Crack Patterns in Drying Laponite–NaCl Suspension: Role of the

Article Cite This: Langmuir 2018, 34, 6502−6510

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Crack Patterns in Drying Laponite−NaCl Suspension: Role of the Substrate and a Static Electric Field Sudeshna Sircar,† Moutushi D. Choudhury,†,‡ Sanat Karmakar,† Sujata Tarafdar,† and Tapati Dutta*,†,§ †

Condensed Matter Physics Research Centre, Physics Department, Jadavpur University, Kolkata 700032, India Centre for Advanced Studies in Condensed Matter and Solid State Physics, Department of Physics, Savitribai Phule Pune University, Pune 411007, India § Physics Department, St. Xavier’s College, Kolkata 700016, India Langmuir 2018.34:6502-6510. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 09/11/18. For personal use only.

‡

ABSTRACT: We report the formation of crack patterns in drying films of Laponite−NaCl solution. Crack patterns that develop upon drying aqueous Laponite−NaCl solution change drastically as the amount of NaCl is varied in the solution. In this work, we have investigated the effect of NaCl on drying films of aqueous solution of Laponite under two conditions: (i) when the film is bounded by a wall, as in Petri dish experiments and (ii) when the film does not have any boundary, as in experiments with droplets. In order to obtain insights into the effect of the substrate, the experiments have been done with two different substrates of different hydrophobicities, polypropylene and glass. The formation of crack patterns has been explained on the basis of the wetting and spreading properties of the solution on these substrates and the effect of salt on colloidal aggregation. In this work, we have shown that the presence of salt in aqueous Laponite solution can induce crack patterns depending on the nature of the substrate. Another important aspect of this work is the role of NaCl in crack inhibition in desiccating films of aqueous Laponite, in the presence of static electric field. This effect can be utilized to suppress undesirable crack formation in many applications.

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double layer coupled with fluid flow and forces resulting from gradients in the electric field. In the aqueous medium, the particles share complex interactions among themselves because of their anisotropic shape, electrophoretic mobility, dissimilar charges on the edges and the faces, and screening of the charges due to the presence of the ions in the liquid media. These interactions may lead to repulsion between the faces and attraction between the edges and the faces of the particles. In addition, there could be van der Waals attraction among the particles. All of these interactions and shape effects are responsible for the microstructure of LP suspension, which evolves as a function of time, causing changes in its viscoelastic character. Also, as the LP particles acquire charges in solution, it is expected that they will respond to the application of an external electric field. The work reports three aspects of our findings. The first part of the paper reports the substrate effect on the crack patterns when aqueous solution of LP and NaCl is allowed to dry in the form of films. Petri dishes of polypropylene (PP) and glass are used for this purpose. It is observed that the substrate has a strong role in determining the final crack pattern of aqueous LP of different salinities.

INTRODUCTION The effect of salt on aqueous solutions of colloids has been an active area of research for the past decade, having wide applications in medical diagnostics, inkjet printing, and cement industry, to name a few.1,2 Some colloids, in particular clays, belonging to the smectic and montmorillonite groups, swell in contact with water. This property of swelling clay is utilized to seal microcracks in concrete and cement. However, the swelling property of the clay is inhibited when exposed to saline water.3,4 Therefore, it is important to study in a systematic manner the effect of salt on crack patterns that develop in desiccating colloids such as clay. In this work, we report the effect of adding NaCl on the crack patterns developed during desiccation of aqueous solution of the synthetic clay Laponite (LP), under different conditions. In order to analyze the crack patterns observed in desiccating films of aqueous LP, it is necessary to understand the behavior of LP in water. LP particles are highly anisotropic oblate particles that develop a net negative charge on their flat faces and a small positive charge on the rim of the particles,7 when dissolved in deionized water to form an aqueous solution. The charged state of the particles is dependent on the pH of the solution8,9 and can change with ageing.10,11 An electric double layer of counterions develops around the particles. When subject to direct (dc) or alternating (ac) fields, the colloidal particles exhibit a wide range of phenomena6,12 which arise from particle polarization, motion of the ions in the electric © 2018 American Chemical Society

Received: February 13, 2018 Revised: May 4, 2018 Published: May 11, 2018 6502

DOI: 10.1021/acs.langmuir.8b00501 Langmuir 2018, 34, 6502−6510

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Langmuir In the second part of this work, we report crack patterns that develop on droplets of aqueous LP of different salinities as they desiccate. LP is a synthetic nanoclay which when dissolved in deionized water forms a gel. This gel, when left to dry in a petri dish of PP or glass, cracks around the edges in a random manner. However, without the bounding wall, the LP gel dries to form a transparent, brittle sheet that does not crack at all. In our current work, we report the remarkable crack patterns produced with the addition of small amounts of sodium chloride (NaCl) to the LP gel, with or without the presence of a bounding wall. The role of salt on crack patterns in drying droplets has been earlier reported by Pauchard et al.13 who worked on droplets of silica solution of different salinities. No report is however available on the role of salt concentration on the crack pattern in clay droplets, though the effect of salt on evaporating droplets of poly(ethylene oxide) polymer14 and polytetrafluoroethylene particles15 has been reported. It is expected that the droplet shape along with the ionized state of colloid particles in solution shall affect the final crack pattern. Crack patterns of desiccating aqueous LP in the presence of electric field has been an area of ongoing research.5,6The third aspect of this paper reports the interesting effects observed when LP solutions of different salinities are allowed to dry in a static electric field. The final crack pattern in all of the three cases is determined by the combined effect of salt concentration, substrate, the wetting and spreading property of the solution on the substrate, and the evaporation rate determined by the curvature of the film interface, apart from the usual ambient conditions of temperature and humidity.

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Figure 1. (a) Experimental setup for the application of direct field (dc) to the Petri dish systems with CP configuration and (b) experimental setup for applying dc field to the droplet system with CP configuration as well. In both the cases, the substrate is of PP. this set of experiments are prepared in the same manner as stated earlier in this section. With an increase in salt concentration in the LP solutions, the time required for complete gelation to occur increases considerably. For the dc field experiments, complete gelation of the solution is required before the electric field can be switched on. As the cracking process is very sensitive to temperature and humidity, the measurements for all the different salt concentrations need to be taken within a single day so as to maintain identical ambient conditions. Therefore, with a gelation time increasing from about 30 min for 0.05% salt to 4−5 h for 0.3% salt, it becomes difficult to perform the experiment for higher salt concentrations within a single day. An electric field (dc) of 10 V is also applied to droplets of different concentrations of LN solution on the PP substrate. A circular ring of Al wire surrounding the outer edge of the droplet forms the peripheral electrode. The inner electrode is formed from an Al wire placed vertically at the center of the droplet (Figure 1b). Characterization Measurements. A pendant drop technique was employed to analyze the surface tension of the LP gel with different concentrations of salt in ambient air at 25 °C and a relative humidity of 45%. The measurements for adhesion and spreading coefficient have been calculated from the contact angle measurements of the LN solutions from eqs 3 and 4, respectively. The contact angle is measured using the pendant drop method using an automated optical contact angle goniometer and the data analyzed using software SCA 20 (Data Physics Instruments, GmbH, Germany). The measurement of the contact angle is taken at a point where the solution has already gelled, so that it is equivalent to measuring the contact angle of the gelled film used in our cracking experiments. The pH of LP solution with different concentrations of salt was measured by a digital pH meter (MIFA Systems, India) maintaining identical ambient conditions of the room. Dilute LN solutions were prepared in molar ratios of 0.2, 0.4, 0.6, 0.8, and 1.0. These solutions were used to measure the hydrodynamic radius, zeta potential, conductivity, and mobility of the samples using Zetasizer Nano ZS (Malvern Instrument, UK). The experiments were repeated three times, and their average values were plotted against the molar ratios (Figure 2).

EXPERIMENTAL SECTION

Desiccation Experiments. We observe desiccation cracks in films of aqueous LP gel containing different concentrations of NaCl under two situations: (i) when the film is enclosed by a boundary which we call the “Petri dish experiments” (PD) and (ii) without the presence of a boundary which we call the “droplet experiments (DE)”. For PD experiments, a pure LP gel is formed by the addition of 2.5 g of LP powder to 40 mL of deionized water and stirred by a magnetic stirrer for about 30 s. The solution is immediately poured into a petri dish and allowed to dry for 4−5 days. To prepare the LP−NaCl (LN) solutions of varying salt concentrations (0.1, 0.2, 0.3, 0.4, 0.6, 0.8, and 1.0%), we first prepare the salt solution by adding the required amount of NaCl to 40 mL of deionized water, for example, for 0.2% LN solution, we add 0.08 g of LP to 40 mL of deionized water, and so forth. To this, we add 2.5 g of LP powder and stir it for about 1 min before pouring the solution into the petri dish. Instrumental errors in the measurement of mass and volume of the samples are in the limit of ±0.001 g and ±2 mL, respectively. To check for substrate effects, we use petri dishes of PP and glass, each of diameter 9 cm. For DE experiments, the LP and LN solutions are prepared in the same manner as above and deposited on both the substrates as drops, each drop of volume 0.5 mL. To check for the effect of droplet volume on the crack pattern, the DEs on the PP substrate were also performed with 1.0 mL volume. The error in volume measurements for droplets is about ±0.001 mL. The temperature and relative humidity during desiccation vary between 24−31 °C and 45−66%, respectively. LP RD is procured from Rockwood additives and sodium chloride from Merck, India. Electric Field Experiments. The effect of a static electric field on the crack patterns in LN experiments is studied by applying a radial field to the PP petri dish system. The outer and inner electrodes are made by wrapping the vertical walls of the petri dish with an Al foil and placing a small Al rod at the center, respectively (Figure 1a). The crack pattern formed on the application of a direct electric field (dc) of 100 V for the center positive (CP) configuration of the system is observed for different concentrations of NaCl. The LN solutions for

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RESULTS AND DISCUSSION Characterizations. Figure 2a−d shows the effect of salt concentration in aqueous LP solution on colloidal particle diameter, the zeta potential, conductivity, and mobility, respectively. These measurements were done on dilute solutions with different molar ratios of LP and NaCl. Although zeta potential and dynamic light scattering experiments are performed at much lower concentrations than those used in dessication experiments, it is useful to know the electrostatic behavior as well as the hydrodynamic radius of LP particles in an aqueous solution. Individual smectite platelets of LP aggregate or flocculate into colloidal particles. The colloidal particle size decreases with the decrease in salt concentration as is evident from Figure 2a. Though this is a pointer to greater flocculation of the particles with the increase in NaCl concentration, it may be mentioned that the NaCl crystals grow on the flocs adding to their total size. If the thickness of

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Figure 2. Variation of (a) aggregate diameter, (b) zeta potential, (c) conductivity, and (d) mobility with NaCl concentration. The error bars are indicated in the graphs.

Figure 3. Variation with NaCl concentration of (a) contact angle, θ; (b) adhesion, ΔW; (c) spreading coefficient, S; (d) surface tension, γ; (e) pH; error bars are indicated within graphs; and (f) time evolution of a drop of 0.4% LN solution, where contact angle measurements are done by the pendant drop method, at time instants of (i) t = 0, (ii) t = 20, (iii) t = 40, and (iv) t = 45 min.

the double layer is very small compared to the particle size, that is, κa ≫1, the von Smoluchowski equation relates the electrophoretic mobility, μ, to the electrical potential at the shear plane of every colloidal particle, that is, the zeta potential, ζ, by

μ=

ε ζ η

μ=

2ε ζ 3η

(2)

This is the Huckel approximation generally used for nonaqueous measurements. Our measurements show that the zeta potential and mobility, Figure 2b,d, show exactly the same variation with salt concentration. The zeta potential shows a maximum of −52 mV for the ratio LN = 0.6. It is well known16 that a higher magnitude of ζ, whether positive or negative, points to greater stability of the colloidal solution. The zeta potential reaches a minimum of 0 mV for the ratio LN = 0.2. It is clear that the increase in salt concentration leads to aggregation or flocculation of the particles, thus destroying the stability of the colloidal solution.

(1)

Here, κ is the inverse of the Debye length, a is the particle radius, η is the dynamic viscosity, and ϵ is the dielectric constant of the solvent. For small particles in low dielectric constant, eq 1 becomes 6504

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Figure 4. (A) Crack pattern of dry LN solution in a Petri dish. The salt percentage is mentioned along the horizontal axis; 0.3% LN shows hierarchical cracking; (B) time evolution of the crack patterns for 0.2% LN solutions is shown for the PP substrate, after (i) 50 and (ii) 70 h of desiccation and glass substrate, after (iii) 24 and (iv) 30 h of desiccation.

Figure 5. Variation of crack ped number with NaCl concentration for (a) PD systems for different substrates and (b) DE systems of two different volumes, on the PP substrate.

Petri Dish Experiments. The experiments were performed on petri dishes of PP and glass. The pure LP gel on desiccation cracks randomly, starting at the edges of its bounding walls for both PP and glass. However, the scenario changes completely once salt, that is, NaCl is added to the system. As the gel dries, cracks start to appear in a specific manner with increasing salt concentration. With the rise in the salt level, the gelation time of the system increases as well, ranging from a few seconds for LP to several hours for 1% LN solution. The LP gel with no salt is completely transparent, even after drying. Even though 0.2% LN solution retains the transparent nature, it turns opaque for all higher concentrations of salt. Complete desiccation takes about 5−6 days, at the end of which the crack patterns are formed (Figure 4). Comparison of the crack peds formed for the two substrates shows that the peds formed on the glass substrate tend to curl up at the edges on complete drying. Further, at the particular concentration of 0.6% NaCl, there is a minimum in the number of crack peds, that is, the tendency to crack decreases. This occurs for both PP and glass as seen in the graph of Figure 5a. A comparison between the plots for the two substrates shows that at 0.3% concentration of salt, the number of crack peds, Np, peaks for both, though Np for glass is far higher than that of PP. It is noteworthy that at this concentration, the crack pattern shows a distinct hierarchical pattern for both the substrates. For all other concentrations, the variation of Np shows the same nature for both surfaces, with ped numbers always being higher in the case of glass.

Figure 3a displays the variation of contact angle of the LN solution of different salt concentrations on PP and glass. On the glass substrate, the contact angle is acute ∼35°, whereas for PP, the contact angle is ∼98°. Increasing the salinity has almost no effect on the contact angle. Figure 3b,c displays the adhesion and spreading of the solution of different salinities for each of the substrates, respectively. Knowing the contact angle, θ, of the LN solutions on both the substrates, glass and PP and the surface tension, γ, the spreading parameter, S, and the adhesion energy per unit area, ΔW, can be calculated as follows

S = γ(cos θ − 1)

(3)

ΔW = γ(cos θ + 1)

(4)

On both glass and PP, adhesion shows a general tendency to increase with salt concentration, reaching a maximum at 0.6% salt concentration on glass. The spreading of the solution on the glass substrate shows a slow decrease with increasing salinity. On the PP substrate, the spreading shows a minimum at 0.6% salt concentration. The surface tension of aqueous LP of different salinities increases steadily to a maximum ∼57 N/m at a salt concentration of 0.6% (Figure 3d). Thereafter, it remains almost constant with increasing salinity. Figure 3e displays the variation in the pH of the solution at different salinities. Figure 3f(i−iv) shows the time evolution of a drop of LN solution during the measurement of surface tension by the pendant drop method. 6505

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Figure 6. (A) (a,g) show droplets of pure LP on PP and glass substrate, respectively, without any formation of cracks; (b−f) crack patterns of dry LN solutions for 0.5 mL droplet on PP; (h−j) zoomed portions of crack patterns for LN droplets on the glass substrate. Salt percentages are mentioned for each figure. For (a−g), one small green box has the dimension of 1 mm × 1 mm; (B) appearance of cracks on 0.2% LN solution droplets on the PP substrate after (i) 1 h 50 min, (ii) 3 h, and (iii) 3 h 15 min, and on glass substrate after (iv) 2 h 50 min, (v) 3 h 20 min, and (vi) 3 h 50 min.

Figure 7. Crack pattern of dry LN solution in a droplet of 1.0 mL, on PP for different salt concentrations. Salt concentrations are mentioned in (a− e). For each figure, one small green box has the dimension of 1 mm × 1 mm.

Droplet Experiments. Pure droplets of aqueous LP when allowed to dessicate on a glass or PP substrate dry without cracking. The addition of even a small percentage of salt produces crack patterns on both substrates. Figure 6 shows the final crack patterns obtained by desiccating 0.5 mL volume of aqueous LP with different concentrations of NaCl. The droplets on PP, Figure 6A(a−f), show a coffee-ring effect17 at salt concentrations greater than 0.4%, followed by a transparent border. The number of peds, Nd, decreases and becomes more irregular in shape as the salt concentration increases. The opacity of the peds also increases with increase in salt concentration. By contrast, the same volume of droplets, having identical salt concentrations as before, when placed on a glass substrate has a larger average diameter. Each droplet shows an opaque border. At lower salt concentrations of ∼0.2%, the dry droplets show a hierarchical crack pattern with larger peds at the center that branch into smaller peds at the periphery. The peds are polygonal structures with straight edges. At salt concentrations >0.4%, this hierarchical structure of cracks is destroyed. The

peds have irregular edges, become more opaque, and move toward a uniform size distribution with increasing salt concentration. Figure 7 shows the final crack pattern when the experiment was repeated on the PP substrate with a higher volume of 1 mL. The coffee-ring effect is not discernible till concentrations are >0.8%. The transparent border observed in the case of smaller droplet volume of 0.5 mL is absent. Figure 5b displays the effect of droplet volume on the variation of ped number with salt concentration. Nd shows a maximum at ∼0.4% of salt concentration as compared to 0.2% for 0.5 mL of the solution. At every salt concentration, the nature of the variation is similar with the number of peds always higher for 1 mL than 0.5 mL. Figure 8a−e shows scanning electron microscopy (SEM) images of the dried film with different salt concentrations. The increase in the average size of the aggregates indicates that the LP particles have flocculated with crystals of NaCl on the surface. It appears that the average size of a floc is the largest for 0.6% of NaCl concentration. Although the cubic crystals of NaCl is clearly visible on the surface of aggregates for all salt 6506

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Figure 8. (a−e) SEM images of the crack ped surface of films of LN solutions of different concentrations after complete drying, that is, no further crack production. The concentrations are indicated on the images and (f) pyramidal growth of NaCl crystals on crack peds for 0.6% salt concentration.

Figure 9. Optical microscopy images of different aspects of crack pattern for 0.4% NaCl concentration developed on the PP substrate. (a) Particles stick to the substrate between cracks; (b) dendritic growth of NaCl crystal growth on the film surface; and (c) periphery of the dried droplet showing particles constituting the coffee ring.

Figure 10. Desiccation crack pattern on aqueous LP solution containing different concentration of NaCl, in the presence of dc electric field. The central electrode is anode, and the peripheral electrode is cathode. The different concentrations of NaCl are (a) 0.05, (b) 0.15, (c) 0.2, and (d) 0.25%, where the direction of propagation of cracks has been marked by red arrows and (e) variation of time of appearance of first crack vs salt concentration. The experimental data is denoted by symbols. The dotted line denotes the best-fit line.

Desiccation Cracks of LP and NaCl Solution in dc Field. The crack patterns obtained on desiccating aqueous solution of LP containing different concentrations of NaCl are displayed in Figure 10a−d. In the absence of any salt, desiccation cracks in the presence of a dc electric field emerge from the central anode and proceed radially to the cathode. However, the addition of as little as 0.05% of NaCl changes the crack pattern drastically. A displacement of mass of the gel which is still soft occurs from around the anode. This displacement proceeds outward toward the cathode in the manner of a circular wave. The opacity of the outer region increases with the increase in salt concentration. The inner depressed region is translucent in nature. Cracks that emerge

concentrations, the SEM image for 0.6% also shows pyramidal crystal structures of NaCl on the film surface (Figure 8f). Figure 9 shows optical microscopy images of different aspects of the dried film ped in the case of 0.4% NaCl concentrated film on the PP substrate, as a case study. The cracks that develop are not “clean” as particle deposits stick to the substrate between crack walls (Figure 9a). The dendritic aggregation of NaCl crystals on the surface of the crack peds is visible in Figure 9b. Figure 9c shows the deposition of particles at the periphery of the droplet forming the “coffee-ring” effect. The transparent zone between the “coffee ring” and the peds indicates that the droplet on PP dried following a stick-slip movement. 6507

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Figure 11. (a) Black line shows the variation in the square of the radius (cm2) ≈ area of the crack ring for different LN solutions formed by the application of a dc field in the PD system. The red plot shows the time instant, tN (min) at which the system attains the maximum applied dc voltage and the current drops to zero. Both the sets of data have been normalized by their maximum values for a better comparison of their nature of variation and (b) energy supplied by the direct electric field to the system has been calculated from the current−voltage data recorded for the different salt concentrations of the LN solutions in the PD system. The variation of the electrical energy, the ring area, and the time shows a similarity in nature.

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DISCUSSIONS For identical salt concentration, the crack pattern and characteristics of desiccating films of the LP solution are different for petri dishes of PP and glass (Figure 4). This difference is attributed to the different spreading and wetting of the solutions on different substrates. The addition of NaCl increases the number of cations in the solution which is responsible for neutralizing part of the negative surface charge on the faces of the LP particles. This reduces the repulsion between LP particles. The particles start forming aggregates through flocculation, and the solution loses its stability as is evident from the decreased value of the zeta potential. The SEM images of Figure 8 show the aggregates increasing in size with increasing salinity up to 0.6% of salt concentration, after which they appear to decrease a little in size. This is responsible for the maximum in ped number at 0.2% where aggregate size is seen to be minimum because cracks can develop only along the aggregate boundaries. By the similar argument, the maximum size of the aggregate corresponding to 0.6% yields the minimum number of crack peds. The excess NaCl in solution starts crystallizing on the ped surface as tiny cubes following the dendritic growth pattern (Figure 9b), as reported by Choudhury et al.18 Xu et al.19 suggest that the effects of added salts are two-fold: (1) compressing the double layer between the particles and the substrate, leading to increased number of adsorbed particles and (2) reducing the electrokinetic effect and hence the radial velocity of the mobile particles. The spreading coefficient of aqueous LP with 0.6% NaCl on PP is a minimum (Figure 2c). This may be responsible for the supersaturation of the solution which in turn causes unequal and rapid crystallization of the salt about the edges rather than the faces of a crystal cube, leading to the pyramidal crystals. Figure 2d shows that a maximum of surface tension value is reached at 0.6% NaCl concentration. A higher value of surface tension is likely to suppress evaporation. It is possible that this affects the rate of crystallization in the solution allowing greater time for the NaCl crystal to grow around a nucleating centre. This probably explains the occurrence of the pyramidal NaCl crystal aggregates that are found only at the salt concentration of 0.6%. For any given salinity, the adhesion and spreading of the solution are far greater on glass than those on PP (Figure 3b,c). With contact angle between glass and the different solutions being much 90°. The adhesion

from the anode are only confined to the inner depressed region. As the salt concentration increases, the mass displacement wave pushes toward the peripheral cathode, making the outer elevated region thicker. It is observed that the rate of increase in the area of the inner depressed region coincides with the increase in the voltage drop across the system (Figure 11a). We calculated the electrical energy supplied to the system during this time (Figure 11b) from measurements of current and voltage. It is evident that the energy is utilized in the displacement wave that propagates from the central anode to the peripheral cathode. Figure 12a,b shows SEM images of the

Figure 12. (a) SEM micrographs of LN solutions placed in an external electric field (dc) of 100 V near the central positive electrode and (b) SEM micrographs of a portion of the same system that is just outside the crack ring formed due to the electric field. The NaCl crystals can be clearly seen in (b), which are completely absent in (a).

regions near the anode and the opaque outer ring, respectively. Although almost no NaCl crystals are seen in the inner ring region, these are clearly visible in the outer region. An interesting observation is that with an increase in the salt concentration of the LN solution, the length of the crack propagating from the anode is inhibited (Figure 10a−d). Another noteworthy point is that with increasing salt concentration, the time of appearance of first crack increases exponentially (Figure 10e), until crack growth is suppressed as is evident from Figure 10d. Thus, it appears that the addition of NaCl to aqueous LP has the effect of suppressing crack formation in desiccating LP films in the presence of dc electric field. We restricted our experiments to 0.3% NaCl concentration as the gelling time required for higher salt concentration increases hugely. The dc field cannot be switched on until the film has gelled. 6508

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even a small percentage of NaCl has a drastic effect on the crack pattern. Figure 10e indicates that the time of appearance of the first crack increases with salt concentration. This is supportive of the fact that cracking in the presence of dc electric field seems to be inhibited with increased salinity.

between the substrate and the solution in the case of glass generates shear stress between consecutive layers of the film that dries top down. It is well-established that thin films generate more cracks than thicker films.20 Lazarus21 has shown that cracks interact mutually as soon as the spacing between them is smaller than 10 times the thickness of the layer. A simple Griffith-type balance between the elastic deformation energy and the fracture bulk, and debonding, captures a broad number of observations. The observations range from squareroot or linear increase of the spacing with the thickness, to its decrease with loading until saturation. Adhesion is identified as playing a key role in these behavior changes. In this context, the greater spreading ability of the same volume of solution makes a thinner film on glass than that on PP. The greater number of peds observed on the glass substrate than that on PP, Figure 5a, is therefore explained. Examination of the crack peds on glass reveals that their edges curl upward as they dry. This happens because once the peds are formed, drying also commences from along the edges of the thin peds, whereas the central part of the peds remains attached to the substrate. If the drying rate is faster than the rate at which the system relaxes, the threshold strain can be reached faster and peeling can happen from along the edges as is observed. As spreading is lower on PP, the peds are relatively thick. The rate of drying is slower than the rate at which the system relaxes and this curling up of the edges is absent. In the case of droplet drying of the solution on the PP substrate, a ring of particles marks the initial boundary of the droplet. The triple line of contact is not pinned to the substrate. This is not unexpected given the high angle of contact and low adhesion between the substrate and solution. With the increase in salt concentration, the transparent quality of the gel observed at 0.2% concentration turns opaque because of aggregation between LP particles. As the droplet dries, the contact line contracts, drawing the particles to the thickening central region. As a result, the number of cracks becomes fewer. The droplet on the glass substrate has a larger average radius, following its greater spreading coefficient on glass. This ensures that the droplet height is much less than that on PP. Further, the pinned triple line of contact ensures a pronounced “coffeering” effect manifested by the thick opaque outer boundary. For lower salt concentration, ∼0.2%, the peds follow a hierarchical pattern being smaller in size along the periphery and join to form larger peds at the center. The low amount of NaCl is not sufficient to form a thick gelled “foot” at the periphery. The natural curvature of the droplet ensures that the edge remains thinner than that on the central part of the droplet. This enables smaller and more numerous cracks to develop along the edges, whereas the cracks at the center remain thicker and fewer in number. However, with the increase in salt concentration, the thick “gelled foot” advances inward creating a thinner film at the central part of the drying droplet. The central thinner film breaks up into numerous tiny peds as the film dries to release the shear stress that builds up because of the greater adhesion between the glass and solution. For both PP and glass substrates, it is noted that as the salt concentration increases, the crack ped edges change from straight lines to a more irregular line as cracking now occurs along the aggregate boundaries. The most remarkable effect of desiccation of LN solution in dc electric field is the inhibition effect that increasing salinity has on crack formation (Figure 10). The difference between Figure 10a and the other images of Figure 10 is testimony that

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SUMMARY AND CONCLUSIONS In this work, the effect of NaCl on drying film of aqueous solution of LP has been investigated under two conditions: when the film is bounded by a wall as in PD and when the film does not have any boundary as in DEs. The experiments have been done with two different substrates, PP and glass. The former is less wetted than the latter by the solution. The crack patterns have been explained on the basis of the wetting and spreading properties of the solution on these substrates and the effect of salt on colloidal aggregation. It is worth recalling that the aqueous LP droplet shows no cracking and can crack only when bound by a boundary.22 The strong cohesion between LP particles enables drying of the droplet while shrinking as a whole. The force of adhesion between the solution and substrate is weaker than cohesion. We show in this work that the addition of salt to aqueous LP solution destabilizes the solution and weakens cohesion considerably. Crack patterns now develop depending on the substrate type. The third important aspect of this work is the role of NaCl in crack inhibition in desiccating films of aqueous LP in the presence of dc electric field. This effect can be utilized to suppress undesirable crack formation in many applications. This effect is quite drastic though we have not yet been able to propose a clear understanding of the reason. We hope to be able to provide a suitable answer in our future work.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sujata Tarafdar: 0000-0003-0207-5455 Tapati Dutta: 0000-0002-8641-9712 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the SEM facility of the Physics Department of Jadavpur University, Kolkata (configuration no. QUO-35357-0614 funded by FIST-2, DST Govt. of India). M.D.C. thanks SERB, India, for providing financial support through NPDF (PDF/2016/001151/PMS).

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REFERENCES

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