Contact Line Region Research Articles

An experimental investigation of the heat transfer dynamics during evaporation of a single liquid drop on a sapphire substrate

The paper is devoted to an experimental study of the heat transfer dynamics during evaporation of a single liquid drop on a heated horizontal surface, which is a sapphire glass coated with a high heat-resistant black graphite paint. The method employed in research can be used to study the heat and mass transfer processes in the gas-liquid-solid contact line region with maximum heat transfer coefficient. Its particular feature as compared to the previously known methods is the solution of the initial-boundary problem for the heat conductivity equation, which in terms of mathematics is a correct problem. Using the thermography method, the sapphire surface temperature fields after single drop impingement are determined. The data obtained will be used to calculate the heat flux density in the region of the contact line of the drop.

Particle monolayer assembly in evaporating salty colloidal droplets

Ring-shaped deposits can be often found after a droplet evaporates on a substrate. If the fluid in the droplet is a pure liquid and its contact line remains pinned during the process, the mechanism behind such ring-shaped deposition is the well-known coffee-stain effect. However, adding small amounts of salt to such a droplet can change the internal flow dramatically and consequently change the deposition mechanism. Due to an increase of surface tension in the contact line region, a Marangoni flow arises which is directed from the apex of the droplet towards the contact line. As a result, particles arrive at the contact line following the liquid-air interface of the droplet. Interestingly, the deposit is also ring-shaped, as in the classical coffee-stain effect, but with a radically different morphology: particles form a monolayer along the liquid-air interface of the droplet, instead of a compact three-dimensional deposit. Using confocal microscopy, we study particle-per-particle how the assembly of the colloidal monolayer occurs during the evaporation of droplets for different initial concentration of sodium chloride and initial particle dilution. Our results are compared with classical diffusion-limited deposition models and open up an interesting scenario of deposits via interfacial particle assembly, which can easily yield homogeneous depositions by manipulating the initial salt and particle concentration in the droplet.

Micro-scale heat transfer modelling of the contact line region of a boiling-sodium bubble

The use of boiling liquid metals such as sodium is attractive for providing a near-isothermal heat source for engineering applications. However, previous use of boiling sodium as a coolant in nuclear reactors and as a heat transfer fluid in solar thermal applications has shown that the boiling process is unstable. To stabilise the flow, it is imperative to gain a better understanding of the boiling phenomena. An integral part of the boiling process is the evaporation of the region where the liquid-vapour interface meets the heater wall, referred to as the contact line region. The heat transfer modelling of this region formed below a single bubble in nucleate pool boiling of sodium is considered in this study. A contact line model previously developed for high Prandtl number flows is extended by including the effect of an electron pressure component which is unique to liquid metals. The assumptions made in the model are critically assessed to determine their validity for modelling micro-scale evaporation in sodium. The model was used to show that the evaporative heat flux from the contact line region in sodium can be up to six times larger compared to a high Prandtl number fluid FC-72 for a superheat of 15 K, owing to the high thermal conductivity of sodium. Furthermore, a study on the influence of specific characteristics of sodium — high boiling superheat and presence of an electron pressure — showed that the evaporative heat flux increases with increasing superheat and decreases with increasing electron pressure.

Magnetic suppression of the coffee ring effect

This paper reports the evaporation and drying characteristics of ferrofluid droplets on a PDMS (Polydimethylsiloxane) substrate under the influence of magnetic field. The effect of magnetic field strength and asymmetry of the external magnetic field with respect to the axis of the droplet is studied. Goniometric measurement is used for visualization of the evaporating droplets. Confocal microscopy is used for imaging the drying pattern and particle motion inside the droplet. The magnetic field strength is controlled by varying the distance between the magnet and the substrate. COMSOL Multiphysics software is used to report the magnetic field and magnetic force distribution. The uniform deposition of drying pattern is observed under the influence of magnetic field. The effect of magnetic field on evaporation dynamics is attributed to the modification in drying pattern. The evaporation of the droplet takes place in constant contact radius (CCR) mode, constant contact angle (CCA) mode, mixed mode, and late pinning mode. Both sliding of the droplet and receding of the contact line is observed during evaporation when the center of magnet is not aligned with the center of the droplet. The suppression in duration of the CCR and CCA modes of evaporation and increase in duration of mixed mode evaporation is observed at higher magnetic field strength. The initial contact angle decreases and receding contact angle increases with increasing strength of magnetic field. This is attributed to lower concentration of nanoparticle in the contact line region resulting in lower frictional resistance to the contact line motion. Hump formation takes place at the apex region of the droplet, which depends on the strength of the magnetic field and local concentration of nanoparticle. The hump formation is followed by splitting of the droplet.

Evaporation Momentum Force and Its Relevance to Boiling Heat Transfer

Abstract As a liquid evaporates into its vapor, the vapor phase leaves at a higher velocity than the approaching liquid and exerts a net momentum force on the evaporating interface. This force is especially relevant in the contact line region where liquid temperature is higher than the bulk liquid, and local saturation temperature is reduced due to curvature effects. These factors result in an increased evaporative flux resulting in higher evaporation momentum force that can influence the interface motion and bubble trajectory. This force provides a new mechanism for enhancing boiling heat transfer by altering the individual bubble trajectory. In microchannels, it can lead to flow instability. These effects are critically evaluated in this paper and their relevance to bubble growth and heat transfer phenomena during pool and flow boiling is presented. Two nondimensional groups K1 and K2, respectively, representing the ratio of evaporation momentum force to inertia and surface tension forces, have been used in modeling heat transfer and interface motion. Evaporation momentum force has been successfully applied in modeling critical heat flux (CHF) in pool and flow boiling, analyzing instability during flow boiling in microchannels, controlling individual bubble motion, and enhancing CHF and heat transfer coefficient (HTC) during boiling on flat surfaces as well as tubular geometries.

Lattice Boltzmann Modeling of Condensation Heat Transfer on Downward-Facing Surfaces with Different Wettabilities.

The model of vapor condensation heat transfer on downward-facing surfaces with different wettabilities is built by a two-dimensional (2D) lattice Boltzmann method. Dynamic evolution of condensate microdroplets on different wettability surfaces is simulated and the influence on heat transfer performance is analyzed. Moreover, the mechanism of a heterogeneous wettability surface enhancing condensation heat transfer is explored by investigating the condensate behaviors in the process of condensation. The numerical results indicate that as the contact angle of the homogeneous wettability surface increases, the initial nucleation time of the condensate is prolonged, while the departure time of the condensate is reduced significantly. The temperature adjacent to the gas-liquid interface, especially in the three-phase contact line region, is much higher than elsewhere due to the release of latent heat during condensation. Coalescence and detachment behaviors of condensate droplets cause the average heat flux to fluctuate locally with time. For the hybrid wettability surface, if the proportion of hydrophobic regions is small, the condensation heat transfer performance will be deteriorated. However, increasing the hydrophobic-hydrophilic ratio has a positive effect on enhancing heat transfer. It is found that a critical hydrophobic-hydrophilic ratio exists to optimize the heat transfer performance. For the gradient wettability surface, directional migration induced by capillary force facilitates the removal of condensate droplets, thereby enhancing the condensation heat transfer. Furthermore, a larger wetting gradient benefits to further improve the heat transfer performance. The results are valuable for optimally designing the heat transfer enhancement of vapor condensation on functionalized surfaces with heterogeneous wettability.

Evaporation Dynamics in Buried Nanochannels with Micropores.

Cross-connected buried nanochannels of height ∼728 nm, with micropores of ∼2 μm diameter present at each intersection, are used in this work to numerically and experimentally study droplet-coupled evaporation dynamics at room temperature. The uniformly structured channels/pores, along with their well-defined porosity, allow for computational fluid dynamics simulations and experiments to be performed on the same geometry of samples. A water droplet is placed on top of the sample causing water to wick into the nanochannels through the micropores. After advancing, the meniscus front stabilizes when evaporation flux is balanced with the wicking flux, and it recedes once the water droplet is completely wicked in. Evaporation flux at the meniscus interface of channels/pores is estimated over time, while the flux at the water droplet interface is found to be negligible. When the meniscus recedes in the channels, local contact line regions are found to form underneath the pores, thus rapidly enhancing evaporation flux as a power-law function of time. Temporal variation of wicking flux velocity and pressure gradient in the nanochannels is also independently computed, from which the viscous resistance variation is estimated and compared to the theoretical prediction.

Modeling of the finite boundary limit of evaporation flux in the contact line region using the surface plasmon resonance imaging

The present study aims to visualize a contact line region including evaporating thin film (ETF) and adsorbed film, and to suggest a new approach for estimating a finite evaporation flux based on a diffusion-limited model. The profiles that cover part of the ETF and the adsorbed film were obtained using the surface plasmon resonance imaging (SPRi) technique. The contact line region film profiles were obtained using an extrapolation method to find the inflection point, as the inward boundary condition. We successfully visualized the ETF at the sub-micron scale that exists near the droplet edge at which the fastest evaporation occurred. The width and thickness of the contact line region were determined. Also, the present study suggested a mathematical expression for the average value of evaporation flux over the area of the contact line region. The results showed that the average evaporation flux (AEF) decreases with an increase of droplet volume; this is because the local evaporation becomes greater as droplets become smaller. Moreover, this approach could be used to estimate the finite evaporation rate that occurs in the contact line region, as well as to impose a finite boundary limit instead of a singular value at the droplet edge.

Directional spreading of a viscous droplet on a conical fibre

If a droplet is placed on a substrate with a conical shape it spontaneously starts to spread in the direction of a growing fibre radius. We describe this capillary spreading dynamics by developing a lubrication approximation on a cone and by the perturbation method of matched asymptotic expansions. Our results show that the droplet appears to adopt a quasi-static shape and the predictions of the droplet shape and spreading velocity from the two mathematical models are in excellent agreement for a wide range of slip lengths, cone angles and equilibrium contact angles. At the contact line regions, a large pressure gradient is generated by the mismatch between the equilibrium contact angle and the apparent contact angle that maintains the viscous flow. It is the conical shape of the substrate that breaks the front/rear droplet symmetry in terms of the apparent contact angle, which is larger at the thicker part of the cone than that at its thinner part. Consequently, the droplet is predicted to move from the cone tip to its base, consistent with experimental observations.

A comprehensive comparison between substrate heating and light heating induced nanofluid droplet evaporations

A comprehensive comparison between substrate heating and light plasmonic heating induced nanofluid droplet evaporations is presented. The substrate temperature was kept as 36.5 °C for substrate heating, but a 1400 W/m2 irradiation flux emitted by a xenon lamp was used for light heating. Both two heating modes yield constant contact diameter evaporation. Because light irradiation reorganizes nanoparticles to generate dynamically varied heat source in nanoscale due to the plasmonic effect, the two heating modes are found to display three differences. First, droplet surface temperature decreases monotonically from contact line to apex for substrate heating, but for light irradiation, droplet surface temperature displays nonmonotonic variation including a contact line region CLR and a bulk volume region BVR. Temperature gradient is significant in CLR but does not exist in BVR. Second, the substrate heating generates both radial flow and Marangoni flow in the whole droplet. Thus, coffee-ring and dispersed nanoparticles are observed on substrate. On the other hand, for light heating, the Marangoni flow is only confined in CLR due to the local temperature gradient there, creating most of nanoparticles deposition near the contact line. Third, because the droplet surface area decreases versus time, the evaporation rate reduces to behave the non-linear variation of droplet volume for substrate heating. The situation is changed for light heating induced droplet evaporation. The enhanced plasmonic heating in CLR due to nanoparticles deposition there offsets the effect of droplet surface area decreasing. Thus, light heating would give rise to the constant droplet evaporation rate, which is distinct to substrate heating. This work enhances the fundamental understanding of the droplet evaporation dynamics under the condition of light heating induced plasmonic heating effect.

Interaction of advancing contact lines with defects on heated substrates.

We consider an advancing contact line traveling over a region of locally modified wetting or thermal substrate properties. A lubrication-type model is developed to account for coupling of viscous flow, evaporation, surface tension, and disjoining pressure. Stick-slip-type behavior is found for a range of conditions as the contact line passes over the defect and explained by a temporary increase in the local stresses disrupting the liquid supply into the contact line region. A simple estimate of the degree of contact line slowdown is obtained and compared with the numerical simulation results. Tangential stresses arising from the action of the electric field on the interfacial changes are accounted for in our model; neglecting them would lead to an overprediction of the time of interaction between the contact line and the defect. Increasing the substrate temperature uniformly has little effect on contact line motion, but local increase of the temperature enhances the tendency of the contact line to be pulled back by the defect, an effect explained by the Marangoni stresses.

Multiscale Characteristic in Symmetric/Asymmetric Solar-Driven Nanofluid Droplet Evaporation.

Driven by nanoparticle plasmonic heating, sessile droplet evaporation presents challenges on the coupling mechanisms between time-spatial heat source distribution and flow/temperature fields in a droplet. Here, symmetric/asymmetric solar-driven droplet evaporation is investigated. An infrared camera captures droplet surface temperatures in the micrometer scale after correction. An optical three-dimensional profiler quantifies nanoparticle deposition in the nanoscale. We show that droplet surface temperatures do display a nonmonotonic variation trend. Based on measurements, we are able to decouple the droplet into a contact line region (CLR) and a bulk volume region (BVR). The CLR volume is two to three magnitudes smaller than the droplet volume. The temperature gradient is significant in CLR, but flat temperature exists in BVR. Radial flow in BVR transports nanoparticles from the droplet body to the contact line, while Marangoni flow in CLR stabilizes nanoparticles there. Light energy is also decoupled based on its wavelength band. It is found that CLR dominates the visible energy absorption, but BVR has a weak contribution. Top light heating causes symmetry temperatures and a coffee-ring profile along the circumference. However, side heating yields higher temperatures and more nanoparticles deposition on the sunny side than on the night side. The above findings are valid when the initial droplet volume and incident irradiation flux are changed on the hydrophilic wall. The hydrophilic wall and hydrophobic wall maintain the evaporation modes of constant contact diameter and "stick-slip", respectively. The present paper enhances the understanding of light-induced droplet evaporation from the multiscale point of view.

Electrically Controlled Localized Charge Trapping at Amorphous Fluoropolymer-Electrolyte Interfaces.

Charge trapping is a long-standing problem in electrowetting on dielectric, causing reliability reduction and restricting its practical applications. Although this phenomenon is investigated macroscopically, the microscopic investigations are still lacking. In this work, the trapped charges are proven to be localized at the three-phase contact line (TPCL) region by using three detecting methods-local contact angle measurements, electrowetting (EW) probe, and Kelvin probe force microscopy. Moreover, it is demonstrated that this EW-assisted charge injection (EWCI) process can be utilized as a simple and low-cost method to deposit charges on fluoropolymer surfaces. Charge densities near the TPCL up to 0.46 mC m-2 and line widths of the deposited charge ranging from 20 to 300 µm are achieved by the proposed EWCI method. Particularly, negative charge densities do not degrade even after a "harsh" testing with a water droplet on top of the sample surfaces for 12 h, as well as after being treated by water vapor for 3 h. These findings provide an approach for applications which desire stable and controllable surface charges.

Heat flux distribution beneath evaporating hydrophilic and superhydrophobic droplets

An experimental investigation of the heat and mass transfer to an evaporating hydrophilic water droplet using thin-foil thermography and droplet shape analysis is reported. These results have been compared with that of a superhydrophobic evaporating droplet. The hydrophilic droplet initially evaporated with a pinned contact line before unpinning and evaporating with a receding contact line. The largest heat flux is observed at the contact line region for both droplets. The hydrophilic droplet evaporated 34% faster than its superhydrophobic counterpart due to its greater contact line length, liquid-gas interface temperature and solid-liquid surface area for the majority of its evaporation. In general, the hydrophilic droplet dissipated a greater total power due to its larger contact line length and solid-liquid surface area, while the superhydrophobic droplet had a greater average heat flux due to its larger contact line length density for the majority of its evaporation time. The average heat flux to the evaporating droplets was demonstrated to vary as a linear function of the contact line length density.

Numerical Investigation of Shape Effect on Microdroplet Evaporation

Abstract As electronic devices continue to shrink in size and increase in functionality, effective thermal management has become a critical bottleneck that hinders continued advancement. Two-phase cooling technologies are of growing interest for electronics cooling due to their high heat removal capacity and small thermal resistance (<0.1 k cm2/W). One typical example of a two-phase cooling method is droplet evaporation, which can provide a high heat transfer coefficient with low superheat. While droplet evaporation has been studied extensively and used in many practical cooling applications (e.g.,, spray cooling), the relevant work has been confined to spherical droplets with axisymmetric geometries. A rationally designed evaporation platform that yields asymmetric meniscus droplets can potentially achieve larger meniscus curvatures, which gives rise to higher vapor concentration gradients along the contact line region, and therefore, yields higher evaporation rates. In this study, we develop a numerical model to investigate the evaporation behavior of asymmetrical microdroplets suspended on a porous micropillar structure. The equilibrium profiles and mass transport characteristics of droplets with circular, triangular, and square contact shapes are explored using the volume of fluid (VOF) method. The evaporative mass transport at the liquid–vapor interface is modeled using a simplified Schrage model. The results show highly nonuniform mass transport characteristics for asymmetrical microdroplets, where a higher local evaporation rate is observed near the locations where the meniscus has high curvature. This phenomenon is attributed to a higher local vapor concentration gradient that drives faster vapor diffusion at more curved regions, similar to a lightning rod exhibiting a strong electric field along a highly curved surface. By using contact line confinement to artificially tune the droplet into a more curved geometry, we find that the total evaporation rate from a triangular-based droplet is enhanced by 13% compared to a spherical droplet with the same perimeter and liquid–vapor interfacial area. Such a finding can guide the design and optimization of geometric features to improve evaporation in advanced microcooling devices.

An experimental investigation into the spread and heat transfer dynamics of a train of two concentric impinging droplets over a heated surface

Extensive studies of two concentric droplets consecutively impinging over a thin heated foil surface are carried out to compare the spread and heat transfer dynamics of a single drop, and drop-on-drop configurations using high speed imaging and infrared thermography. Millimeter-sized deionized water droplets (2.80 ± 0.04 mm) are impinged upon a heated Inconel surface (thickness of 25 μm) from a fixed height corresponding to a Weber number (We) of 50 ± 2 and Reynolds number (Re) of 3180 ± 90 with a flow rate of 20 droplets per minute. Surface temperature is chosen as a parameter, and is varied from 22 °C (non-heated) to 175 °C. Temperature and heat flux distributions associated with droplet-surface interactions are obtained, and the outcomes of the process are measured in terms of spread diameter, droplet input heat transfer, dynamic contact angle, and surface mean temperature. A decline in the droplet heat transfer for drop-on-drop impingement is observed for all temperatures investigated in the present work. This is attributed to the surface pre-cooling by the initial droplet and also to the reduced surface area-to-volume ratio i.e., increased spreading film thickness. High heat transfer rates are observed around the three-phase contact line region, especially during the receding phase of the droplet, for both configurations, confirming the significance of contact line evaporation in droplet-hot wall interactions. Theoretical models predicting the maximum spread factor and corresponding input heat transfer into the droplet are identified from the literature, and found to be in good agreement with present experimental observations.

Role of Surfactant in Evaporation and Deposition of Bisolvent Biopolymer Droplets.

Inkjet printing of biopolymer droplets is gaining popularity because of its potential applications in regenerative medicine, particularly the fabrication of tissue-regenerative scaffolds. The quality of bioprinting, which affects cellular behaviors and the subsequent tissue formation, is determined by the solvent evaporation and deposition processes of biopolymer droplets, during which instantaneous local viscosity and surface tension changes occur because of the redistribution of the biopolymer inside the drop. Such dynamics is complex and not well understood. Most biopolymer inks also contain multiple solvents of distinct evaporation rates, further complicating the system dynamics. Using high-speed interferometry, we directly observe in real time the instantaneous drop shape of inkjet-printed picoliter gelatin drops containing glycerol and water. It is observed that, for bisolvent gelatin drops with surfactants, highly viscous gelatin and glycerol accumulated near the pinned contact line at an early stage suppress the evaporation-driven outward flow and create a stagnation zone near the contact line region. Lower surface tension at the contact line, because of its high local surfactant concentration, as compared to the drop apex induces a strong Marangoni recirculation, which in conjunction with a stagnation zone in the contact line region causes the instantaneous drop shape to transition from a spherical cap to a volcano shape during evaporation and resulting in a volcano-like deposition profile. In contrast, the suppressed evaporation outward flow together with a weak Marangoni flow leads to a domelike deposition for the case without surfactant. The role of surfactant in polymer drop deposition with water-only solvent is also investigated and compared against that of bisolvent drops. For the single-solvent case, the deposition profile is found to shift from a coffee-eye shape in the presence of surfactant to a uniform deposition without surfactant. The results reveal new insight into the complex role surfactant plays during polymer drop evaporation and deposition processes.

Experimental investigation of evaporation from asymmetric microdroplets confined on heated micropillar structures

Droplet evaporation is ubiquitous both in everyday life and numerous engineering, biomedical, and environmental applications. For example, the large amount of latent heat associated with the liquid-vapor phase change process makes droplet evaporation an ideal solution for cooling high-powered electronic devices. Over the past 100 years, many new theories and models of droplet evaporation have been proposed. Still much of the fundamental transport physics remains elusive, which makes it challenging to describe diverse evaporation behavior by a universal mechanism. In particular, evaporation from asymmetric droplets remains grossly unexplored due to their rarity in nature. However, recent advances in micro- and nanoengineering technology have made it possible to tune and maintain droplets in non-spherical geometries, where the interfacial transport rate becomes highly anisotropic along the circumferential direction. Such a characteristic represents a distinctive feature from evaporation of capped-spherical droplets and has not been comprehensively explained by existing theories. This paper exams the evaporation rate and heat transfer performance of continuously–fed water microdroplets confined on heated silicon micropillar structures with circular, square, and triangular cross sections. The evaporation experiments are performed at substrate temperatures ranging from 60 °C to 98 °C. The droplets exhibit a capped spherical shape on the circular micropillar but an asymmetric geometry on non-circular micropillars. Our experimental results show that increasing levels of droplet asymmetry significantly enhance evaporative transport. Specifically, for a 60 °C substrate, a triangular micropillar has 45% greater heat transfer coefficient than a circular one; at 98 °C, the increase reaches 71%. This enhancement is validated by multiphase numerical simulations which show agreement with experimental values. The enhanced heat transfer coefficient from asymmetric droplets is attributed to a reduced conduction resistance inside the droplet and smaller diffusion resistance at the droplet interface, both of which result from the non-uniform geometric features of the droplet along the circumferential direction. Finally, a thermal resistance network is developed to demonstrate the variation in the local droplet thickness and interfacial curvature along the circumferential direction for droplets confined on circular, square, and triangular micropillars. Asymmetric droplets yield smaller thicknesses near regions with high curvature, which results in a smaller conduction resistance and higher fraction of heat transport near the contact line region compared to a capped spherical droplet. Furthermore, with increasing droplet temperature, the thermal resistance associated with vapor diffusion is reduced significantly, leading to greater enhancement of evaporative heat transport. These findings not only provide insights in the evaporation behavior of asymmetric droplets, but also provide important guidance for relevant applications, such as the design of evaporative cooling systems.

Investigation of the confinement effect on the evaporation behavior of a droplet pinned on a micropillar structure

Evaporation of sessile droplet suffers from reduced evaporation rate due to the confinement of vapor diffusion imposed by the bottom substrate. However, it is possible to change the evaporation behavior of a droplet by suspending it from the bottom substrate, in particular, supporting the droplet on a micropillar. This is expected to enable diffusion transport in the downward direction that will subsequently enhance evaporative transport. In this study, we investigate the diffusion confinement effect imposed by the bottom substrate and the side wall of the micropillar through numerical simulations and experimental investigation. The approximate solutions for total evaporation rate and local evaporative flux were subsequently derived from the total evaporation rate predicted by the simulation results. The simulation results, agreeing within 5% with the experimental measurements, show that increasing the micropillar height enhances the total evaporation rate from the suspended hemispherical droplet. This enhancement is due to a dramatic improvement of the local evaporation rate near the contact line region as micropillar heights increase. The micropillar heights examined for maximum evaporation rates were observed under substrate temperatures from 60–98 °C. The increasing pillar height leads to smaller vapor diffusion resistance but greater conduction resistance.

Drying pattern and evaporation dynamics of sessile ferrofluid droplet on a PDMS substrate

The present study reports the evaporation dynamics, deposition pattern, effect of magnetic field and mechanism behind the pinning or depinning of the contact line of sessile ferrofluid droplet on a PDMS substrate. The concentration of ferrofluid has been varied as 0.0%, 0.1%, 1.0%, and 5.0%. Optical profilometry, confocal microscopy, micro-PIV and goniometry measurements have been carried out. The average evaporation rate increases with increase in ferrofluid concentration i.e. about twice increase in evaporation rate is observed at 5% ferrofluid concentration compared to 0.0% concentration. Crack formation takes place inside the coffee ring of the drying pattern. The crack size i.e. length and width increase with increase in ferrofluid concentration, which can be correlated to the drainage of solvent by the capillary action and formation of cluster. The contact line motion is a function of ferrofluid concentration, which influences the diameter of the coffee ring. The width of the coffee ring, the contact line velocity of the droplet during the receding phase and total drying period decrease with increase in concentration of ferrofluid. The contact line stops receding if the frictional force exceeds the horizontal component of surface tension force at a critical particle density in the contact line region. The application of magnetic field leads to uniform deposition pattern of the drying droplet, which can be correlated to higher duration of mixed mode evaporation of the ferrofluid droplet.