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What is Effusion and explain in detail?
In physics and chemistry, effusion is the process in which a gas escapes from a container through a hole of diameter considerably smaller than the mean free path of the molecules. Such a hole is often described as a pinhole and the escape of the gas is due to the pressure difference between the container and the exterior. Under these conditions, essentially all molecules which arrive at the hole continue and pass through the hole, since collisions between molecules in the region of the hole are negligible. Conversely, when the diameter is larger than the mean free path of the gas, flow obeys the Sampson flow law.
In medical terminology, an effusion refers to accumulation of fluid in an anatomic space, usually without loculation. Specific examples include subdural, mastoid, pericardial and pleural effusions.

Etymology
The word effusion derives from the Latin word, effundo, which means "shed, pour forth, pour out, utter, lavish, waste."

Effusion into vacuum
Effusion from an equilibrated container into outside vacuum can be calculated based on kinetic theory. The number of atomic or molecular collisions with a wall of a container per unit area per unit time (impingement rate) is given by:

assuming mean free path is much greater than pinhole diameter and the gas can be treated as an ideal gas.If a small area 
  
    
      
        A
      
    
    {\displaystyle A}
   on the container is punched to become a small hole, the effusive flow rate will be

where 
  
    
      
        M
      
    
    {\displaystyle M}
   is the molar mass, 
  
    
      
        
          N
          
            A
          
        
      
    
    {\displaystyle N_{A}}
   is the Avogadro constant, and 
  
    
      
        R
        =
        
          N
          
            A
          
        
        
          k
          
            B
          
        
      
    
    {\displaystyle R=N_{A}k_{B}}
   is the gas constant.
The average velocity of effused particles is

Combined with the effusive flow rate, the recoil/thrust force on the system itself is

An example is the recoil force on a balloon with a small hole flying in vacuum.

Measures of flow rate
According to the kinetic theory of gases, the kinetic energy for a gas at a temperature 
  
    
      
        T
      
    
    {\displaystyle T}
   is

  
    
      
        
          
            1
            2
          
        
        m
        
          v
          
            
              r
              m
              s
            
          
          
            2
          
        
        =
        
          
            3
            2
          
        
        
          k
          
            
              B
            
          
        
        T
      
    
    {\displaystyle {\frac {1}{2}}mv_{\rm {rms}}^{2}={\frac {3}{2}}k_{\rm {B}}T}
  where 
  
    
      
        m
      
    
    {\displaystyle m}
   is the mass of one molecule, 
  
    
      
        
          v
          
            
              r
              m
              s
            
          
        
      
    
    {\displaystyle v_{\rm {rms}}}
   is the root-mean-square speed of the molecules, and 
  
    
      
        
          k
          
            
              B
            
          
        
      
    
    {\displaystyle k_{\rm {B}}}
   is the Boltzmann constant. The average molecular speed can be calculated from the Maxwell speed distribution
as 
  
    
      
        
          v
          
            
              a
              v
              g
            
          
        
        =
        
          
            8
            
              /
            
            3
            π
          
        
         
        
          v
          
            
              r
              m
              s
            
          
        
        ≈
        0.921
         
        
          v
          
            
              r
              m
              s
            
          
        
      
    
    {\textstyle v_{\rm {avg}}={\sqrt {8/3\pi }}\ v_{\rm {rms}}\approx 0.921\ v_{\rm {rms}}}
   (or, equivalently, 
  
    
      
        
          v
          
            
              r
              m
              s
            
          
        
        =
        
          
            3
            π
            
              /
            
            8
          
        
         
        
          v
          
            
              a
              v
              g
            
          
        
        ≈
        1.085
         
        
          v
          
            
              a
              v
              g
            
          
        
      
    
    {\textstyle v_{\rm {rms}}={\sqrt {3\pi /8}}\ v_{\rm {avg}}\approx 1.085\ v_{\rm {avg}}}
  )