question: What causes a suction cup to stick to a surface?
comment: Vacuum. A suction cup works by creating space where there was no space and does not allow the outside atmosphere to come in. This imbalance of pressure allows the air pressure from outside the suction zone to press on the cup and hold it to the smooth surface to which it is attached. This is similar in principle to how a straw works. A straw works when a person lowers the air pressure in their mouth by sucking and be outside atmospheric pressure, which is greater at that point, presses the liquid down and then up through the straw.
passages: BULLET::::2. No air leaks into the gap between the cup and the surface.
  
  BULLET::::3. The pulling force is applied perpendicular to the surface so that the cup does not slide sideways or peel off.
  
  Section::::Artificial use.
  
  Artificial suction cups are believed to have first been used in the third century, B.C., and were made out of gourds. They were used to suction "bad blood" from internal organs to the surface. Hippocrates is believed to have invented this procedure.
   Self-sealing suction cup
  
  The self-sealing suction cup is a suction cup that exerts a suction force only when it is in physical contact with an object. Unlike most other suction cups, it does not exert any suction force when it is not in contact with an object. Its grasping ability is achieved entirely through passive means without the use of sensors, valves, or actuators.
   Suction cup
  
  A suction cup, also known as a sucker, is a device or object that uses the negative fluid pressure of air or water to adhere to nonporous surfaces, creating a partial vacuum.
  
  Suction cups are peripherial traits of some animals such as octopuses and squids, and have been reproduced artificially for numerous purposes.
  
  Section::::Theory.
   Section::::Use in Robotics.
  
  The self-sealing suction cups have been incorporated in robots to improve their passive grasping capabilities. Due to the design of the suction cups, a central vacuum source can be used to effectively generate suction force from the cups and reduce the number of actuators and sensors for the robot.
   When the user ceases to apply physical pressure to the outside of the cup, the elastic substance of which the cup is made tends to resume its original, curved shape. The length of time for which the suction effect can be maintained depends mainly on how long it takes for air or water to leak back into the cavity between the cup and the surface, equalizing the pressure with the surrounding atmosphere. This depends on the porosity and flatness of the surface and the properties of the cup's rim.
  
  Section::::Calculations.
   BULLET::::- To affix objects to nonporous vertical surfaces such as refrigerator doors and tiled walls, and mooring ships
  
  BULLET::::- To move large smooth objects such as panes of glass, automobile windscreens and raised floor tiles
  
  BULLET::::- In toys such as Nerf darts
  
  BULLET::::- In toilet plungers
  
  BULLET::::- By urban climbers, to scale buildings with smooth exterior surfaces
   In 2015, several improvements were made to the design of the self-sealing suction cup to improve its grasping capabilities. The previous design demonstrated the following flaws:
  
  BULLET::::1. The design required a high overall cup height, which increased the bulk of the device.
  
  BULLET::::2. The design was relatively fragile with respect to hyper-extension as well as torsional and shear forces.
  
  BULLET::::3. The internal geometry made it difficult to remove the support material inside, varying the quality of the seal.
   The force required to detach an ideal suction cup by pulling it directly away from the surface is given by the formula:
  
  where:
  
  This is derived from the definition of pressure, which is:
  
  For example, a suction cup of radius 2.0 cm has an area of formula_3(0.020 m) = 0.0013 square meters. Using the force formula ("F" = "AP"), the result is
  
  "F" = (0.0013 m)(100,000 Pa) = about 130 newtons.
  
  The above formula relies on several assumptions:
  
  BULLET::::1. The outer diameter of the cup does not change when the cup is pulled.
   The self-sealing suction cup was first developed in 2010 by a collaboration of researchers from the U.S. Army Research Laboratory (ARL), the Edgewood Chemical Biological Center at Aberdeen Proving Ground, and the University of Maryland.
  
  Section::::Design.
  
  The design of the self-sealing suction cup was initially inspired by the suckers of the octopus and its ability to pick up different sized items by individually actuating its suction cups based on the item’s size and physical features.
   During object grasping testing where the grasping range was examined, the test rig successfully grasped about 80% of the objects attempted. These items consisted of the following: TV remote, pill bottle, glue stick, eyeglasses, fork, disposable bottle, toothpaste, coffee mug, bowl, plate, book, cell phone, bar of soap, paper money, mail, keys, show, table knife, medicine box, credit card, coin, pillow, hairbrush, non-disposable bottle, wallet, magazine, soda can, newspaper, scissors, wrist watch, purse, lighter, compact disc, telephone receiver, full wine bottle, full wine glass, light bulb, lock, padded volleyball, wooden block. (4) As a demonstration of the cups’ strength, the ARL researchers were able to pick up a full bottle of wine using only four of the dime-sized suction cups.
   BULLET::::- Nema C88 is a vacuum pen that simplifies the application of Etest® strips. The applicator is held like a pen and the evacuation hole is covered with the fingertip to create suction. The suction cup is placed on the strip to lift it up and then position onto the agar surface. The strip is released by removing the finger tip from the evacuation hole.
  
  Section::::References.
  
  Etest has 3,000 scientific references in which it has been tested. A few key references are as follows:
   Section::::Strainers.
   BULLET::::- The Conundrum. This is a modified doumbek which has several suction cups attached to the head. A long rubber band is wound around the suction cups. Tension on the band and placement of the suction cups controls pitch.
   A force-displacement test that compared the performance between the self-sealing suction cup, an identical suction cup, and a commercially available suction cup found that the internal structures of the self-sealing cup allowed more force to be exerted for the same displacement compared to the other cups. However, under identical conditions, the self-sealing cup achieved a maximum force of 12.5 N while the commercially available cup achieved a maximum force of 12.9 N.
   The self-sealing suction cup has been subjected to a series of tests to determine the quality of its performance. A flexible test rig with four dime-sized suction cups and plastic ribs connected with rubber tubes was created for force-displacement and testing.
   The working face of the suction cup is made of elastic, flexible material and has a curved surface. When the center of the suction cup is pressed against a flat, non-porous surface, the volume of the space between the suction cup and the flat surface is reduced, which causes the air or water between the cup and the surface to be expelled past the rim of the circular cup. The cavity which develops between the cup and the flat surface has little to no air or water in it because most of the fluid has already been forced out of the inside of the cup, causing a lack of pressure. The pressure difference between the atmosphere on the outside of the cup and the low-pressure cavity on the inside of the cup keeps the cup adhered to the surface.
   It was designed so that, when used as part of a suction cup array, the suction cups that don’t come in contact with the object remain sealed. By having only the suction cups that are in direct contact of the object to exhibit suction force, the researchers were able to minimize leak points where air could enter and increase the pressure that each active cup receives, maximizing the suction force. As a result, an array of self-sealing suction cups can grasp and pick up a wide range of object sizes and shapes. This comes in contrast to conventional suction cups that are typically designed for one specific object size and geometry. In addition, suction cups of various sizes have been manufactured, ranging from the palm of a hand to the point of a fingertip.
   A seal quality test measured the pressure generated from each self-sealing suction cup. The results showed that an array of four cups maintained a pressure of 93.8% atmospheric. The test also demonstrated that not all the cups were equally efficient at sealing after object contact. However, this could be the result of variation in the cups’ prior usage.
   Section::::Usage.
   To determine the viscosity of a liquid, the cup is dipped and completely filled with the substance. After lifting the cup out of the substance the user measures the time until the liquid streaming out of it breaks up, this is the corresponding "efflux time".
  
  On paint standard specifications, one denotes viscosity in this manner: "efflux time", "Zahn cup number".
  
  Section::::Conversion.
  
  One can convert efflux time to kinematic viscosity by using an equation for each cup specification number, where "t" is the efflux time and "ν" is the kinematic viscosity in centistokes.
   Dorian Gibbs of Los Angeles, California had the earliest workable cup holder that attaches to a flat edge in the U.S. with US 5,842,671 granted Dec. 1, 1998. 
   Hard suction hose predates steam or gas powered fire engines and has been available since at least 1888, sometimes referred to as "spiral suction hose". Modern apparatus are commonly equipped with flexible suction as opposed to hard suction, due to the relative ease of use associated with flexibility, as well as the increasing rarity of employing suction hoses to pressurized water sources.
  
  Section::::Diameter.
   Section::::Variants of temperature play.:Fire play.:Fire cupping.
  
  Fire cupping is where the air inside a cup (almost always glass) is heated then placed on the skin - the cooling air creates a low-pressure pocket that pulls the skin partially into the cup. Experienced cuppers can create varying strengths of suction by controlling the heat of the cup.
   The LSM uses a distance function to locate different fluids. A distance function is that whose value represents the smallest distance from the point where it is being analyzed to the interface. This distance function is identified by isolines (2D) or isosurfaces (3D), showing that  the negative values refer to one of the fluids, positive values refer to the other and the
  
  zero value corresponds to the position of the interface.
  
  But, how Heaviside function its inserted in the "Compact Level Set Method?"
   When a large enough surface area of the object is pressed onto the tape, considerable force is necessary in order to detach the object by pulling it off. The mechanism is similar to that of a suction cup, where a well attached cup with a radius of 2 cm (with no air left between the cup and the surface) would require a force of about 130 newtons to detach it from the surface. For micro-suction tape, the necessary force is proportional to the attachment surface area. Compared to a suction cup, the force is smaller for the tape (assuming the same attachment surface area) because only a portion of the surface area contains craters that form vacuum chambers, and the maximal force factor is determined by this portion.
  