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Name: Muhammad H.
Status: student
Age: 18
Location: N/A
Country: N/A
Date: 2001-2002


Question:
Consider a drop of water lying on a table. Slowly move your finger close to the drop and watch closely. Before your finger touches the water drop the water drop just rises and sticks to your finger. Why and how does the finger attract the drop without touching it?


Replies:
Good question. I have a few related plausible explanations but do not know which one is the right or the main answer. One needs verification, for example, under a microscope, and/or by setting up a low voltage between the finger and water in an experiment as you can infer from the following.

First, although a water droplet is typically smooth, a finger is not. It is thus possible that in fact you make physical (or close to physical) contact with the surface of water thought small peaks on the finger. These can be either from dead skin on the finger or from contaminants in the air or other origins. Eye cannot see dimensions under say 10 m and all one needs for the flow of water to finger is a stand of dead skin or a particle 10 m in diameter and a few tens of microns in length. It is also possible that such particles may be attracted to water electrostatically in which case they rise off the finger bed and connect or attach to water.

Another but related explanation is that there is a higher concentration of vapor on the surface of water at the boundary with air. This mixed medium may provide the means for either or both electrostatic and surface tension forces to attach water molecules (and thus the droplet) to your finger without water and skin first touching.

Other explanations include molecular attractions that are significant when the distance between objects is on the order of a few m and also the capillary forces on the finger. Also, note that finger and its surface do move even though they are not perceptive motions.

You can test all of these by using a microscope, a finger, a smoothed surface metal needle or rod (such as stainless steel), a means to charge and also measure the charges in the two contacting objects (water and finger/rod) and perhaps a means to heat the water (to increase evaporation to test the second hypothesis). This is a good question and makes a project that one can learn a few things along the way. If you further research it, I suggest you post your findings on the web.

Ali Khounsary, Ph.D.
Advanced Photon Source
Argonne National Laboratory


If the table is not metallic and is not grounded, and if you are not grounded to the same electrical potential as the drop of water, you and/or the drop may have sufficient static electrical charge to cause what you observe.

Vince Calder



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