What is it that is meant by microfluidics within flowers?

It is not a metaphor: the stem of a flower is filled with small canals carrying water (and whatever is dissolved in it) to the top. Microfluidics, in general, is concerned with controlling small volumes of liquid, such as a drop of water, through channels only a fraction of a millimeter in diameter. Those channels are already present in a plant, and rather astonishingly, they are engineered.

What is the reason behind the change of color of white flowers when they are dipped in coloured water?

Because the dye flows with the water through the stem’s internal microchannels, it finally reaches the petals. The staining increases with time, in part because more dyed water passes through, and in part because dye molecules adsorb to plant tissues. The outcome is an apparent map of fluid transport; however, it occurs at the microscale.

What is the transport structure of the plant in this experiment?

Mostly the xylem. Xylem is a system of long, narrow, and relatively straight channels dedicated to the water movement in the lower sections of the plant to the leaves and petals. When you think of a microfluidic engineer, you are dealing with an inherent microchannel network with geometry, surface chemistry, and flow mechanisms.

What is the pressure, pumping, or something that causes the flow to go upward in the stem?

No pump in the usual sense. The movement is largely driven by capillary action (and the larger transpiration process). Water can be drawn upwards in narrow channels by adhesion (water preferring the channel walls) and cohesion (water molecules to one another). Exposed ends Evaporation above ground. It also contributes to evaporation by continually drawing water through the system. The plants use this evaporation, often called transpiration.

What should I do to make it multicolored?

It can be done very straightforwardly: divide the stem along its length, and place each portion in a separate colored water set. The procedure typically involves a straight incision of approximately 10 cm and ten drops of food coloring in each small jar of water. Change after 1-2 hours and more striking after 12-24 hours.

The interesting bit (which is also a little unpredictable) is that the initial color limits may be quite strong, almost as individual lanes of circulation, and then shift slightly depending on the reconnection of channels within the stem.

Will the color disappear if I take the coloured flower and put it again into clear water?

Not completely. The dye molecules can bind to petals and stain internal pathways, so they may remain visible even when the dyed water is replaced with clear water. It is an effective lesson to keep in mind when microfluidics experiments are involved as well: adsorption and binding can produce memory effects even when you assume that you have flushed a system.

But what happens to the flower that has not received any water, but has already absorbed dye?

It dries out. Water can evaporate through the open ends of channels (both at the cut stem and in the upper tissues). The flower can not dry out; it still has color, basically a snapshot of the dye location. It is aesthetically pleasing, yet it also quietly demonstrates a real transport principle and shows how evaporation can be a formidable force at small scales.

Does that mean that microfluidic flow is confined to tubes and rigid channels?

Not at all. Microfluidic transport may occur through porous materials via wicking. Imagine paper towels, fabrics or any structure of fine pores created by fibers. Water moves in these materials not through man-made microchannels but by capillary action, wetting areas, and moving toward drier areas.

Scientifically speaking, what is the experiment of the walking rainbows demonstrating?

It is a standard example of microfluidics made of paper and diffusion-based mixing. You have jars with paper towel bridges, colored jars of water, and wicks that allow water to wick through the paper into a glass of water. Over time, the transparent dish turns colored, and two primary colors appear; they can combine to form a secondary color (e.g., yellow + blue = green).

It consists of two processes piled atop each other: capillary transport through the porous paper network and molecular diffusion that drives concentration gradients and mixes the dyes in the receiving jar.

So why should the researchers be interested in these unsophisticated demos- beyond outreach or education?

They simulate actual design issues in lab-on-a-chip systems, including capillary-driven pumping, porous microfluidics, surface interactions (adsorption/binding), and transport coupled to evaporation. Such kitchen-table experiments involve cheap analogs of phenomena you would subsequently struggle with (or pursue to advantage in microfluidic chips) in diagnostics, sample preparation, and passive pumping designs.