-saturated elastic network of particles that relieves drying-induced strain by cracking. These cracks display intriguing intermittent motion originating from the deformation of arrested crack tips and aging of the elastic network. The dynamics of a single crack exhibits a universal evolution, described by a balance of the driving elastic power with the sum of interfacial power and the viscous dissipation rate of flowing interstitial fluid.

Junction formation during desiccation cracking

In order to provide a sound physical basis for the understanding of the formation of desiccation crack networks, an experimental study is presented addressing junction formation. Focusing on junctions, basic features of the network determining the final pattern, provides an elemental approach and imparts conceptual clarity to the rather complicated problem of the evolution of crack patterns. Using coffee-water mixtures a clear distinction between junction formation during nucleation and propagation is achieved. It is shown that for the same drying suspension, one can switch from the well-known symmetric triple junctions that are unique to the nucleation phase to propagation junctions that are purely dictated by the variations of the stress state. In the latter case, one can even manipulate the path of a propagating crack in a deterministic fashion by changing the stress state within the suspension. Clear microscopic evidence is provided for the formation of propagation junctions, and material inhomogeneity is observed to be reflected by a broad distribution of angles, in stark contrast to shrinkage cracks in homogeneous solid films.

Cracks and topological defects in lyotropic nematic gels

We report on the effects of the coupling of nematic order and elasticity in anisotropic lyotropic gels consisting of large nematic domains of surfactant coated single wall carbon nanotubes embedded in a cross-linked N-isopropyl acrylamide polymer matrix. We observe the following striking features: (i) undulations and then cusping of the gel sidewalls, (ii) a nematic director field that evolves as the gel sidewalls deform, (iii) networks of surface cracks that are orthogonal to the nematic director field, and (iv) fissures at the sidewall cusps and associated topological defects that would not form in liquid nematics.

Why do drying films crack?

Understanding the mechanism by which films fail during drying is the first step in controlling this natural process. Previous studies have examined the spacing between cracks with predictions made by assuming a balance between elastic energy released with a surface energy consumed. We introduce a new scaling for the spacing between cracks in drying dispersions. The scaling relates to the distance that solvent can flow, to relieve capillary stresses, as a film fails. The scaling collapses data for a range of evaporation rates, film thicknesses, particle sizes, and materials. This work identifies capillary pressures, induced by packed particle fronts travelling horizontally across films, as responsible for the failure in dried films.

Silver halide colloid precursors for the synthesis of monolayer-protected clusters

A new method for the synthesis of monolayer-protected silver clusters (MPCs) based on the two-phase reduction of a stable negatively charged silver bromide sol is described. Phase transfer of the colloid to toluene is accomplished using tetra-n-octylammonium bromide as the phase transfer reagent. The advantage of this synthesis is to uncouple the formation of the silver halide colloid from its transfer and reduction in the organic phase, thus allowing control over each reaction step. The silver colloid in toluene was reduced with aqueous borohydride in the presence of 4-bromobenzenethiol as the passivating agent. The UV-visible absorption spectra indicate the intermediate formation of Ag(core)AgBr(shell) clusters during reduction. The resulting MPCs have been characterized by optical and transmission electron microscopy, energy-dispersive X-ray analysis, thermogravimetry, and UV-vis absorption spectroscopy. The formation of spiral cracks in the nanoparticulate agglomerates on solvent evaporation was observed. The spectra of thin films obtained by solvent evaporation have been analyzed using an effective medium theory.

Flow and fracture in drying nanoparticle suspensions

Drying aqueous suspensions of monodisperse silica nanoparticles can fracture in remarkable patterns. As the material solidifies, evenly spaced cracks invade from the drying surface, with individual cracks undergoing intermittent motion. We show that the growth of cracks is limited by the advancement of the compaction front, which is governed by a balance of evaporation and flow of fluid at the drying surface. Surprisingly, the macroscopic dynamics of drying show signatures of molecular-scale fluid effects.