Numerical simulation

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Appendix 1.(b). Droplets lying along the x axis

4.5 Numerical simulation

We are limited to produce clusters made of three equal-sized droplets N = 3. Using triplet clusters allows us to study the clustering process without provoking too much complexity. Figure 4.29 shows the experimental observations for the system h1 = 10µm, h2 = 163µm. We set the dispersed and continuous phase pressures fixed and increase the dilution phase pressure. Under low dilution pressure, the three droplets adopt the triangle-like close packing structure (at Pdilution= 20mbar). As the pressure increases, the droplets configuration evolves to linear chain-like shape (at Pdilution= 160mbar) and eventually separate at higher pressure (atPdilution= 180mbar).

Figure 4.29: Cluster structures transition. PDMS system, W=50µm, h1=10µm, h2=163µm. (a)Pdisperse=38mbar, Pcontinuous=11mbar, Pdilution= 20mbar.

(b)Pdisperse=38mbar, Pcontinuous=11mbar, Pdilution= 160mbar. (c)Pdisperse=38mbar, Pcontinuous=11mbar,Pdilution= 180mbar. Scale bar is 100µm.

A more quantified work is carried out for a small system (W=20µm, h1 = 1µm, h2 = 22µm), the size of the droplet decreases by one order of magnitude with R around 2.5µm. Throughout the experiments, we span a range of flow rates and the typical flow velocity ranges from 70 µm/s to 2000 µm/s in the reservoir channel which allows us to estimate the Reynolds number and the Capillary number in our device (the width of the reservoir channel is 600µm). The corresponding Reynolds number Re =ρUL/µvaries between 3×10−6and 9×10−5 and the capillary number Ca =µU/νrange between 4×10−6 and 10−4. The transition of morphologies is observed (see figure 4.30).

We conclude that the close packing structures are favored under high adhesion and low flow field conditions. In order to better understand the flow field influence on the clustering process, we conduct a numerical simulation in the absence of droplets to investigate the flow field in the reservoir channel under different experimental conditions.

4.5 Numerical simulation

In collaboration with Florent Malloggi (CEA Saclay), we carry out this numerical simulation study. Three-dimensional flow simulations are used to compute the streamslines and flow conditions in the microfluidic reservoir in the absence of clusters. The simulations are set in COMSOL Multiphysics which is a commercial software widely used in industries as well as in research laboratories. Its graphical user interface (GUI) and a set of predefined user interfaces with associated modeling tools provide an easy preparation of the simulation and offers clarity in the choice of the governing flow equation without requirement to write them in variational form. We use the 3D Stokes flow module as the experimental Reynolds number throughout our experiments remains below 10−2. The flow geometry mainly consists a rectangular cross-section with three entries: one in the middle for which the flow rate contribution comes from inlet 2 (continuous phase to produce droplets at

Figure 4.30: Clusters morphologies transition. We present here the cluster aggregate velocity as a function of the maximum flow velocity (poiseuille flow in z direction) in the reservoir channel. Triangle structures (black) are obtained under low Umax, chain structures (blue) at highUmax and in between is the oscillatory states (red).

T-junction) and two entries symmetrically located at both sides which represent the inlet 3 (dilution phase).

We simulate the system with the smallest dimensions. The height of the channel is constant and its value ish2=22µm except in the mid inlet channel where the height ish1 1µm. Figure 4.31 shows a perspective view of the design along with the meshed channel.

The domain is meshed with a density of 4 nodes on the channel height edges, 80 on the width and 200 on the length edges (tetrahedral elements).

(a) (b)

Figure 4.31: Design of the system for the 3D-simulation of the flow.(a) geometrical design for the fluid domain. (b) the mesh obtained with Comsol.

For convenience description we define two different regions in our device (see fig-ure 4.32). Region I is the reservoir area near the step and close to the entries where we expected to have an elongation flow. Region II represents the area where the entry ef-fect can be neglected, a stationary flow is expected, a plug flow in xy plan and a poiseuille flow in z. The fluid flows from left to right. It flows through 3 inlets (Wmid=20µm, h1=1µm andWside=80µm, h2=22µm) and enters a larger reservoir channel (W=600µm, h2=22µm).

We simulate the flow with the following parameters:

4.5. Numerical simulation 101

Figure 4.32: Two different regions of the fluid domain.

Flow condition Q middle(nl/min) Q side(nl/min) Experimental clustering observation

NO 1 50 35 close packing

NO 2 50 240 oscillation

NO 3 50 775 chain-like packing

Table 4.2: three flow conditions for simulation

Inlet: We simulate the flow by imposing the flow ratesQmiddle andQside. To compare with the experimental results, we set Qmiddle =50nl/min, varying the Qside progressively from 0 nl/min to 775 nl/min.

Outlet: Outlet is connected to a channel of 1mm length. This channel is left open to ambient pressurePoutlet = 0 mbar.

We consider the fluid as Newtonian (constant viscosity η=10−3P a.s−1) and the flow in a stationary state (no temporal dependence).

We should first recall the experimental results, three behaviors of the clustering are observed according to the flow conditions: when the flow rate of dilution phase is low compared with the one of the middle phase, the clusters adopt close packing morphology;

when the flow rate of dilution phase is high, the clusters form a chain-like structure and in between the two, we observe oscillations of the two configurations. The corresponding flow conditions are shown in the table 4.2. We present numerical simulations of the corresponding three cases to provide comparison with the experimental data and explore the dependence of the flow conditions and morphology of the clusters.

We present in the figure 4.33 the maps of velocity field as well as the streamlines in the middle height of the channel (h=11µm) for the three flow conditions indicated in the table 4.2. It is a projection along the xy plane.

As expected, the velocity decreases when entering in the main chamber. Indeed for a given flow rate the mass conservation givesQ0S0 =Q1S1 with Q the flow rate and S the channel cross section. From the entries to the reservoir, the cross section increases thus Q decreases due to mass conservation. The reservoir channel is a Hele-Shaw cell (with aspect ratio W/h=27), we obtain a creeping flow quickly after a small entry region as expected for all cases.

We use the following notation for the velocity vector:

v=

In the region I, the flow is complex u6=0, v6=0 and w6=0. However we proceed at the following simplification. From the streamlines of figure 4.34 (a) for the flow condition

Figure 4.33: The velocity field (a) and the streamlines (b) for flow condition NO 1; (c), (d) for flow condition NO 2 and (e), (f) for flow condition NO 3.

NO 3, we clearly see that the z-component of the velocity field vanishes after few tens of microns ( 30µm), i.e. w is almost 0 after this distance. The similar range is confirmed for other flow conditions by the simulation. This result is consistent with the measurements of droplet z position with Adaptive Focus Control and with Microscope Confocal Imaging.

Figure 4.34: (a) Zoom of streamlines near the middle inlet. (b) Transversal cut at different positions: x=L=10µm - 25µm -50µm -100µm -200µm -300µm- 500µm.

Moreover since the main contribution to the velocity field comes from side channels this approximation seems correct. Hence the velocity profile can write:

4.5. Numerical simulation 103

v=

u(x, y, z) v(x, y, z)

0

We made several transversal cut velocity profiles at different positions(see figure4.34 (b) for the cut and figure4.35 for the velocities): x=L=10µm - 25µm -50µm -100µm -200µm -300µm- 500µm and for z=h/2=11µm (x=0 is for the beginning of region I).

Figure 4.35: Transversal cuts of the velocity field (z=h/2=11µm). Velocity vector U (a) (x componet) and V(b) (y componet) for flow condition NO 1, (c) and (d) for flow condition NO 2 and (e) and (f) for flow condition NO 3.

After L=300µm the flow adopts a plug like profile, i.e. v is almost 0 and u is almost uniform, as shown in figure 4.35. In the region II, the velocity profile writes:

v=

u(x, y, z) 0 0

Since the clustering formations take place in the mid-plan of the reservoir channel in the region II, where the flow field is plug flow in xy and poiseuille flow in z, the shear stress can be neglected. We suggest that the dipolar interation is responsible for the dynamical evolution of the droplets towards cluster. This study will be discussed in the preprint paper in chapter 5.

4.6 Conclusion

Through the use of dedicated two-layer microfluidic devices and by coupling T-junction and step emulsification droplet production, we demonstrated the direct assembling of droplets into clusters (of adhesive formulation) within this device. We discussed two regimes: the cluster production and the cluster transport. Various morphologies of clusters were observed and the corresponding experimental conditions were well precised. This mechanism produced high yield monodisperse clusters in a robust way. We proposed some explanations for the droplets generation mechanism and the cluster transport behavior in the microfluidic reservoirs, with support from numerical and modeling work. We mainly focused ourselves on the flow field effect in this chapter. A further investigation on the clustering process by scanning other parameters such as the surfactants, electrolytes will be provided in the next chapter. A theoretical analysis on dipolar interaction will be provided in the preprint in chapter 5.

Chapter 5

Clustering based on

hydrodynamics self-assembly:

towards more complex clusters

5.1 Introduction

In the previous chapter, we discussed simple cluster formation whereas in this chapter, we further carry out a serie of experiments with different formulations to investigate the influence of physical chemistry to the hydrodynamics assembly process. The goal is to use the broaden fluids is attempting to produce more complex clusters. We start with adhesion study, trying to understand the adhesion origin between the droplet interfaces in order to better reinforce their interaction. This study will give us an insight of how resistant those clusters are when they are submitted to hydrodynamic forces as well as the morphology preferences they adopt under different conditions (2D and 3D structures). Then we focus on the hybrid cluster and magnetic clusters synthesis which is potentially interesting to develop directional interactions between the building blocs for complex assembly. We also investigate the solidification of the clusters which is important for material applications. A proof of concept and some perspectives are provided in this chapter for the future studies.

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