In this study, the internal fluid flow and deposition pattern of the evaporating droplet of polystyrene particles placed on substrate was investigated through experiment using Particle image Velocimetry (PIV) technique. Due to the complex nature of biofluids, a comprehensive understanding of the underlying mechanism of patterning is lacking.
2.21 (a) Particle concentration profile inside an evaporating droplet for an example of ambient conditions, (b) Snapshot of a particle settling pattern. 2.22 (a) Particle concentration profile inside the evaporating droplet for the case of symmetric heating of the substrate, (b) snapshot of the particle deposition pattern.
List of table
Nomenclature
Introduction and Literature Survey
Introduction
Shape of the microdroplet
The shape of the microdroplet is governed by the interfacial surface tension while the shape of the large liquid droplet results from a balance between the two forces. The shape of a drop of liquid by placing the drop on a horizontal surface: a) small droplet and (b) large droplet.
Interfacial forces near the contact line
The contact angle between a liquid droplet and a solid surface can change with surface roughness. Suppose there is a small displacement dx of the contact line of a drop placed on a rough surface as shown in Figure 1.3.
Evaporation of droplet
Evaporation model
Evaporation rate
Heat flux through the interface of the droplet
Internal fluid flow
If the fluid is heated sufficiently large enough (higher temperature gradient across the layer), then only the system becomes unstable and convective motion is induced by gravity from the low-density region to the high-density region as shown in Figure 1.7a. These forces drive currents from the region of lower surface tension to the region of higher surface tension as shown in Figure 1.7b.
Dimensionless numbers
Grahshoff number
Marangoni number
Bond number
Coffee ring stain
Review of the literature
This confirms the importance of physiochemical properties of biological fluids in addition to the mechanisms of particle transport in pattern formation. The proposed work is to understand the internal fluid flow pattern, particle transport and subsequent particle deposition pattern of an evaporator.
General pattern formation and dynamics of the evaporating droplet
They found that the convection and diffusion mass transfer of the particle in the bulk fluid affects the deposition rate on the substrate. Their experimental results indicate that the particle deposition in octane droplet may be localized mainly in the center instead of at the edge of the contact line.
Drying pattern
A liquid droplet placed on an imperfectly homogeneous surface, the dynamics of the contact line showed "stick-slip" behavior. It was shown that the contact line carried the frictional force as the particles were transported from the center to the edge of the droplet.
Factors affecting dried pattern
The previous theoretical models that assumed the potential flow in an evaporating droplet predicted velocity vectors moving from the center of the droplet, parallel to the planar substrate, to the edge, as shown in Figure 1.14a. Deegan et al. (1997) argued that the evaporation rate near the edge of the drop is much higher than that at the center, leading to greater solvent loss at the edge compared to the center.
Surface temperature distribution
RN can be defined as the ratio between the thermal resistance of the substrate and the thermal resistance of the liquid droplet. The temperature distribution relative to the thermal conductivity and contact angle of the droplet.
Flow visualization of evaporating droplet
Outline of the thesis
In Chapter 2 we investigate the internal fluid flow and particle transport in an evaporating droplet under different conditions of external heating of the substrate. The contact line of the droplet subjected to external heating at the droplet interface was found to be loose during the process. Most studies reported that the surface temperature gradient reverses at the critical angle of the droplet.
Microdroplet evaporation on heated substrate
Introduction
- PIV principle
- Image analysis for velocity field
- Evaluation of particle concentration fields
- Experimental Set up
This is due to the small size of the droplet and the density of the particle used in our experimental studies, which was almost the same as the liquid density. The principle of cross-correlation analysis is to shift the second query window by an amount approximately equal to the local displacement of the particles. This code is used to find the location of the particles and the total number of particles inside the droplet.
Numerical simulation
In a confined disc-shaped drop attached to a substrate, the only area through which evaporation occurs is a thin circular strip in contact with air. In the above equations, v is velocity, P is pressure, ρ is fluid density, β is coefficient of thermal expansion of water, µ is kinematic viscosity, T is. Therefore, the temperature change is almost linear on the droplet-free surface and the underlying substrate.
Results and discussion
The differences in the results of the two networks turned out to be negligible, confirming the network-independent solution. The results obtained from simulations were analyzed to understand the velocity field and circulation pattern.
Analysis of height and contact line during evaporation
Time sequence of the images of the droplet (a) Ambient condition (b) Symmetric heating of the substrate (c) Asymmetric heating of the substrate.
Velocity field
The temperature profile on the left half of the droplet free surface was assigned as a linearly varying profile between the left edge and the tip temperature. Similarly, the right half of the free surface was assigned a linearly varying temperature profile between the temperature of the top and the right edge. Therefore, an asymmetric temperature gradient is created on the free surface of the droplet, the direction of which is exactly opposite to that in the previous case.
Strength of Marangoni and buoyancy convections
Numerical simulations were also performed to verify that the strength of the Marangoni convection is dominant within the evaporating droplet. These results show that the strength of the Marangoni convection is much stronger compared to the buoyancy flow. Velocity field inside the droplet for the case of ambient condition recorded at (a) 60 s and (b) 180 s respectively.
Particle concentration inside the evaporating droplet
The particle concentration (Ni/NT) indicates the number of particles dispersed in a bowl per unit area at any time (Ni) divided by the total number of particles per unit area initially present in the droplet (NT). The evolution of the concentration profile indicates that a ring-like spot will be formed after complete evaporation of the droplet. The image of the particle deposition pattern of the 3D drop placed on the heated surface is shown in Figure 2.22b.
Effect of particle concentration
Velocity vectors superimposed over the velocity field contour colored by its magnitude during the evaporation of a droplet containing 5% polystyrene particles, recorded at (a) 90 s (b) 200 s, respectively. Velocity vectors superimposed over the velocity field contour colored by its magnitude during the evaporation of a droplet containing 8% polystyrene particles, recorded at 110 s (b) 190 s, respectively.
Conclusion
Microdroplet evaporation with surface heating
In order to investigate in detail the effect of external heating of a liquid droplet, an investigation was carried out on a heated droplet on the upper free surface using a wire heating element. This heating element was inserted on top of the droplet as shown in Figure 3.1a. Schematic diagram of localized heating of a droplet. a) Localized heating at the top of the droplet, (b) localized heating on the left side of the droplet.
Results and discussion Analysis of contact line
The depth of the tip of the element at the apex of the point was ~0.4 mm. Initially, the tip of the heating element was found to adhere to the surface without spots. When the droplet surface is heated to the left, the liquid/vapor interfacial force ie.
Velocity profile
Schematic diagram shows the mechanism of action of local heating of the droplet-free surface (a) Point heating at the tip of the droplet; (b) Point heating on the left side of the drop and (c) The interfacial forces on the contact line of the drop. The velocity fields from the experiments are shown in the left column of the figure. The initial height of the drop was 2.8 mm. In this case, the droplet temperature at the left edge is higher than at the top and right edges.
Particle concentration profile and pattern formation
To compensate for the loss of mass due to evaporation, the contact line of the droplet with the lower substrate recedes over time. In Figure 3.6b we have shown the evolution of particle concentration profile for the case of left heating of the droplet. On the other hand, concentration dynamics were found to be strongly influenced by the contact line of the droplet.
Flow reversal in evaporating droplets on heated substrates
Introduction
Experimental procedure
These images were used to analyze the contact angle, velocity field and concentration of particles in the evaporating droplet. a) Schematic diagram of experimental setup.
Results and Discussion Temperature profile
The temperature therefore decreases monotonically from the top to the edge of the drop at time 170 s. It was observed that the temperature at the edge was higher than the top temperature of the droplet. As a result, surface tension will drive the liquid from the edge to the apex along the surface of the droplet.
Concentration profile and particle deposition pattern
Conclusion
Four distinct flow patterns were revealed at different stages of the process within the droplet. The flow pattern is significantly affected by the surface temperature distribution of the evaporating droplet. Two symmetric counter-rotating flow patterns observed in the initial stages are driven by the monotonic temperature gradient increasing from the apex to the edge of the droplet.
Evaporation of bacterial suspension drops
Preparation of biological tracers
Experimental procedure
Results and discussion Velocity profile
The contact line of the droplet was observed to be attached to the bottom substrate during the evaporation process. In the next step, we performed experiments in the presence of bacterial chemoattractant in the form of sugar crystal placed on the center of the substrate. When the sugar crystal was placed on the bottom of the substrate, we noticed that the crystal was completely dissolved in the medium by 1000 s.
Concentration profile
It can be noticed that during the early time, the concentration of the bacteria in all the containers is almost the same. The concentration of the live bacteria within the containers (1, 2, 9 and 10) which are near the edges increases while the concentration of the central trays (3-8) decreases over time. We observed that the concentration in bins on the left (1-6) decreases while the concentration within the bins on the right (7- 10) increases with time.
Pattern formation
Figures 5.12c and 5.12d show the deposition pattern when a sugar crystal was placed on the center of the substrate. On the contrary, the sugar crystal does not affect the dynamics inside the droplet-containing suspension of dead bacteria. However, a ring-like deposit (similar to Figure 8a) was observed when the drop of suspension of dead bacteria was dried in the presence of sugar crystal located on the right as shown in Figure 5.12f.
Evaporating droplet on curved surface
The material and thickness of the substrate were also the same, except that the geometry of the substrate is concave. As soon as a liquid droplet was placed on the surface, images of the droplet were captured by a CCD camera. These images were used to determine the contact angle, internal fluid motion, and average particle density of the evaporating droplet. a) Schematic diagram of the curved geometry of the substrate.
Results and Discussion
- Contact angle and surface temperature distribution
- Velocity field
- Concentration profile
- Particle deposition pattern
The contact line of the droplet was found to be unfastened as the evaporation progressed over time. As mentioned above, the droplet contact line recedes as evaporation progresses over time. It was found that the direction of the internal liquid flow inside the evaporator was reversed when the droplet remained on the heated substrate.
Suppressing the coffee stain effect: how to control colloidal self-assembly in evaporating droplets using the electric field. Coffee ring as a low-resource diagnostic: detection of histidine-rich plasmodium falciparum protein-II malaria biomarker using Ni(II)NTA gold-coated polystyrene particle paired ring. Analysis of Marangoni stress effects on microflow in a sessile evaporating droplet.
Research output
