WO2019132829A1

WO2019132829A1 - Passive micro mixer for microfluidic systems

Info

Publication number: WO2019132829A1
Application number: PCT/TR2018/050760
Authority: WO
Inventors: Ender YILDIRIM
Original assignee: Cankaya Universitesi
Priority date: 2017-12-27
Filing date: 2018-12-04
Publication date: 2019-07-04
Also published as: TR201722168A2

Abstract

The invention subject to the application is related to a passive micro mixer for microfluidic systems that provide homogenous and rapid mixing of fluids by means of its zigzag structure which is formed of a series of contractions which connect circular wells that are located at the connection area of two inlet channels at the direction of flow.

Description

PASSI VE Ml CRO Ml XER FOR Ml CROFLUI Dl C SYSTEMS

Technical Field of the I nvention

The invention subject to the application is related to a passive micro mixer for microfluidic systems that provide homogenous and rapid mixing of fluids by means of its zigzag structure which is formed of a series of contractions which connect circular grooves that are located at the connection area of two inlet channels at the direction of flow.

Known State of the Art ( Prior Art)

Microfluidic systems are related to the behaviour of microfluids, their precision control and manipulation that has been limited to a scale below millimetres in terms of size and small in terms of geometry. Microfluidic systems are related to multi disciplinary fields in which several sciences such as engineering, physics, biochemistry, nanotechnology and biotechnology intersects and they comprise practical applications in the design where low volume fluids are processed for multiplexing, automation and high throughput screening.

Microfluidic systems enable the controlled operation at micro and nano levels, and make it easier to sensitize and improve analysis methods. Microfluidic systems are frequently used in chemistry and medical labs.

One of the most important applications of microfluidic systems is the lab-on-a-chip ( LOC) technology.

Chemical and physical analysis of very small volumes of samples can be carried out rapidly and accurately in parallel on a few mm sized chip.

It is possible to combine different function such as determination of single cancer cells, diagnosis of possible diseases, blood cell count using a single drop of blood by means of unique and complicated LOC designs. Microfluidic systems are also used for separating valuable fluids. I n fluid mechanics the Reynolds number ( Re) is the ratio of inertial forces to viscous forces, of a fluid and as a result this value provides the relative significance of these types of two forces to each other under certain fluid conditions. Due to this reason, the Reynolds number is used to define different flow regimes such as laminar flow and turbulent flow.

The Dean number ( De) is a nondimensional parameter defined in the flow along curved channels. If a fluid reaches a curve while it is moving along a straight channel, this curve leads to the change of direction of the main stream of the fluid particles. Flow speed shall decrease as the pressure increasesat the vicinity of the convex wall due to an inverse pressure gradient which arises from the curvature This will lead to a secondary motion that is superposed onto the primary flow, as a result the fluid located at the main axis of the channel is pushed towards the outer side of the channel and the fluid close to the wall is curled through the main axis of the channel. I t is anticipated that this secondary motion will appear as a pair of counter rotating vortices which are called Dean vortices.

Some of the present micro mixers operate by means of an external source (magnetic field, electric field, and pressure fluctuation) . These types of micro mixers are called“active micro mixers”. Although the performances of active micro mixers are generally very good, they are not preferred due to their energy requirements. Micro mixers that are obtained by altering the channel geometry are called “passive micro mixers”. Although these types of micro mixers show lower mixing performances in comparison to active micro mixers, they are preferred as they provide practical usage because they do not require an external source. Several passive micro mixers are often designed to shorten the diffusion distance required for mixing the fluids.

The invention of the patent document numbered CN105148781 of the state of the art, is related to an axially symmetrical, logarithmic spiral type cross shaped micro mixer. I n this invention subsequent contractions and expansions are available. However these contractions and expansions are aligned along the axial flow and they do not form any kind of zigzag structure. Therefore it is not possible for Dean Vortices to be formed at high flow speeds contrary to the subject matter of the invention.

I n the WO 2004105153 numbered patent document of the known state of the art, a zigzag structure enhances the mixing efficiency similar to the proposed invention. However the effects of the zigzags are not determined in this patent. When the present applications of the known state of the art are taken into consideration, it can be seen that an application having the same features of passive micro mixers for microfluidic systems subject to the invention is not observed.

Brief Description of the I nvention and I ts Aims

According to the invention two miscible fluids that are moving in different channels are joined together in a mixing channel. The mixing efficiency is enhanced due to the shortening of the diffusion distance between the fluids while the fluids are passing from the contractions that have been placed subsequently in zigzag form on the mixing channel. Moreover the zigzag formed structure triggers Dean Flow at a speed that his higher than a certain flow speed and this leads to enhancement of mixing efficiency.

An aim of the invention is to enhance mixing efficiency by shortening the diffusion distance regionally by means of the contractions and expansions on the mixer

Another aim of the invention is to enhance the mixing efficiency by triggering Dean Flow depending on micro mixer geometry when a certain Reynolds number is exceeded. As a result, it is possible to obtain mixing efficiency that is higher than 80% with flow regulation arising completely from channel geometry without the need for any external factor in high Reynolds numbers ( 10-100) for micro flows.

Definitions of the Figures that Describe the I nvention

The figures that have been prepared in order to further describe the passive micro mixer developed for microfluidic systems by means of this invention are presented below.

Figure 1 - Schematic view of the micro mixer

Figure 2- View of a implemented micro mixer and the mixing processes.

Figure 3- Light intensity profile which shows the mixture at the outlet of the micro mixer at a high Re number flow.

Figure 4- Micro mixer efficiency for different Re numbers.

Figure 5- Formation of Dean Vortices depending on the increase of Re number. Aspects/ Sections/ Parts Constituting the I nvention

The parts/ sect ions/ aspects shown in the figures that have been prepared in order to better describe the passive micro mixer for microfluidic systems developed with this invention have each been numbered and the references of all numbers have been listed below.

1 . 1 st fluid inlet channel

2. 2nd fluid inlet channel

3. Contraction

4. Mixing channel

5. Micro mixer outlet

6. Opening of the 1 st fluid inlet channel

7. Opening of the 2nd fluid inlet channel

8. 1 st fluid

9. 2st fluid

Detailed Description of I nvention

I n the passive micro mixer for microfluidic systems subject to the invention, The 1 ^st fluid (8) and the 2^nd fluid (9) which are miscible move in two different paths separately and the 1 ^st fluid enters through the inlet channel opening (6) and moves through the 1 ^st fluid inlet channel (1 ) and the 2^nd fluid enters through the inlet channel opening (7) and moves through the 2^nd fluid inlet channel (2) respectively; following this the fluids merge together inside the mixing channel (4) having contractions (3) in subsequent zigzag form placed thereon ( Figure 1 ) .

The mixing efficiency increases since the diffusion distance between the fluids (8, 9) decreases as the fluids (8, 9) flow through the contractions (3) on the mixing channel (4) . These contractions (3) also lead to narrowings and expansions along the mixing channel (4) . This situation causes circulations in the flow, which increases the efficiency of the mixing process. Additionally the zigzag structure triggers the Dean flow above a certain flow speed and causes the mixing efficiency to increase further.

The contractions (3) have been obtained by opening cavities in the form of circular wells such that the distances between the two successive wells is smaller than the diameter of the wells. As a result the width of the contractions (3) and in turn the diffusion distance can be changed by changing the distance between the wells . Moreover the number of micro wells that have been opened successively can be increased to increase the number of the contractions.

The effect of mixing of a fabricated micro mixer has been shown in Figure 2. I n this test water coloured with food dye and colourless distilled water is used. The light intensity depending on colouring shows the mixing performance. The light intensity changes sharply between the fluids at the inlet, but varies about an average value at the micro mixer outlet. Figure 3 shows the light intensity distribution along the micro mixer at high Re numbers.

This shows that mixing occurs. For the ideal mixing, normalized value of the light intensity depending on colouring needs to be 0.5 uniformly across the channel. The mixing efficiency can be calculated by using the below mentioned formula according to this normalized average value.

I n this equation /,· shows the normalized grayscale light intensity of /th pixel across the channel (4) section viewed on a frame recorded at the opening (6) of the micro mixer. I_avg is the average of the normalized grayscale light intensity of all pixels across the section. If the variance of the normalized grayscale light intensity about the average of 0.5 value is small, this shows that mixing performance is high. Figure 4, shows the change of the mixing performance that has been calculated as described above with respect to the Re number. I n Figure 4, the mixing performance starts to increase after a certain Re number value (Re= 7) . Considering the dimensions of the micro mixer of concern (200 pm channel (4) width, 100 pm channel (4) height, 45° zigzag angle, 50 pm knuckle (3) width) , the De number is calculated as 10 at this Re value, according to the formula below.

I n this equation D_h denotes the hydraulic diameter of the channel (4) (for a channel with rectangular cross-section D_h = 2( wh)l ( w+ h) , where w is the channel (4) width and h is the channel (4) height) and /?denotes the radius of curvature of the flow axis due to the zigzag. According to Sudarsan and Ugaz (2006) , De= 10 is accepted as a critical value, and flow above this value creates Dean vortices. Figure 5 shows the flow velocity field at channel (4) section for different Re numbers (Re = 7 and Re = 20) obtained by computation fluid dynamics model. The arrows shown in Figure 5, denote the flow velocity. It is shown that Dean Vortices are being just formed when Re = 7 ( De = 10) and that Dean Vortices have developed when Re = 20 (De = 29) .

Computational fluid dynamic analyses show that mixing efficiency increases with the number of contractions (3) . The mixing efficiency which was around 70% when the number of contractions (3) was 45, increases above 80% when the number of contractions (3) was increased to 55.

Claims

CLAI MS

1. A passive micro mixer for microfluidic systems characterized in that it comprises a 1 ^st fluid inlet channel opening (6) and a 2^nd inlet channel opening (7) where the miscible fluids (8, 9) enter therein from two different paths respectively, the 1 ^st inlet channel ( 1 ) and the 2^nd inlet channel (2) where the fluids (8, 9) reach after they pass through the fluid inlet channel openings (6, 7) , zigzag placed successive contractions (3) through which fluids (8, 9) flow, and a mixing channel (4) on which the contractions (3) are located .

2. A passive micro mixer for microfluidic systems according to claim 1 , characterized in that the contractions (3) are obtained by opening two circular micro wells such that the distance between the centres of the circular micro well is smaller than the diameter of the wells.

3. A passive micro mixer for microfluidic systems according to claim 1 , characterized in that the mixing efficiency can be controlled by changing the number of contractions (3) and the width of contractions (3) .

4. A passive micro mixer for microfluidic systems according to claim 1 , characterized in that in micro flows with high Re numbers between 10 to 1 00, high mixing efficiency is obtained in cases where the De number exceeds 10 depending on the geometry of the mixing channel (4) .