Inside flowers, Microfluidics!

Explore the movement of water – on the micro-scale!

Water moving through a garden hose or up a straw are common and easy to see examples of fluid flowing in a channel (macroscopic). On the other hand, microfluidics is the flow of very small volumes of liquid (that can be less than a droplet!) through channels that are only a fraction of a millimeter wide. Such thin channels are abundant in biology and play important roles in transporting fluid in living organisms, e.g., around the body (capillaries are the smallest blood vessels) and in plants (water travels in tiny channels from the roots, up the stem and to the tips of the leaves).

Flow on the micro-scale can be hard to see! A classic tool to help visualise very small channels and the fluid inside is food dye. For example, the small channels in plants are visible in the cut end of a celery stalk after soaking in water mixed with food dye. A similar experiment can be done using flowers with some fun results.

Material

• White flowers
• Sharp blade
• Food colour
• Small jars (2-3 per flower)
• Water

Method

• Add 10 drops of food colour to 2 small jars of water
• Cut the stem of a white flower lengthways (make the incision straight and about 10 cm long)
• Put each end of the stem into a jar of different coloured water
• Observe the petals after 1-2 hours and again after 12-24 hours. What changes do you see?

To think about

• What happens if you make 2 parallel incisions and insert the ends into three jars of different coloured water?
• What will happen to the coloured flowers if you transfer them to a jar of clear water (will the colour disappear)?
• What will happen to the coloured flowers if you transfer them to an empty jar (where does the water go)?

The science: movement of fluid in microchannels

Observation: The petals become coloured. The colour becomes stronger over time.

Explanation: capillary action in the “veins” of the plant (called xylem for water transport), a network of thin, straight channels that carry water from the roots to the leaf tips.

How does liquid move in channels? Fluid flow is driven by forces. Common examples include gravity (water in a stream flows downhill, the gravitational pull of the moon causes tides on earth) or external pressure (as in the case of a garden hose or a straw). In plants, water movement occurs by capillary action. Capillary driven flow is the movement of water up thin tubes due to a combination of molecule adhesion and cohesion. Adhesion describes the property of the liquid molecules sticking to the walls of the channel. Cohesion is the ability of molecules of the same type, e.g. water, to stick to each other. In plants this directional flow is called transpiration and is aided by evaporation at the open ends of the channels in parts of the plant that are above ground.

In this experiment, the molecules of food dye will bind to the petals and stain the channels even after the coloured water is replaced by clear water. If no water is left in the glass, the flower will dry out as the water evaporates from both the top and bottom open ends of the channels. Dried flowers will remain coloured!

What if the channel is not a tube?

Microfluidic movement does not have to be guided through a classical tube or trough. Materials with wicking properties will do the job, like paper towel or the hem of your jeans in the rain! This experiment explores the movement of water through micropores made by networks of closely woven fibres.

Material

• Small jars (3)
• Food colour
• Paper towel strips
• Water

Method

• Fill three small jars with water
• Add 10 drops of food dye in primary colours to two of the jars (leave the third jar clear)
• Cut a sheet of paper towel into multiple strips about 2 cm wide
• Soak strips of paper towel in coloured water
• Connect the jars of coloured and clear water using a bridge of paper towel strips
• Observe the jar of clear water after 1-2 hours. What changes do you see?

The science: movement of fluid through micropores

Observation: movement of coloured water up the paper and into the jars of clear water. After time the clear water will turn a mix of the two primary colours.

Explanation: (paper microfluidics) the porous network of fibres that makes up the paper acts like a wick to absorb water from a wet area to a neighbouring dry area. The dye molecules (colour) are dissolved in the water and are mobile and free to diffuse through the liquid to an area of higher to lower dye concentration. The two primary colours mix by diffusion in the jar to create a secondary colour (e.g. mixing yellow and blue makes green).

This project was presented during the European Research and Innovation Days. It is the first annual policy event of the European Commission, bringing together stakeholders to debate and shape the future research and innovation landscape.

Lisa Muiznieks, PhD
Marie Curie H2020 Individual Fellow. Elvesys Microfluidics Innovation Center.