A microfluidic study of nucleation and crystal growth

This is a short review article based on a research paper titled “Microfluidic control of nucleation and growth of CaCO3” authored by Lei Li , Jesus Rodriuez Sanchez, Felix Kohler, Anja Royne, and Dag Kristian Dysthe from the Physics Department of Oslo University. The experiment demonstrates a novel method of studying the nucleation and precise growth rate of Carbonate crystals using pressure-driven microfluidic flow control equipment, and how it is achieved. Throughout the experiment, the researchers have developed and rigorously tested the procedure for crystal growth.

Abstract

A new procedure for the study of nucleation and growth of CaCO3 crystals has been developed and tested extensively. The original research demonstrated that precise flow control plays an integral role in this process, and how it can be achieved. One of the advantages of this particular process is that thermodynamically unstable single crystals of polymorphs can be studied accurately to obtain precise growth rates. This research paper also illustrates at low supersaturations, in the absence of 2D nucleation, the growth rate of Calcite crystals is observed to be 5 times higher than figures observed in batch methods, and almost 2 times larger than the measurements taken with a AFM (Atomic Force Microscope). Taking into consideration the huge interest in calcite growth for environmental science, geoscience and industry, it is important to study the reasons behind this inconsistency in growth rate constants for different methods. The procedure demonstrated in the original research paper can easily be applied to many other minerals.

Introduction

The Earth’s crust is approximately made up of 4% Calcium Carbonate. CaCO3 also acts as one of the major long term sinks for the global carbon dioxide cycle.^[1] A major proportion of the earth’s oil reservoirs are also stored in carbonate reservoirs ^[2], making it one of the most studied biomineral. ^[3] CaCO3 has various industrial applications, with it being used in paper, plastics, paints, coatings, personal health, food production, building materials and construction. The study of biomineralization, geological, industrial and climatic processes involving calcium carbonate mostly depends on the detailed knowledge of Calcite crystal growth rates ^[4,5], through fundamental research on the subject. ^[6]

Most existing studies dealing with the nucleation and crystal growth of CaCO3 include the segmented flow-mixing of reagents, in which case the nucleation takes place within droplets. Very few studies explore the path of continuous -flow conditions. The unconventional nature of this research paper is the use of a pressure-driven flow controlled microfluidics to prompt nucleation and crystal growth of CaCO3 from the surrounding solution under a closed environment.

Aim and objectives

The primary objective of this study is to nucleate calcium carbonate crystals in a limited area to achieve the following results:

• High resolution imaging access
• Controlling which polymorph to keep in the system for study
• Keeping the saturation conditions in check at the growing crystal surface.
• Allowing slow growth of crystals from the nuclei
• Measurement of growth rates
• To avoid the blockage of our microfluidic device due to crystal growth elsewhere in the system.

Materials & Methods

Two types of microfluidic devices were assembled for this experiment.

In Fig.1 the crystal nucleation and growth is observed using a channel dimension of 120±2 µm wide and 45 µm high, the distance from the first to second junction is lc=50 mm and the length between the second junction to the outlet is 10 mm.

Fig.2 shows  the pattern used for flow stability experiments where the channel dimensions have a width of 70±2 µm, 45 µm height, and the distance between the junction and the outlet is set at 20mm.

For the purpose of nucleation and crystal growth of carbonate crystals – pure water, CaCl2 solution and Na2CO3 solution are introduced through separate inlets. They are then allowed to meet at the junction which is then followed by the mixing of the solutions by diffusion. Nucleation of a crystal was performed in two stages – Deionized water was introduced into the channel from inlet 2. Following this, 2mM CaCl2 and Na2CO3 solutions were introduced from inlet 1 and 3 to achieve a CaCO3 concentration of c=0.8mM. This concentration avoids any nucleations. After a stable flow has been attained, 10mM CaCl2 and Na2CO3 solutions were injected. Once nucleation is observed, and crystals start sticking to the PDMS surface the flow from inlet 4 and 5 are stopped.

Flow rate control is a crucial factor towards achieving the stable concentration necessary for crystal growth. The gas pressure control system includes an Elveflow OB1 MK3 controller which has both flow rate and pressure control modes, a Elveflow MUX and Elveflow Flow sensors. The Elveflow OB1 is responsible for the input pressure, Pi. The three inlet flow resistances are controlled by the Elveflow OB1 in flow rate mode, to keep all flow rates constant. The syringe pump system comprises a syringe pump; a BD plastic syringe, a Hamilton glass syringe, Elveflow MUX flow valve and Elveflow flow sensors.

Key findings

The stability of the CaCO3 concentration, c is found to have a directly proportional relationship with the volume fraction in the flow. Therefore, it is key to the precise measurements of crystal growth rates. In this experiment, the flow stability tests were performed for 5 hours with 2 water input channels at 1 µl/min and 1 input channel with dyed water at 2 µl/min in four different configurations – gas pressure controlled fluid flow with a) Pressure control b) flow control and syringe pumps with c) plastic syringes and d) glass syringes.

From figure 3, a constant deviation from the expected flow fraction value of 0.5 can be observed that when a syringe pump is used for flow control. For these reasons, that have been discussed in the research paper, all crystal growth experiments have been performed using the Flow Control mode (FC).

Nucleation occurs fast, when the saturation index, ∑ = 1.9, and some nuclei attach to the PDMS or the glass surface. The calcite and vaterite nuclei are shown in Figure 4, in the middle of the channel. A vaterite crystal nucleus chosen for growth is shown in 6C and Figure 6D shows the same crystal after 23 hours of growth.

In terms of studying the growth rates of Calcite crystals, many experiments were conducted with the reported setup. Figure 5 showcases the results of one of these experiments where solution concentrations were rapidly increased and then decreased to obtain the growth rate and check it’s stability.

Conclusion

To sum up the experiment, in simpler terms, a new experimental method to study the nucleation and growth of CaCO3 crystals was developed and thoroughly tested. The experiment demonstrated how the crucial role that precise flow control plays.

If you’re interested in achieving this level of precision in microfluidic flow control, the OB1 MK3+ pressure-driven flow controller was used as the flow control system within this experimental setup.

Through this experiment, researchers will be able to obtain accurate growth rates of single polymorph crystals without any additional assumptions.

To delve deeper into the findings of the author and study the fine details of the experiment, please visit the research paper here.

References