PowerPoint Presentation

Drawing pinsMichal Hledk7.7. Drawing pinsA drawing pin (thumbtack) floating on the surface of water near another floating object is subject to an attractive force. Investigate and explain the phenomenon.

Is it possible to achieve a repulsive force by a similar mechanism?2Video of attracting+picture of pins

3ContentDeformation of water surfaceAttraction of pinsMechanismCalculating the motionTheory vs. experimentsRepulsion of objectsMechanismAttracting/repelling boundary41. Deformationof water surface5Forces analysisGravityBuoyant forceSurface tension

Force equilibrium6Force equilibrium

Only unknown quantitiesEq. (1)7Water displacementAbsent water compensated by surface tensionPressure hydrostatic8Water displacementResulting function:Dominic Vella, L. Mahadevan, The Cheerios effect, (2005)``

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9Finding contact angle

Eq. (1):

Predicted angle:

10Contact angle measurementAnalyzing sizeof shade of the pin

Distantlight source

Pin on water11Measuring the contact angleApplying Snells law, fitting contact angle

(size of the shade)Contact angle:12Shape of water surface[mm][mm]

132. Attraction of pins14Why are they attracting?2 pins on water inclined to each otherPotential energy of water and pin decreases as pin descends

Mass of a pin > mass of water displaced15Determining the accelerationHorizontal motion:

1616Slope of one pinGiven by the deformation of water surface by the other pin

Our approximation:

17Drag forceAssuming

position

Video analysis and fit

18Fitting the drag coefficientDistance passed x [m]Time [s]

19Acceleration distance in timeDependence of acceleration on distance and velocity

Numerical solution20Attracting experiment

21Theory vs. experimentDistance of the pins [mm]Time [s]223. Repelling objects23

Repulsive forceObject wetted by water acts downwards

object floats up

24Repelling objectsPlastic caps from pins float upwardsThere is a critical mass does not repel or attract

Behavior depends on weight25

Both caps wetted by water26

+ A little weight on the yellow cap27Empty cap and a cap with a weightTime [s]Distance between the caps [cm]0,027g0,041g0,062g0,162g0,204gGreater mass stronger repulsion28ConclusionWe explained the mechanism offloating, attraction, repulsion

Determined the deformation of water surface

Described the motion quantitativelytheory correlates with experiments

Found the border between attraction/repulsionThank you for your attention29Appendices7. Drawing pins

Drawing pin dipole attracts different objects on different sides31`Water displacementBoundary conditions:

Solution:Dominic Vella, L. Mahadevan, The Cheerios effect, (2005)

32Critical massWater is not deformedSurface tension resultant force = 0

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Critical massPin caps bent edgesMass theoretically:Depth:Mass experimentally:

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