SURFACE ENERGY OF SPREADING CELLS 



179 



Fig. V-2. Cell immersed in plasma in equi- 

 librium position with respect to glass surface. 

 6 = contact angle. 



In this environment the cell is spherical and possesses potential surface 

 energy equal to 4irr 2 Tcp ergs per square centimeter, where Top is the 

 tension attributed to the cell-plasma interface. The spherical cell is 

 then allowed to come in contact with a glass plate G shown in Fig. V-l, 

 which is also immersed in 

 plasma. As the cell makes 

 contact with the glass sur- 

 face it exchanges a cell- 

 plasma (CP) interface, of 

 area to, 2 , for a cell-glass (CG) 

 interface of equal area (Fig. 

 V-2). If the cell spreads 

 over the glass surface, this 

 area increases until the cell 

 has expanded and formed a 

 layer one molecule thick. 



The fundamental question is whether the cell will always spread to 

 monomolecular thickness or whether an intermediate equilibrium posi- 

 tion will be attained, owing to the character of the interfaces. Fenn's 

 [1921] theoretical analysis of this situation showed that for a given 

 environment the surface energy of the cell, while spreading, would 

 decrease and reach a minimum value when the cell attained its equi- 

 librium position on a given surface. 



To verify this analysis the spherical cell C is represented as in Fig. V-l. 

 This cell with cell-plasma interface and of unit radius is about to come in 

 contact with the surface G in the presence of the plasma P. As the cell 

 touches the glass surface it assumes the shape shown in Fig. V-2. The 

 problem is to determine the height h of the distorted cell above the hori- 

 zontal surface G at equilibrium in terms of its surface energy. In its 

 new environment it now possesses cell-plasma (CP), cell-glass (CG), 

 and glass-plasma (GP) interfaces differing in area from those in Fig. V-l. 



In assuming the equilibrium position on the glass surface, part of the 

 cell-plasma interface, of area to, 2 , is replaced by an equal area of cell- 

 glass interface. At the circular edge of contact between the three media 

 are located the tensions T G p due to the glass-plasma interface, T GC due 

 to the glass-cell interface, and Tqp due to the cell-plasma interface. 

 At the junction of these three phases the cell-plasma interface makes an 

 angle with the glass-cell interface, which defines the contact angle. 



The cell, if free to spread, must decrease its potential surface energy in 

 the act of spreading. This act of spreading enlarges the glass-cell con- 

 tact area until equilibrium has been attained. In this state the surface 

 energy is at a minimum. The surface energy of the contacted cell is 



